elict111111 - worldradiohistory.com · 2019. 7. 17. · items as photographic flash gun tubes,...

ELICT111111

COMPUTERSUnderstanding theirSecrets

MOCK STEREOSplitting your MusicalAffinities

BUDGET EEPROMPROGRAMMERPut Processing Powerin your Pocket

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High TechnologyTest Equipment

30 -Range Digital Multimeter

£6995Features front -panel socket for trail rtor and capacitor tests.Low hatters indicator, diode check junction and continuitysounder.Measures to 1000 VD(', 750 VAC, 10 amps AC 1 DC current, 20megohms resistance.20µF capacitance and transistor gain. Requires 9c battery . 22-194

MICRONTA

Probe StyleA utoranging

Multimeter

11995Data hold function enables you to freeze the display and to remove it from the circuitlitr more convenient reading.Measures. to 400 volts A('/ D(' and resistance in K -ohms- up to 2 megohms.Includes 2 button batteries. Overload protected. With carrying case 22-165

13.8v DC 12 AMP

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Regulated Power Supply

£599513.S 1/1)( Regulated Supply.Ideal for use with HAM transceivers. 5A continuous./2A intermittent. 15A surge. 240 VA(', 50 Hz.Fused 22-7001

For The Best InHigh Quality Electronics

Over 400 Tandy Stores And Dealerships Nationwide.See Yellow Pages For Address Of Store Nearest You.

InterTAN U.K. Ltd., Tandy Centre, Leamore Lane,Walsall, West Midlands, WS2 7PS. Tel: 0922 710000

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NEXT MONTHWe show you how to: tune in to theradio times with our Rugby -programmedautomatic clock receiver, clean up yourmotoring with a rear wiper controller, puteprom programming under micro-processor control, and excitingly exploreinductors and capacitors in our BasicElectronics adventure.

* SO DON'T MISS OURMARCH 1990 ISSUE

* ON SALE FROM FRIDAYFEBRUARY 2ND

* YOU CAN'T BEAT OURVALUE

* OR OUR CELEBRATEDOFFERINGS!

PRACTICAL ELECTRONICSVOL 26 NO 2 FEBRUARY 1990

CONTENTSCOMPETITION RESULTSCELEBRATED WINNERS! 60An Acorn A3000 computer, 10 Cirkit digital mu!tTheters, years and years of PE subs!

CONSTRUCTIONAL PROJECTSPE MODEM by Kevin Kirk 12PE puts you on the hi -tech comms line, building your own high quality, modest costmachine that's suitable for use with virtually any computer!

SWITCHED EEPROM PROGRAMMER by Malcolm Harvey 27How to build a really low cost programmer that puts microprocessing control withinreach of all constructors' pockets!

MOCK STEREO AND EQUALISER by John Becker 32An Easi-Build example of how one into two will go if you stop at three first! Turn yourmono music into simulated stereo, with a touch of potted equality thrown in!

MOTOR SPEED CONTROL by Andrew Armstrong 40An Ask PE answer to putting you ergonomically on course to the 19th with a powerfuldriver. Suitable for non -golfers too!

INGENUITY UNLIMITED by Enthusiastic Readers 42Three novel ideas: a Hex Keyboard, a Car Burglar Alarm, and an RF Relay.

SLAVE FLASH by Chris Brown 45Chris and his multiple flash -gun genie brighten up the photogenics and earntheir Brownie points!

SPECIAL FEATURESCOMPUTERS - PART ONE by Mike Sanders 21From Ascii to Zilog, this is the story of the hows and whys, and the rise and rise ofcomputers - and you don't need a degree in logic to follow it!BASIC ELECTRONICS - PART TWO by Owen Bishop 35Discovering how electronics works, and why! A closer examination of resistance isthis month's theme.

CROSSWORD - THE ANSWERS! by Anthony H. Smith 48Cross-checking our techno-cryptic blankety-blanks!UCT PERFORMANCE - PART TWO by Anthony H. Smith 49Contact bounce avoidance, frequency multiplying schemes, selective arming, andother special techniques for improved uct measurements.

REGULAR FEATURESEDITORIAL by John Becker - Beware the cat! 9

LEADING EDGE by Barry Fox - CD video 8

INDUSTRY NOTEBOOK by Tom Ivall - Two cultures revisited ...57

SPACEWATCH by Dr Patrick Moore - Junk advert space 46

READERS' LETTERS - and a few answers 38

PRODUCT FEATURESNEWS AND MARKETPLACE - what's new, where and when 4

PCB SERVICE - professional PCBs for PE Projects 52

ARMCHAIR BOOKSHOP - haven for practical bookworms 58

ADVERTISERS' INDEX - locating favourite stockists 62

PE TAKES TECHNOLOGY FURTHER - BE PART OF IT!

N EIS

TEXTURISEDNICAM VCRFerguson, proudly claiming tohave been the market leader in

video for over a decade, appearequally proud of their new Nicamhifi vcr. Aimed at the mid -pricesector of the market. the Videostar

FV37 is a slimline, three -headmachine with an innovative Fastext-capable lcd handset, and retailing ataround £499.

The major benefit of the FV37H,say Ferguson, is its ability to decodeand record Nicam digital stereotransmissions and, in audio long playmode, to record up to eight hours ofhifi stereo sound. A Peritel (Scan)connector and separate audio phonosockets ensure easy connection tovirtually any other audio-visualproduct.

Plugging an optional teletextadapter into the rear socket allows itto receive and display teletext pageson screen, even on tv sets without

text. And what a boon the provisionof this teletext can be, enabling as itdoes the vcr to be easilyprogrammed. Simple buttonselection on the remote controlhandset moves a cursor around theteletext programme pages. Thedesired programme info is thenstored in the machine's memory,giving probably the simplest methodyet of programming the vcr. It's alsopossible to record subtitles with thissystem, offering an extremelyvaluable benefit to the hard ofhearing.

There's much more to the FV37than there's space to publicise. Formore details, rush down to yournearest Ferguson retailer, or contactFerguson direct at Cambridge House,Great Cambridge Road, Enfield,Middx, EN1 IND. Tel : 01-363 5353.

CONTROL TORED LEADER

If you're looking for a unit that willcontrol high intensity infrared

lamps and modules, look into Lux-Therm's new model LX25 variablepower controller.

The LX25 utilises a phase anglefired triac circuit, which allowscontrol over the voltage to the lampswithout any noticeable lamp flicker.Its use enables the intensity of IRlamps to be controlled from zero tomaximum lamp voltage up to a totalresistive load o f 25 amps.

J.F. Butler's the hot -man to handleyour enquiries at Lux -ThermProducts, Unit M, Portway IndustrialEstate, Andover, Hants, SPIO 3L U.Tel : 0264 51347.

CATALOGUE

Continuing our alphabetical browsethrough advertisers' literature

Rotalink are definitely a company you should contact if it's motors thatdrive your robotic ambition! Rotalink's product review and catalogue is full ofdetailed information on a wide range of miniature motors and associatedproducts. Included are synchronous, dc, uni-directional, reversible and steppingmotors, gear boxes, cycle timers and relays. Outline drawings and specificationtables are given for each product, making your selection of the best item a verysimple procedure. Rotalink Ltd, Cropmead, Crewkeme, Somerset, TAI8 7HQ.Tel : 0460 72000.

Service Trading have been a valuable source of unusual products for manyyears. Their range, as shown in their recently received literature, includes suchitems as photographic flash gun tubes, strobe kits, rheostats, variable -voltagetransformers, bilge pumps (!), UV and halogen lamps, water to heat exchangers,and eprom erasing kits, just to mention a few. Service Trading Co, 57 BridgmanRoad, Chiswick, London W4 5BB. Tel : 01-995 1560.

Sony is a household name for audio and vision consumer products and therecently received catalogue details the full range available. It is well illustratedwith colour photographs, and in addition to listing the product specs, other text isincluded that makes the booklet nearly as much like a magazine as a catalogue.The ranges include tv receivers, video recorders, hifi systems, radio receivers,speakers, turntables, headphones, cassette recorders, alarm clocks, tapes andaccessories. The full range of Walkman's is also well covered. Did you knowthat the Sony Walkman has just celebrated its 10th anniversary? The catalogue isentitled A Sense of Perception, and has a £2 price marker on it. Sony (UK) Ltd,Sony House, South Street, Staines, Middx, TW18 4PF. Tel : 0784 67000.

STC Instrument Services' latest catalogue includes full colour photographs ofall the products detailed. There are over 300 pages covering computer systems, testgear (from meters to oscilloscopes), counter timers, power supplies and a largeselection of technical books. This is a catalogue that should be in the workshop ofany serious electronics designer or constructor. STC Instrument Services, DewarHouse, Central Road, Harlow, Essex, CM20 2TA. Tel : 0279 641641.

STC Mercator have sent their massive catalogue. This too ought to be on theshelves of anyone seriously involved in electronics. In addition to passivecomponents such as capacitors and resistors, it includes many products that areless widely available. Such products include inductors, emi filters, varistors,vacuum fluorescent displays, resonators and speech systems. STC Mercator,South Denes, Great Yarmouth, Norfolk, NR30 3PX. Tel : 0493 844911.

Tandy's catalogue of consumer and electronic products is always a goodread. You've probably already picked up your latest copy from your local Tandystore, but if you haven't, nip down now and get one! The contents of its 140 pagesare too full to list, but from aerials to watches through computers to videosyou'll find a wealth of products to make your mouth water! If you don't have aTandy store in your area (unlikely!) contact their head office : Intertan UK Ltd.Tandy Centre. Leamore Lane, Walsall, WS2 7PS. Tel : 0922 710000.

Three -Five Semiconductor Ltd is a company formed in 1985 after thepurchase of National Semiconductor's optoelectronics division. Their recent leafletlists the range of optoelectronic products available, including lamps, digits,optocouplers, lcd and led displays, bargraphs, clock modules, and much more. Formore information contact III -V direct at Suite 32, Cherry Orchard North,Kembrey Business Park, Kembrey Street, Swindon, SN2 6UH. Tel : 0793 618835.Alternatively, contact their main distributors, Hawke Components, tel : 01-9797799.

4 PRACTICAL ELECTRONICS FEBRUARY 1990

N EIS

DISHING OUT COLDCOMFORTT -No you dread the thought ofI./trudging through the snow on ableak night in order to clear the snowfrom the satellite tv receiver in yourback garden?

Well, Mandoval Coatings maywell have the cool answer to cleaningup the Sat -Soaps. They've announcedthe launch of Vellox, a uniquehydrophobic coating speciallyformulated to repel water fromsatellite dishes. Surfaces coated withVellox refuse water to such an extentthat a visible air layer, recognised as asilver sheen, can be seen between thewater and the treated surface. Waterin contact with these surfaces formstiny droplets which literally rollacross the surface in an almostfrictionless manner.

Originally developed for the USAir Force, Vellox has been keepingcommunications equipment free ofice, snow and water for over five

years. It is a two component productcomprising a range of primers andtopcoats which can be adjusted totake account of almost any surface. Akit is available which containseverything necessary to prepare andcoat a satellite dish up to 90cm indiameter. Other uses for Velloxinclude applications as diverse as theprotection of radar equipment, pcbs,and even cardboard packaging!

It seems that whenever ice, snow,water sheeting or high humiditypresent a problem, Vellox can providean answer. I wonder if it wouldprotect an Editor caught out on hisbike in a storm?

Graham Ellicott might know if Ifind the time to ask him - he's theman you should contact for moreinformation about Mandoval CoatingsLtd, at Lawn Road, Carlton-in-Lindrick, Nr Worksop, Notts, S8I9LB. Tel : 0909 730059.

If you are organising any event to do withelectronics, big or small, drop us a line, weshall be glad to include it here.

PLUMBING PE'SDEPTHS1 Dear John", writes Mike Everett,

"I have enclosed details of aProton Magnetometer we havedeveloped. You may not remember,but I spoke to you some 18 monthsago on the subject, and you sent acopy of PE's original article of Oct 70which provided the inspiration todevelop it.

"It is now perhaps the highestspecified unit for under £10kanywhere worldwide. I thought youmight be interested, if only from theinspirational point of view."

Yes, Mike, I do remember. ThePE issue is the one with FoundingEditor Fred Bennett on the cover,testing out the original on the Thameswaterfront! I am always delighted toknow of products and careers thathave been inspired by PE.

Mike Everett also sent complete

specifications of the WreckmasterProton Magnetometer. Very healthyand interesting reading they make.Applications for the detector includescuba diving, fishing, salvage,surveying, archeology andengineering. It allows rapid, positivelocation of existing and new wrecksfrom distances of up to 300 metres.The unit measures 340 x 300 x140mm, is microprocessor controlled.and has an lcd readout. As you canhopefully see from the photo, it has aselection of several water repellantcontrol pads, and is reputedlyinflatable -boat proof. (That indeed isa harsh environment to design for!)

The Wreckmaster sells at £1695plus post, and is available fromCapricorn Marine Technology, 19Pickeridge Close, Taunton, Somerset,TA2 7HN. Tel : 0823 278093.

Please note : Some events listed here may be trade or restrictedcategory only. Also, we cannot guarantee information accuracy, socheck details with the organisers before setting out.

Mar 7-8. Laboratory 90. G-Mex Centre, Manchester. 0799 26699.

Mar 9-10. London Amateur Radio Show. Picketts Lock Centre,Edmonton, North London. Advance ticket sales and trade enquiries toThe Secretary. LARS, 126 Mount Pleasant Lane, Bricket Wood, HertsAL2 3XD. 0923 678770.

Mar 28-29. Laboratory, Science & Technology Show. KelseyKerridge Sports Hall, Cambridge. 0799 26699.

Apr 9-11. Cable and satellite exhibition and conference. Olympia,London. 01-486 1951.

Apr 4-5. Drives, Motors, Controls. New Century Hall, Manchester.0799 26699.

Apr 24-26. British Electronics Week. Olympia, London. 0799 26699.

Jun 26-28. Infrared Technology. Wembley Conference Centre. 079926699.

Sep 25-27. British Laboratory Week. Olympia, London. 0799 26699.

RECK MRS T ER

PRACTICAL ELECTRONICS FEBRUARY 1990 5

N E-IWLS

HIGHCHAPARRALN ew to the UK is the home

satellite tv receiver fromChaparral Communications. TheMonterey 20 will enable users tochoose from the 30 or so availableUS and European national tvchannels, including Sky andSuperchannel. Microprocessorcontrolled, the fully integratedreceiver and dish positioner havebeen especially designed to meet theneeds of the European market.

Chaparral claim to be the firstcompany to offer a satellite receiverfor Europe with an audio/videoswitcher. The switcher accepts threeinputs which can accommodate

satellite decoders, vcrs, laser disks,video games, cd players, securitycameras, or any other audio -videosource.

The receiver's auto track featuresimplifies set-up by allowing theinstaller to automatically locatesatellites, while the favourite channelselector enables users to quickly tuneto their preferred programmes. Thesuggested retail price in the UK is£995, including vat.

You can find out more about theMonterey 20 from Steve Chilver,Chaparral Communications, 10Campbell Road, Hanwell, LondonW7 3EA. Tel : 01-579 6587.

CHIP COUNTOf special interest to PE readers this month are a new eprom, an lcdmodule and a 1 Mbit sram.

1MBIT EPROM 27C210

Philips (the new name for Mullard) have added a new higherperformance lower power 1Mbit eprom to their range. This addition ofthe 27C210 now extends the range from 64K to 1Mbit.

The 27C210 is organised as 65536 words of 16 bits and is currentlyoffered with an access time of 200ns. It is available in windowedceramic and plastic 40 -pin dil as well as 44 -pin plcc packages. (Mostreaders will probably only be interested in the more usual plastic 40 -pinversion.) The new eprom is designed to be used with the new familiesof 16 -bit microprocessors and avoids the need for multiple devices. Italso increases system performance when used to replace older, sloweror smaller nmos roms and eproms. The 27C210 is manufactured usingPhilips' 1 micron cmos process which ensures a low powerconsumption (1001.1A standby) and high noise immunity.

All Philips eproms use the Quick Pulse Programming Algorithm, acapability which allows the device to be programmed in less than 12seconds. Programming can be carried out on any standard programmer.

B&W LCD PANEL LM296D XBF

TOOL -UPPRECISELY!

Who's heart doesn't race at thethought of new tools for the

workshop? You're not a true diyer ifit doesn't!

Maplins' new tool set shouldcertainly get the adrenal in flowing,and the price won't give you a heartattack. The photo shows the tools inthe set, and the specially designedblack plastic box, with slide -on lid,that contains them.

The long nose pliers have serratedjaws, plus sprung insulated handles,as do the cutters. There are six flat -

blade and six Phillips screwdrivers ina range of good sizes, plus serratedgrip steel tweezers and a pearlcatcher. A nice touch is the inclusionof a glass magnifier and of course ahammer: no workshop's completewithout one! This one has both plasticand metal faces.

The price for the FK52G microtool set is only £9.95 including vat,and is available from any of Maplin'snationwide shops, or through theirhead office at PO Box 3, Rayleigh,Essex, SS6 8LR. Tel : 0707 554161.

Hitachi have announced a new lcd graphics panel that uses theiradvanced third generation supertwist technology, cold fluorescent lamp(cfl) backlighting and TAB packaging technology to achieveexceptional high visual performance combined with small size, lowweight and high reliability.

The LM296DXBF is a true black -on -white, 640 x 400 display, fullycompatible with the popular ega graphics format. It is ideal for use inmany applications where crts have traditionally been used. Hitachi saythat the display uses an additional plane of liquid crystal cells to cancelout the colour effects associated with earlier supertwist technologies,while enhancing the performance for a high contrast ratio of 15:1, and awide viewing angle of 50 degrees. Coupled with the bright, whitebackground by the cfl backlight, this results in a high quality b&wdisplay that matches the visual performance of crt monitors.

Containing all the necessary row and column drivers, the displayalso has an integral inverter that allows the cfl backlight to oper atefrom a 12V supply. The module interfaces directly to the HD63645advanced lcd controller, or the HD66840F lcd-video interfacecontroller, which converts standard RGB signals into lcd drive signals.

1MBIT SRAM HM628128

For some time Hitachi have had a IM sram available, but only in thesop package (largely unavailable to the diy market). They have nowintroduced the more conventional dil package version which shouldhave greater PE reader appeal.

The HM628128 is organised as 128K x 8 and is manufactured witha cell area of only 5.2 x 8.6 micrometres, allowing 6,400,000 transistorsand resistors to be placed on a chip measuring just 5.7 x 14.4

millimetres! This sram will find particular appeal among users of fastmicroprocessors who want to minimise wait states. Srams are preferredto drams where speed is critical or where battery operation is essential.

MANUFACTURERS' ADDRESSES

Hitachi Europe Ltd, 21 Upton Road, Watford, Herts, WD1 7TB. Tel:0923 246488.

Philips Components Ltd, Mullard House, Torrington Place, LondonWCIE 7HD. Tel : 01-580 6633.

PRACTICAL ELECTRONICS FEBRUARY 1990

N E -M/ -S

CLEVAR TESTERNew from TMK Instruments is the

Multi-Vartest, a singleinstrument which is suitable for awide variety of uses on both singleand 3 phase electrical networks.Some applications include thecalculation of Power Factorcorrection and the measurement ofActive and Reactive energy with anindication of harmonic component.Measurements and their parametersare set up by simple front paneltouchpad programming. AC currentand voltage, Cos Phi, kW, kWh,kVALh, and kVACh are displayed ona 16 character alphanumeric liquidcrystal display.

Battery operated, this handheldinstrument includes a clip -onammeter for the two current ranges of2 to 200A, 20 to 1500A as well astest leads for phase rotation and the500V voltage range. Other accuratemeasurements include a percentage

voltage variation from a pre-set valueand the overcurrent as a percentagethat is absorbed by networkcapacitance and harmonic effects.Active energy is shown as kWhconsumed since the readingcommenced and the elapsed time inminutes. Most results can be shownas a maximum or minimum value andare average values taken over anadjustable period of 1 to 20 seconds.

Fully guaranteed this trulyportable tester is housed in a sturdymoulded case and measures just 210x 116 x 32mm and weighs 450gm.Supplied accessories include a qualitycarrying/storage case.

The price of the Multi-Vartestexcluding vat is £399.00.

For more information, contact:TMK Instruments, Building 3, GECEstate, East Lane, Wembley, Middx.HA9 7PJ. Tel: 01-908 3355.

PATENTLYGOOD NEWS

With thirty million specificationscomprising complete holdings

for all the major patenting bodies andmany documents from other issuingbodies throughout the world, theBritish Library is regarded by manyas the patent source.

The British Library's PatentExpress service already providesphotocopies rapidly and at acompetetive price not only to the UK,but to every part of the globe. Aspart of its continuing drive toimprove the quality of service itprovides, Patent Express has justlaunched its own newsletter

exclusively for account customers.The aim in launching the

Newsletter is to improvecommunications with customers. Itwill report on items of interest tocustomers, everything from improvedmachines being brought into use, to,changes in the law, and will givethem the opportunity to talk backabout issues with which PatentExpress may be able to help.

It costs nothing to be a PatentExpress account customer, so if youwant to keep truly up-to-date withthis unique information sourcecontact Richard Garner on 01-323-7929. Alternatively, write to SRISMarketing and Public Relations, 25Southampton Buildings, ChancheryLane, London WC2A 1AW.

DIGITALSTEREO TVUPDATE

You'll recall that Barry Fox looked1 at the change over to Digital

Stereo TV Sound in PE Oct 89. TheIBA television transmitters coveringthe London area and a large part ofYorkshire became fully operationalfor Nicam digital stereo sound from11 September on both ITV andChannel 4.

The high -quality digital soundsystem, known as Nicam 728, hasbeen developed jointly by thebroadcasters and receivermanufacturers. Initially equipped for

stereo will be the Crystal Palace(London) and Emley Moor(Yorkshire) main transmittingstations, together with associatedrelay transmitters.

At the same time, the programmedistribution network linking Channel4's studios with all the regional ITVprogramme companies is beingequipped with dual -channel sound -in -sync arrangements. The same digitalcoding system will carry the signalsall the way from the studio to thehome ensuring extremely highquality, comparable to that of thecompact disc.

During 1990, the IBA will beextending the availability of Nicamtransmissions to reach about 75% ofthe UK population by the end of theyear.

THEGROWLERRETURNS

Now available from ClareInstruments is an Armature and

field winding tester known by the oldschool as a "Growler". Model V180is designed to indicate faultconditions in armatures, field coils,rotors and stators and is sufficientlysensitive to pick up single short oropen circuit windings from thesmallest to the largest power toolmotor.

The latest test set from Clare iscompact, easy to use and mainspowered. VI80 uses a probe head,bearing two sets of pole pieces, one

pair to induce a small current into thewinding under test and the secondpair to detect that current. The signalcable connected to the probe headreturns the signal to the maininstrument, where it is displayedusing a sensitive analogue meter.Short circuit turns are indicated as anincrease in meter reading, while opencircuit turns are shown as a reductionin the reading. Full adjustment of themeter sensitivity gives a variety oftest methods.

Fully guaranteed, the VI80 issupplied ready for use in a sturdycarrying case with the operatinginstructions permanently affixed inthe lid. The price is £285 plus vat.

For further information contact:Clare Instruments Ltd, Clare Works,Woodsway, Goring By Sea,Worthing, Wc'a Sussex, BNI2 4()Y.


C D Video, the re -launch ofLaservision video disc, has beenconfused by use of the CDV tag on

all disc sizes, 5in, 8in and 12in. This is onlyone of many reasons why CDV has failed totake off in Europe, while video disc has soldvery well in Japan and not too badly in theUSA.

Don't shed too many tears. AlthoughCDV has digital sound (the justification forre -launch) the picture signal is still fmanalogue. The future lies with discs whichstore digital video. In this respect, Europe(except France, which looks likely to "buy"CDV) may well benefit from the failure ofLaservision and CDV. We have a clear deckfor digital video on 5in discs.

BURIED BITS

Although few people realise it there arealready 5in cds in the shops which havedigital video buried in the bitstream alongwith stereo sound. Talking Heads' albumNaked was the first. The system is called

LEADING channel can also carry a code number toidentify a disc. But many record companiesare not using this code. Likewise the Qchannel carries a "digital copy prohibited"flag. This can be set to stop a digital taperecorder copying digital data from the disc.Curiously, not all the record companies havebeen using this facility either.

The information bits for the remaining sixchannels (R,S,T,U,V,W) convey text andgraphics. They stream at only around 45kilobits/second and offer only around 20megabytes of 8 -bit ASCII coding for anhour-long disc.

Hence the result on screen is often trivial,simply still pictures and running text, whichsoon become tiresome. The Talking Heads cdgives a clue to more constructive uses of cdgraphics. As the music plays, guitar chordsare displayed if a tv screen is connected tothe cd player. Likewise foreign opera discscan contain the libretto in a choice oflanguages. The disc can also generate MIDIcodes to control electronic musicinstruments.

Already the cd-rom and CD -Interactive

CD VIDEOCD+Graphics, or BGV (BackGround Video).JVC was the first to start selling CD Graphicsplayers, in the USA, last July (1989). Theyconnect to a tv set, as well as a hi-fi system.

All cd audio discs have part of the datastream allocated for "sub -codes" which indexthe disc and tell the control programmingcircuitry of a cd player where music tracksbegin and end. The sub -codes can also carrytext and graphics. But until recently thefacility was not exploited.

One problem is that it costs money toproduce the graphics programme materialand unless there is decoding hardware on themarket there is little point in the recordcompanies making the investment. Bylaunching hardware, JVC hoped to break thevicious circle. A launch for Europe isplanned for next spring (1990).

But CD+G is limited in scope because ofthe limited number of bits per secondavailable in addition to digital stereo. Thefinal count is 97% of the usable dataavailable for music and 3% for sub codes.Here's how it breaks down and adds up.

SAMPLE FRAMES

For a music disc, the sound is sampled at44100Hz, or 44.1kHz. Each sample isdescribed in a 16 bit code word. The codewords, with right and left channelsinterleaved, are grouped together in "frames"and error correction bits added. So are thesub codes, and bits to synchronise the wholesystem. The data streams off disc at4.3218MHz although only around half thebits represent music or usuable data.

BY BARRY FOXWinner of the

UK Technology Award

The picturebehind the

sound is thefirst step

towards multi-media cd.

There are eight sub code streams, labelledP,Q,R,S,T,U,V,W. The sub code data rate isaround 60 kilobits/second.

The P bits will be used in the future byvery simple compact disc players, withvirtually no fancy features. So far no playersuse the P sub code channel. They all use theQ channel.

The Q channel bits tell the player aboutthe disc track (0-99) and index numbers, theygive information on pauses, timing, whetherthe disc is in stereo or four channelquadraphonic, whether the sound has beenrecorded with pre -emphasis, whether it is anaudio disc or a cd rom data disc and so on.

KUDOS

Without the Q sub code channel, a cdplayer would not be able to work. The Q

formats reproduce much higher qualityimages, with a degree of cartoon animationby turning over the entire data stream tovarious mixes of computer code, text,graphics and compressed sound.

VIEWING HOUR

The long term aim is make the 5in disc aone hour video disc!

There is nothing new in convertingmoving video pictures into digital code andrecording them; professional video taperecorders are already available whichperform this trick. The difficulty is that thedigital code needed to record the picturestreams so fast that there is no hope of acompact disc recording, and replaying, it inraw state. The only way is to compress thedata so that the bit stream reduces to thesame rate as cd audio.

This is seen as an important challenge, notso much to provide movies on 5in discs, butpave the way to a "hyper media system"where sound, graphics, text and movingvideo are all combined on the same carrierand accessed from the same computer workstation.

The technology is daunting and there arenow several different lines of research, allincompatible. Recently, a Working Group (8)of the International Standards Organisationand International ElectrotechnicalCommission met at JVC's new Kurihamaresearch and development centre atKanagawa, near Tokyo. Next month I willreport on the technology they saw anddiscussed.


MOS chips have been with usfor many years and it would,perhaps, seem reasonable to

assume that most readers will be atease in handling them. True, up to apoint. Naturally, one does not expectnewcomers to know instinctively that,though requiring moderately respectfulhandling techniques, the use of cmosdevices should not be the cause forunnecessary concern. One wouldexpect, however, that experienced con-structors would regard cmos handlingas second nature.

The fallacy of the latter expectationwas brought home to me only a fewdays ago. A reader told me that he hadstarted investigating electronics back inthe days of valves. He'd made thechange to transistors and standard ttl

logic chips but then job pressures pre-vented him from keeping abreast withimproving technology. Now that he hadretired he wanted to become moreinvolved in electronics, but was reluctantto construct cmos-orientated projectssince he had heard that the chips couldeasily be killed by static electricity.

His concern, and that of any readerwho has not yet used cmos for similarreasons, is almost totally unjustified. Inthe course of my own constructionaland experimental work, I have seldomkilled a cmos chip, even though onmany occasions I have grossly mis-treated them, violating the proceduresstated in manufacturers' data books.

The instructions given in some data

PRACTICAL

ELECTRONICS

BEWARETHE CAT

books can run to several pages, cover-ing a variety of situations, most ofwhich only apply to the assembly andhandling of cmos on a commercialscale. For example, the use of earthingwrist straps and work pads by opera-tors is detailed at some length. In acommercial situation you would proba-bly also need to reduce static build upby using ionized air blowers, anti -staticsprays, and room humidifiers. Similarly,brush and spray cleaning of cmos prod-ucts would be severely frowned on, andyou might be heavily reprimanded if

you didn't place your assembly in avapour degreaser immediately upon its

removal from antistatic or conductivecontainers!

These requirements, and many oth-ers, although absolutely valid in a com-mercial environment, are of little rele-vance to the average diy constructor.The most important, but very simplerule you as a diyer should observewhen handling a cmos device, is thatyou make sure you have dischargedany static electricity from your bodyimmediately prior to touching it. Sinceyour soldering iron ought to be earthed,preventing any electrical charge build-ing up on it, all you need to do is tobriefly touch the cooler end of the iron'smetal part every few minutes while han-dling cmos chips. Alternatively, simulta-neously touch a finger and the iron toanother metallic surface. In my ownworkshop I have another technique - I

touch the bare metal surface of a testunit which I know is adequately earth-ed.

There are three other simple precau-tions to take with cmos. Never insert orextract chips when the power is on.Never allow power supply or other volt-ages to exceed those stated in circuitdiagrams or data sheets. Keep staticgenerating items away from the chips;such items include many man-madefibres and plastics, wool, hair and fur.There's no need to let the last precau-tion dictate what you wear, but I strong-ly suggest you keep the cat off theworkbench!

THE EDITOR

Editor:John Becker

Sub -Editor:Helen Armstrong

Technical Illustrator:Derek Gooding

Advertisement Executive:David Bonner

Office Manager:Louise Hewett

Production Manager:David Hewett

Deputy Publisher:Tom Robson

Publisher:Angelo Zgorelec

Practical Electronics,Intra House, 193 Uxbridge Road,

London W12 9RATel: 01-743 8888

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Readers' EnquiriesAll editorial correspondence should be

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We regret that lengthy technical enquiriescannot be answered over the phone.

AdvertisementsThe Publishers of PE take reasonable

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© Intra Press 1990. Copyright in all drawings, photographsand articles published in PRACTICAL ELECTRONICS is fully

protected, and reproduction or imitations in whole or in partare expressly forbidden. All reasonable precautions aretaken by PRACTICAL ELECTRONICS to ensure that theadvice and data given to readers is reliable. We cannot,however, guarantee it, and we cannot accept legalresponsibility for it. Prices quoted are those current as wego to press. All material is accepted for publication on theexpress understanding that the contributor has the authority

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Published on 1st Friday of each month by Intra Press. 193 Uxbridge Road. London WI2 9RA. Printed in England by Andover Press Ltd. Andover, Hants. Distributed by Quadrant Publishing Services (11-6618526. PRACTICAL ELECTRONICS is sold subject to the following conditions, namely that it shall not, without the written consent of the Publishers first having been given. be lent, resold, hired out or otherwise

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TECHNOMATICTechno House 468 Church Lane, London NW9 8TO.

Tel: 01-205 9558 Fax: 01-205 0190

Prices are subject tochange without noticPlease add carriage(a) £8.00 (Courier)(b) £3.50(c) £1.50(d) £1 .00

rchimedes Computer steinsAll Archimedes systems are fitted with new RISC OSsystems.

Model Basic Colour*A3000 £649 £849310 £899 £1099410/1 £1199 £1399420/1 £1699 £1899440/1 £2499 £2699PC Emulator £60

Carriage£12/system £8/computer*Colour monitor can be Acorn AKF I I or PhilipsCM8833

Technomatic Special DealTo get you going on any of the aboveArchimedes systems you purchase from us,we will contribute 10% of its cost towardsany additional hardware or softwarepurchase you make from us or to pay forextended finance if you require the facility.

The following MultiScan Monitors can be suppliedat special prices when purchased with a computer:TAXAN 770+ £419(a) MTS9600 £375(a)

CM1686 16" £1499(a)(for the extra HiRes modes on 400 series)

Extended FinanceWe can offer extended finance for 11

months under our special offer detailedabove or for 24/36 month period at 12.75%(approx APR 26°4 Please send for details.Technomatic is a licensed credit broker.

R140 UNIX SYSTEMTechnomatic is an authorised dealer.

Pse send for details

Techno 410/1 Upgrade Specials3Mb RAM + 49Mb HD (fitted) £8403Mb RAM + 40Mb HD (fitted) £7201Mb RAM + 20Mb HD (fitted) £325(All hard drives are with fast access times)These prices apply only when purchased with Arc 410/ I( Your discount entitlement can be used towards paying forany of the above kits)

EXPANSION SYSTEMSRISCOS kit (305/310/440) £29(b)1Mb RAM upgrade (410) £149(c)3Mb RAM upgrade (410) £429(b)2Mb upgrade (420) £290(b)1Mb RAM upgrade A3000 £199(b)3.5" Int Drive Upgrade(please specify 305/310 or 410)

£118(b)

External Drive Adaptor £30(c)External 5.25" Drive with psu (40/80T) £90(b)Acorn 20 Mb Upgrade (305/310) £449(a)Techno 20Mb upgrade kit (410) £199(a)Techno 40Mb upgrade kit (410) £349(a)Techno 49Mb upgrade kit (410) £465(a)

Acorn Backplane (2 slots)TeChnoLog Backplane (4 slots)Fan for TechnoLogAcorn ROM poduleWe have a large range of expansion cards in stockplease send for details.

Techno Ext HD upgrade (300 & 400 series) POA£37(c)£39(c)£8(d)

£51(c)

MULTI 1;O PoduleA low cost card provides: Video Digitiser, SoundSampler, a full RS232 interface, 3 ROM skts &Joystick interface within RISCOS environment.Details on request. £117(b)

rchimedes, SOFTWARE

LANGUAGES & PROG. TOOLSANSI C/ISO PASCAL/FORTRAN 77PROLOG X/LISPTWIN EditorArchimedes AssemblerSoftware Dev. ToolboxRISC BASIC Compiler (SV)

each £95(c)each £175(b)

£27(d)£185(b)£185(b)

£85(c)Chares Toolkit £32(d) Clares Toolkit+ £41(d)

WORD PROCESSORS1st Word Plus £79(c) Graphics Writer £27(d)Pipedream £89(c) P. Spellchecker £43(d)Pipedream Ver 3 (inc Spelling checker) £129(c)

SPREADSHEETSLogistix £95(c) Sigmasheet £57(c)

ACCOUNT SYSTEMMinerva's Suite* each £54(d)(Order Processing/Sales Ledger/Stock Manager/Purch. Ledger/Nom. Ledger)*Requires System Delta plusHome Accounts £41(d)

GRAPHICS/ART/CADPro Artisan £137(b) Artisan £30(d)Artisan Support Disc £17(d)Atelier £90(b) Auto Sketch £55(c)Gamma Plot £52(d) Render Bender £59(b)Presenter £24(d) SVARC-PCB £167(b)SV Solid CAD £41(d) SV Super Dump £20(d)Sv Real Time Solids Modeller £75(c)

NEW PRODUCTStechnoSCAN Scanner 200dpi 59(b)

Genius Digitising Tablet £249

GAMESHoverbod/Missile Control each £12(d)Orion/Freddie's Folly/Jet Fighter each £12(d)Fugitive Quest/Rise in Crime/Overload each £26(d)Zarch £15(d) Conqueror £24(d)Enthar Seven £29(d) Minotaur £9(d)Clares 3 Pack £12(d) Viking £17(d)Thundermonk £12(d) Corruption £20(d)Pacmaria £16(d) Terramex £16(d)

DATABASESSystem Delta Plus £57(c)System Delta Plus Prog Ref Manual £25(d)Reporter*/Mailshot* each £33(d)School Administrator £125(b)Alphabase £37(d)* Requires System Delta plus

MISCELLANEOUSPC EmulatorHEARSAY. Comms PackageMAYA Terminal EmulatorControl (setup utility)(inc VT52, ANSI, Tektronix 4034, Kermit protocols)

Arcomm £29(d) Presenter II

£79(d)£55(d)£29(d)£13(d)

£POA

EMULATED PACKAGESVIEW/Viewsheet/ViewstoreInterword/IntersheetWordwise

each £47(d)each £35(d)

£20(d)

Ex -SERVICE EQUIPMENT

MASTER SERIESAMB15 Master 128 £439(a)The Master package includes: View Word processorand Viewsheet spreadsheet software and EITHEREducational/Fun software bundle withSTARdataBASE package OR A 200K 40T DS singledrive TS200. Please specify your choice.

ADC06 Turbo 65C102 Module £115(b)Technomatic is a Acorn Authorised EconetReferral Centre. We carry a full range ofEconet Accessories in stock.

DISC DRIVES400K/640K per mechanism (all drives)5.25" Double sded (40/80T Sw):TS400 SinglePS400 Single with psuTD800 DualPD800 Dual with psuPD800P Dual with psu plinth mounted3.5" Double SidedTS351 (5.25")PS351 (5.25" Case + psu)TD352 DualPD352 Dual with psuCombo Drives 5.25" + 3.5" & psuPD853PD853P plinth mounted

£80(b)£90(b)

£160(a)£170(a)£185(a)

£69(b)£95(b)

£126(a)£139(a)

£170(a)£190(a)

WINCHESTER DRIVESP3OHD 30Mb Hard Drives £410(a)Other sizes available

PRINTERSEPSON

Lx800 £145)a) LQ500FX850 £285(a) LQ850

£225(a)£405(a)

STARLC10 £149(a) LC1O Colour £199(a)

LC24-10 £269(a)

NATIONAL PANASONICKXP1081 incl master lead £139(a)KXP1124 £269(a)Integrex Inkjet ColourHitachi 672XD A3 Plotter

£515(a)£479(a)

PLOTTERSHitachi 672XD A3 4 pen plotter SpecialRoland DXY880A A3 Flatbed 8 penplotter

£409(a)

£495(a)

EThis advertisement can only show an exampleof the range of products stocked byTchnomatic. So send for our latest free BBCcatalogue providing detailed information andprices on BBC Computer Systems,Peripherals, Software and Books.

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Return to Technomatic Ltd, Techno House,468 Church Lane, London NW9 8TQ.

L PE 11/89i

Our Service department has a number of discdrives, monitors and printers available for sale atreduced prices. Please send for detailed list.

Please mention Practical Electronics when contacting advertisersT TL,

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0.240.240.240.240.240.240.240.240.240.240.240.340.500.240.240.240.240.500.240.240.240.240.240.240.240.240.240.500.800.900.240.240.240.300.350.450.360.420.700.750.360.480.900.350.540.750.900.400.400.450.450.450.700.801.400.500.500.650.500.450.550.550.951.751.400.652.000.661.600.650.650.500.650.750.750.750.750.751.301.50.1.30.1.001.401.000.750.752.001.900.750.750.800.800.750.750.800.800.900.900.900.900.900.700.901.101.101.100.750.750.900.7(0.701.200.751.200.601.250.701.900.80

12:880.801.40

14.001.002.203.703.003.202.001.201.202.101.801.800.500.500.520.500.700.700.751.30

74103797410381741539074153937415395A741539'374154457410465741046774155407410541741060874106107410612741562474106267410628741062974156407415640 174106417410642741S642.174106437415643 1741S644741064574166401741066874166697416670741668274166847416687741568874C92274C92374C92574C926

1.304.500.601.001.001.401.801.201.801.001.007.00

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0.240.250.700.250.600.460.600.240.250.360.600.700.360.560.600.600.800.600.700.300.480.240.900.400.600.750.361.251.001.252.500.702.501.101.002.500.600.560.500.600.601.000.600.600.550.360.350.660.600.600.800.800.854.000.700.860.400.250.240.240.240.240.240.240.460.250.250.240.250.600.751.200.360.900.900.902.700.750.900.360.550.360.963.600.900.361.200.560.550.551.501.101.100.562.200.480.320.601.150.800.700.800.861.000.750.65

453645384539454145434551455345554556455745604566456845694572458345844585472414411144121441614419144901449514500145992212210001

2210240014400854009740098401004010140102401034010440105401064010740108401094011040114401474014017363

4017440175401924019340194402444024540257403734037480C9580C9780C98

2.500.750.750.900.701.002.400.360.502.401.401.402.401.700.450.900.480.601.507.507.503.002.604.204.506.502.003.507.007.000.481.200.360.401.501.251.502.001.201.500.480.553.201.202.252.252.801.001.201.001.001.001.001.001.501.501.801.801.800.750.750.75

SPECIALOFFER

30%DISCOUNT

ONTTL/CMOS/

REGULATOR

it socketspin

26 pin40 pin

7.008.00

TTL 6 ECL

LINEAR ICs407581 12.00ADC0808 11.90AM7910DC 25.00AN103 2.00401 5050 1.00403 8010 4.50403 8910 4.50403 8912 5.00CA3019A 1.00CA3028A 1.10CA3046 0.70CA3059 3.20CA3060 3.50CA3080E 0.70C43086 0.60CA3089E 2.50CA309040 3.75CA3130E 0.90CA31307 1.30CA3140E 0.45CA31407 1.00CA3146 2.25CA3160E 1.50CA3161E 2.00CA3162E 6.00CA3189E 2.70CA3240E 1.50CA3280G 3.00D7002 6.00040I408 8 3.00DAC0800 3.00DAC0808 3.00DG308 3.00HA1366 1.90

17106 6.7517611 0.9517650 4.00

CL7660 2.501.8038 4.00

CM7555 0.90CM755 1.40LC7120 3.001C7130 3.00LC7431 3.50LC7137 3.50LF347 1.201E351 0.60LF353 0.90LF355 0.90LF356N 1.10LF357 1.00LMIOCLH 4.50LM30IA 0.30LM307 0.45LM308CN 0.751M310 2.25LM311 0.60LM318 1.50LM319 1.80LM324 0.45LM334Z 1.151.7413352 1.30LM336 1.60LM339 0.40LM348 0.60LM349CH 3.60LM358P 0.50LM377 3.00LM380N 8 1.50LM380N 1.58LM381N 3.LM382 2.00L44383 045LM384 2.20LM386N 1 1.00LM387 2.70LM391 1.80LM392N 1.10LM393 0.85LM394CH 4.00LM709 0.35LM710 0.481M711 1.00

VOLTAGE REGULATORS1A FIXED VOLTAGE PLASTIC T0220

4. 7

5V6V8V

12 V15 V18 V24 V

7805780678087812781578187024

0.460.500.500.450.500.500.50

-VE7905 0.507906 0.507908 0.507912 0.50/915 0.507918 0.507924 0.50

IA FIXED VOLTAGE PLASTIC 1092

5 V 78105 0.306 V 78106 0.308 V 78108 0.30

12 V 78112 0.OTHER

REGULATORS

LM309K 100LM323K 2.5078H05KC5 A 5 V 5.75781112 7.50

15 V 78115 0.335 V 79105 0.45

12 V 79112 0.5015 V 17115 0.50

SWITCHINGREGULATORS

ICL7660SG3524714947149778540RC41

2.503.003.002.252.501.50

VARIABLE REGULATORSLM305AHLM317TLM317KLM3377LM3507

M 96K

IL11074MCT26MCS2400MOC3020TIL111

2.501.202.402.254.00

LM723N 0.5078HGKC 6.5078GUIC 2.2579HGKC 6.7579GUIC 2.5079MGT2C . 1.40

OPTO ISOLATORSTIL112

2.20 TILII31.00 7I1II61.90 6N1371.50 691390.70

DISPLAYS

0.700.700.703601.75

FND 57 1.00 MAN6610 2.00E90500 111730 1.00 95135881 5.70EN0507 TIL729 1.00 711311 6.50MAN74. 01704 1.00 111729 1.00MAN71 DL707 1.00 711730 1.00MAN3640 1.75 MAN8910 1.50MAN4640 2.00

DISPLAY DRIVERS9368937093741M39141M39151M3916UDN6118

DN6184

2N5777BPX25BPW21

4.504.503.503.503.503.503.203.20

ULN2003ULN2004ULN2068ULN2802ULN2803ULN28047549175492

0.900.902.901.901.801.900.70

70OTHERS0.501.802.90

0RP12ORP6OORP61

1.201.201.20

CPU's RAMS

LM723 0.60 TC9109 5.00 68809 10.00 2016155 4.00LM725CN 4.00 TCA940 1.75LM733 0.66 704I010 2.25LM741 0.22 TDA1022 4.501M747 0.70 1041024 1.10LM748 0.30 TDA11705 3.00LM1011 4.80 7042002 3.25LM1014 1.50 7042003 1.90LM1801 3.00 TDA2004 2.401M1830 2.50 T042006 3.201M1871 3.00 TDA2020 3.20LM1872 3.00 TDA2030 2.50LMI886 6.00 TDA3810 7.501M1889 4.50 TDA7000 3.50LA42917 3.00 TEA1002 7.00

68809E 12.008035 3.508039 4.2080C39 7.0080804 7.5080854 3.00800854 9.008086 22.008087-5 £708087.8. £9680287.8 £15580287-10 £175

17.50

2101 4.0321078 5.002111A35 4002/14 1.502114-3 2.502147 4.004116.15 2.004116.20 1.504164.15 ITU 3.00416415 1.50441615 3.004532.20 2.5041256.15 3.005101/5501 4.00

1M3302 0.90 TLO61CP 0.40 12.00 5514,5114 4.001M3900 0.80 T1062 0.60LM3909 1.00 71064 0.90LM3911 1.80 71071 0.401M3914 3.50 01072 0.70LM3915 3.40 71074 1.10LM3916 3.40 71081 0.3515413600 1.50 71082 0.55M515131 2.30 71083 0.75

12.00TMS9980 14.50TMS9995 18.00

2.502.905.507.50

5516 4.005517 4.0050256.15 4.005116LP-3 3.0062641P.15 2.609810 2.0074S189 1.8074S201 3.50

M515161 4.50 T LoeA 1.00M83712 2.00 71094 2.00

746289 2.2593415 6.00

MC1310 1.50 UA759 3.20 93425 6.176SUPPORT DEVICESMC1413 0.75 UA2240 1.50

MC1458 0.45 UCN5801A 6.00 261 12.002815030 20.00MC14951. 3.00 ULN2003A 0.75

MC1496 0.70 ULN2004A 0.75 3242 8.00RUMS PROMSMC3340 2.00 ULN2068 2.90 3245 4.50

MC3401 0.70 11192802 1.90 6520 3.0028122 4.00MC3403 0.65 ULN2803 1.80 6522 3.50

MFIOCN 4.00 U1512804 1.90 6522A 5.50 24010 2.50MK50240 9.00 UPC575 2.75 6532 4.80 18S030 2.00M19215. 5.00 UPC592H 2.00 6551A 5.50 18SA030 2.00ML922 4.00 UPC1156H 3.00 6821 1.80 745188 1.80MM66221A 3.00 UPC1185H 5.00 68821 3.00 745287 2.25NE531 1.20 XR210 4.00 6829 12.50 745288 1.80NE555 0.22 X142206 4.00 6840 3.75 745387 2.25NE556 0.60 XR2207 3.75 68840 6.00 82023 1.50NE564 4.00 XR2211 5.75 6850 1.80 825123 1.50NE565 1.20 XR2216 6.75 68850 3.00 820129 1.50NE566 1.50 XR2240 1.20 6852 2.50NE567 1.25 ZN404 1.00 6854 6.50NE570 4.00 ZN414 0.80NE571 3.00 ZN419P 1.75'

68854 8.006875 5.00

SPECIALNE592 0.90 ZN423E 1.30NE5532P 1.50 ZN424E 1.30

8154 8.508155 3.80 OFFER

NE5533P 1.60 ZN425E8 3.50 8156 3.80NE5534P 1.20 ZN426E8 3.00 8205 2.25NE5534AP 1.50 ZN427E8 6.00OP O7EP 3.50 ZN428E8 4.50

8212 2.208216 1.60 30%

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T he Modulator/DEModulator is findingincreasing uses in industry, commerceand the home. It is used mainly

where the bandwidth of the carrying mediumis restricted, such as on telephone lines or viathe mains wiring for home control systems.

MODEM PRINCIPLES

Simple modems usually use either fsk(frequency shift keying) or ask (amplitudeshift keying) techniques but are restrictedusually by bandwidth constraints to around1200 baud in any single direction or 300 baudsimultaneously in two directions. Morecomplex modems feature a variety oftechniques to increase the throughput on agiven line these include dpsk (delta phaseshift keying) such as that used on V22, qam(quadrate amplitude shift keying) such as thatfound on V22 bis and tcm (trellis codedmodulation) as found on V32 modems.

WHY A MODEM?

To communicate effectively over atelephone line requires not only thecommunications medium but also a host ofother features. For example, there must besome form of 'handshaking' between each

CLAIRTRONIC9311

4AM<

OS, '4.44*

Photograph of the author's prototype modem board. There are some differencesbetween the prototype and the project we present here, but the photo illustratesthe basic ease of the construction.

PE MODEMend to determine whether the far end ispresent and ready to send and also the speedthat it is to be sent at.

In addition, the telephone line has a topfrequency limit of around 2.5 kHz into whichwe have to fit both the sending (ororiginating) signals and receive (oranswering) signals. The transmit amplitudemust be limited to less that OdB, preferablyat a level of -10dB, which reduces thepossibility of cross talk on the telephonelines. The receiver must be capable ofworking down to around -35dB or better.

The modem must be matched to the line,which presents a complex impedance at anominal 600 ohms. Safety must also beconsidered with all line functions beingseparated from the computer (or dataterminal) by isolated barriers and lastly theremust be some way of setting up a call, sodialling and answering mechanisms must beincorporated.

On the face of it, this appears to be a tallorder, especially when you consider that theunit has to meet other requirements whengoing through approval such as radiatednoise checks, acoustic shock tests, offlinecurrent and ren (ring equivalence number)checks. Plus, it is being connected to amedium over which the user has little controland which changes according to the weather!

This is probably why modems tend to bepretty expensive considering their relativesimplicity.

Kevin Kirkdiscloses the

secrets of modemsand shows you

how to build yourown line-up!

When isdn (integrated digital switchednetwork) comes on stream then all telephonelines will be digital and so you should be able toconnect your computer directly to the line, butyou will have to connect your phone via amodem (called in this instance a codec) somodems will be with us for some time to come.

MODEM DESIGN

Modems require a digital input in the formof a series of is and Os, usually referred to asthe RS232 input. This digital input may takeone of two forms, either synchronous or morecommonly, asynchronous.

Fig 1. Synchronous comms link.


COMMUNICATIONS PROJECT

BIT

A

2 3 C 5 5

t t tSAMPLING POINTS

7 8 9 10

f t f tfDJG17661

Fig 2. Typical databyte (in this case'A'; 8 odd parity, 1stop bit).

Synchronous communications are the mostsimple and fastest form of serialcommunications. They rely on the fact thatfor each serial bit there is a correspondingclock pulse so encoding and decoding thesignal from a parallel computer bus isrelatively simple. In addition, there is noneed for synchronisation 'overhead' so thedata throughput is correspondingly faster.Fig. 1 shows a typical synchronous comms setup, with a parallel in/serial out shift registerbeing used to send the data and a serialin/parallel out -shift register being used toreceive and reconstruct the data under thecontrol of a master clock. Two of thesecircuits are required for a two way link.

This is fine for direct connection betweencomputers and it is also used in faster modemdesigns (from V22 up) but in an fsk modemor in a simple two wire comms link it needsextra channels or wires to work.

In this instance we use an asynchronouslink. This requires the use of a clock at eachend which will time the bit stream. Eachseparate byte of data must be individuallysynchronised so the data byte is preceeded bya low going 'start' pulse. (Fig. 2).

On automatic speed sensing modems thispulse is measured to provide the baud rate.

After one half of the width of this pulse haspassed, and after the end of the pulse, then theinput line is sampled to get the level of bit 1of the byte. This is repeated 7,8 or 9 timesdepending on the protocol at intervals equal tothe width of the start pulse, at which point thestop pulse will signal an end to the byte.

PROTOCOL

The protocol of the byte is its 'makeup'.For example, 7 Even 1 is used by Prestel and8 No 2 is used by Telecom Gold. This meansthat the byte has either seven or eight data bitsin the byte, has odd or no parity and has eitherone or two stop bits (ie, the stop bit is eitherone or two start pulse widths wide).

The parity is a crude form of errorchecking which adds either a 1 or a 0 to thebyte to make up an odd or even number of 1bits. For example, in the letter 'J' the bytewould be 1 0 1 0 0 0 1 so if we had oddparity then the parity bit would be 0; similarlyif it were even then we would add a 1.

Note that the byte is only seven bits longsince ascii only uses the lower seven bits as itonly has 128 different codes. So why peoplepersist in using eight bits is beyond me as it

actually slows down the throughput by over10%!

The asynchronous conversion is usuallyhandled by a device called a uart (universalasynchronous receiver transmitter) which willcheck for parity and framing errors (lack ofstop pulses) automatically and will flag aninterrupt to the processor. Fig. 3 shows atypical uart.

This device will usually provide othercontrol and monitoring lines, usually referredto as 'handshake' lines. These lines are usedfor setting up and controlling of the modemand consist of two outputs and two inputs tothe uart.

These go valid in the following order:

VALIDITY

DTR (data terminal ready output):signals to the modem that the uart (and hencethe terminal) is ready, usually the modem willdrop out if dtr is lost.

DSR (data set ready input): signals thatthe modem is ready.

RTS (request to send output): signals thatthe terminal is ready to send.

CTS (clear to send): signals that themodem has set up the data link and theterminal can proceed to send.

In addition there are two other lines fromthe modem:

CD (carrier detect): signal that the energyis present on the telephone line, signifyingthat the distant terminal is active. Loss ofcarrier will usually terminate the connection.

RI (ring indicate): signals that the phoneline is ringing so the terminal may decide

ENABLE

R/0/

AO

A ,

R/WAND

CHIPSELECT

8

DATABUS BUFFERS

8

Fig 3. Typical UART (6850 in this instance).

TRANSMITDATA

REGISTER

STATUS

REGISTER

8

TRANSMITCONTROL

PARITYGENERATOR

TRANSMITDATA

CONTROL

REGISTER

CTS

DC

RTS

RECEIVEDATA

PRACTICAL ELECTRONICS FEBRUARY 1990 I3

whether to raise dtr and proceed with theincoming call.

These signals are active low and are passedvia RS232 line drivers and receivers. Theseconvert the ttl (0-5V) signals into + or -signals which are then passed between theterminal and the modem. RS232 is an awfulstandard as it is slow and limited to 20 metres.In addition, it must have been designed by the`13' connector industry as it calls for the useof 25 way D connectors, while it only usesabout nine conductors. However, it isreasonably standard so we are stuck with it.

Its main disadvantage is that is itunbalanced: that means that it has one outputand one earth (return) which in turn means thatit is susceptible to noise on the output whichcannot easily be filtered out on the receiver. Abalanced system, however, (like RE422) usesa differential input so that noise induced onone input is not amplified (series moderejection) and noise on both inputs is cancelledout (common mode rejection). So RS422 cancope with line lengths of over one kilometreand speeds of 10 Mbaud compared with speedsof 19.2 kbaud for RS232. Fig. 4 shows thedifference between the two systems.

CONVENTIONS

At this point we have converted out RS232signals to ttl data and control signals inside themodem. The digital signals are then convertedto tones. These tones will correspond to eitherthe internationally recognised CCITT tones(referred to by their recommendation number,eg V21, V23 etc) or using the American BELLtones.

Thus a mark would be assigned (in V21Originate) a tone of 980Hz and a space wouldbe 1180Hz. Similarly, on V21 Answer themark would be 1650Hz and the space would be1850Hz. Note the spacing between 1180Hzand 1650Hz so that interference betweentransmit and receive is minimised. Conventionrequires that the modem making the call (theoriginating modem) uses the low frequencies,or low channel and the modem answering thecall uses the high channel to avoid confusion.

As with everything else in datacomms thisis not always obeyed. For example, in the caseof the 'Hayes' command atr, which allows amodem to originate a call in answer mode, sothat acoustic couplers can be called. Thisseems silly as most acoustic couplers (moreabout these later) can already be switchedbetween answer and originate modes, and inany event, they require the manual interventionof an operator who, if it were me, would bevery put out to be bleeped at by a modem!

CONVERSION

The modem chips on, the market willprovide this conversion function as well asproviding the filtering on the receive to decodethe incoming marks and spaces. Space = 0 andMark = 1, incidentally, refers to the good olddays when the state of the art comms linkeither made a mark or left a space on a piece of

Fig 4. RS232 andRS422 links(respectively). Note:noise may bereduced byinserting 1kresistors on theinputs to the26LS32 amp. DATA

ENABLE

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RS232

14.894k7

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TWISTED PAIR

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DJG1763

moving paper, giving a sort of visual binarycode.

As analogue filters tend to be fairly difficultto make on silicon (not counting switchedfilters) then many modem chips use a fast D/Aand A/D converter on the line side so that thetones are derived from a rom and are decodedusing a dedicated microprocessor rather than apll/filter network.

At higher speeds than V21, which is limitedto 300 Baud, we have to use the data link in onedirection only at any one time. This is referred toas half duplex. The reason for this, as previouslyexplained, is bandwidth on the telephone line. Ifyou look at the frequencies used in V23, of1300Hz for a mark and 2100Hz for a space, youwill see that there isn't space for another channelof the same speed. So what is done here is thatwe use the its and cts control lines to create thehandshaking. It works like this:

One end, say the originating end, isdesignated the master and the answering end isdesignated the slave. The master will call up theslave and after the link is established will raiseits rts line. This has the effect of raising thecarrier detect at the slave modem. After a presettime (usually 100ms) the cts will come backfrom the master modem and it may start to send.When it has finished it drops the rts line whichin turn drops the slave cd and the master cts. Itis now the slave's turn. It may if it wants tosend to the master, raise its its and the commslink is 'turned around', and the set up is asbefore but in reverse.

PACKET SENDING

Many half duplex systems use a 'packet' orXMODEM method of sending whereby a packetof 256 bytes is sent then the line is turnedaround and an acknowledge or checksum signalis sent back to check the integrity of the link.

Half duplex using V23 is an excellent way ofsending data, especially over marginal lines withfar less chance of errors than V22 (which relieson phase changes to a single frequency ratherthan switching between two frequencies). It israrely used, though, except on professionalsystems as it requires a little bit of thinkingabout to set it up properly.

There is a twist in V23 which tends toconfuse people, and that is where it is used onViewdata systems such as Prestel. This isbecause in the bandwidth available there is a

sliver of frequencies available at the low endwhich will allow the use of a low speed 75baud 'Back' or Engineering channel whosefrequencies are 390Hz for a mark and 450Hzfor a space. This is (very cleverly) used byPrestel to allow the user to send controlcharacters and page requests to Prestel whilethe bulk of the communication is from Prestelat 1200 Baud. In effect, this is full duplex butat different baud rates, so V23 can be used forfull and half duplex communications.However, for neatness and accuracy we shallrefer to it as assymmetric full duplex. Thischannel may, of course, be used to send theacknowledgements thus speeding up halfduplex transmissions still further. There isanother form of modulation called amplitudeshift keying which is mainly used in mains -bourne communications systems. It is basicallyCW. It operates on a carrier of 120kHz whichis switched on and off for a mark or a space. Itis covered by a British Standard (BS6839) andnormally uses a packet control system such asKermit (not the frog) but is outside the scope ofthis article. Further details on this may beobtained from: BIMSA, Leicester House, 8Leicester Street, London WC2H 7BN.

CONSTRUCTIONALCONSIDERATIONS

For a modem that forms a constructionalproject, certain design criteria must be met,for example:

It must be simple to build and test, evenfor relative novices to electronics.

It should have communications softwarereadily available for it.

It should meet the InternationalCommunications standards.

It should be capable of being approved foruse on the Public Telephone Network.

It must have a self test capability.

It should be capable of being used withvirtually any computer.

The PE Modem does all this at a verymodest price.


COMMUNICATIONS PROJECT44

WHAT TYPE OFMODEM?

This modem is a dumb unit, which meansthat it does not have a processor in it and socannot interpret software commands directly.This does have some disadvantages, in that itcannot use 'Hayes' software, such as thatavailable on the majority of comms softwarepackages. To use these packages it must be usedas a manual modem, so the dialling etc must bedone manually.

However, not having an internal processordoes have a lot of advantages. For instance,when on line the processor in a. 'clever' unit hasvery little to do (except crash!) so the manualunit is just as good. In fact it can be better sinceit is easier to get off line if there is a problem(flick of a switch), whereas a `Hayes'compatable modem needs software escapecodes, offline commands etc.

Another benefit is that it cannot accidentallybe put on line if the processor crashes.

On balance, the 'clever' modem is easier touse, but the dumb modem is more reliable,cheaper, easier to build and simpler to modifyfor special functions (such as amateur radio,earphone or even cb use).

COMPATIBLESOFTWARE PACKAGES

TABLE 1

This modem will work with any softwarewritten for the following modems, andshould auto -dial and auto -answer:

Voyager 7 and 11Magic ModemEnterpriseApolloCommtel-x

It will also work with software written forthe following modems, but may not supportall the functions, such as automatic speedsensing and selection:

UnicornDemonDesigner

It will also work with any software that hasa manual modem capability, but will notsupport automatic functions such as autodial and auto answer.

THE DESIGN

The heart of the unit is the 7910 modemchip. This is the chip that can be found in thevast majority of modems. If the modemsupports V23 then it usually has a 7910lurking in there somewhere.

It gives the capability of V21 (300 baudfull duplex) and V23 (1200/75 asymmetrical

ORIGINATE ANSWER

Designation Space Mark Space Mark Speed

CCITT V21 1180 980 1850 1650 300

CCITT V23 450 390 2100 1300 1200/75

BELL 103 1070 1270 2025 2225 300

BELL 202 2200 1200 See Note 1 1200 HD

TABLE 2.Systemfrequencies in Hz.

Note: Bell 202 has no official 'back' channel like V23 so can only operate in one direction at a time(half duplex) whereas V23 can be used in both directions at once, albeit at a slower speed on theoriginating side. This is called asymmetric duplex. V21 and BELL 103 are both full duplex somay be used at up to 300 bauds in both directions simultaneously.

duplex) plus a loopback test facility. It has allthe filters, AD converters, etc, required toboth MOdulate and DEModulate the digitalsignals. Since it does all this, all we need todo is hang the various little circuits on thatperform the computer/telephone line/humaninterface functions.

Fig.5 shows the basic block diagram forthe modem. The circuit can be divided intotwo parts, the digital part on the left of thecircuit in Fig. 6 and the analogue/telecom sideon the right, with the 7910 bringing them bothtogether.

The RS232 port on the far left of thediagram is fairly straight forward as it uses aconventional RS232 line driver which has atailored slew rate and current limiting (toprevent damage if the outputs are shorted toearth). It also does level shifting.

The input does not use the conventionalline receiver as I find they are not as flexibleas this cmos design. The 4k7 resistorsprovide the loading (5k in the RS232standard) and the 100k resistors provide acurrent limit into the cmos gate of around120µA. The input voltage is then clamped bythe gate's protection diodes to +5V and OVfrom the original plus or minus 12V. This hasthe advantage of being able to drive the unitfrom direct logic (such as a user port on a

BBC) rather than having to use a properRS232 port. In this instance, a 4V7 zenerdiode on each of the 1488 outputs, with itscathode connected to the output and its anodeconnected to ground will ensure a correct 0-5V input to the logic.

The baud rates are selected by S2. It isshown on the diagram at 300 originate andwill turn anti -clockwise through:

300 Answer1200 Originate1200 Answer1200 Half DuplexTest

Originate, incidentally, means theoriginator of the call (ie, the one who dials).Answer means the one who answers the call.This is a convention used to prevent bothparties using the same set of tones. As a trueexperimenter, you need not stick to thisconvention.

The digital input is converted to tones bythe 7910 and will find its way out on pin 8,which is transmit carrier. Here it goes into acircuit called a duplexer. Essentially, this is afour wire (TC, OV and RC, OV) to two wire(line, return) converter. In conventionaltelephones this task is undertaken by a

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I PIN 11. IC2)

+5V(PIN 1 ICI II 2,PIN 14.1C3,PIN 7 IC44

33011

DOND

ANOV

- 5V(PIN 4 IC1PIN I. IC41

-12Y(PINT IC71

specially wound transformer. This isinconvenient (and nowhere near as elegant) asthis circuit. Its operation is worth describingas it has uses in many other areas such asamateur radio, for example.

Capacitor C35 decouples the 7910 from thecircuit (to prevent dc offsets) and is fedthrough two resistors the first of which (R1)provides the signal path to the Telecom lineand the non -inverting input of the op -amp. R3then goes to the inverting input of the opamp.

The two signals will thus cancel each otherout so that the receiver is not swamped by thetransmitter. The receiver, on the other hand,comes from the line transformer and is only

put onto the non -inverting input so that it isamplified and sent to the receiver. Resistor R2sets the gain of the circuit, in this instance atunity. If you experience low signal levels onthe inputs which cause errors, try increasingR2 to 33k.

On the other side of the transformer lies thegyrator circuit. On many telephones (andsome modems) an inductor is used to draw dccurrent from the exchange to hold the relay on(around 20mA) while allowing the low level acsignals (around - 12dB) to be passedunattenuated to the duplexer. However, aninductor poses a few problems here as it is bigand will need to be shorted out during dialling,

otherwise the back emf will at best providespurious dial pulses and at worst weld the relaycontacts shut.

This circuit features a semiconductorgyrator which mimics a coil yet is less bulkyand can, with the addition of a zener, be usedwithout shorting out during dialling. A versionof this circuit has passed BABT approval, so itworks!

The zener turns the transistors hard onwhen the relay first closes, so providing a lowimpedance path for the dial current. Thisprevents spurious pulses caused by thecapacitor charging. When the voltage reachesthe transistor turn on threshold the zenerreduces the voltage again, thus providing a`knee' which can be interpreted by theexchange as two pulses.

The 10µF capacitor in series with thetransformer is to provide a dc block since itcannot take very much current. The bridgeacts as 'steering' diodes to ensure the rightpolarity on the gyrator.

RL1 is the main online/dial relay.RL2 isused to prevent the bells on parallel connectedphones undesirably tinkling by shorting themout while dialling. This circuit is driven froma simple monostable consisting of IC3A andIC3B which holds the relay on for a presettime (set by C10 and R33) after each validtransition of DTR.

NEATSINN

REG 1

1 2 3

111

1 1 13 2 1

REG 2

0

-110V

240V

1100-0-

T1

FS1

240V.,AC IN

[DJG,aos I

0-0R29 }-

7(18

RD

6

3 2 17 a -11M-*

1 I 03'11 r1;51411 1/:41 C95

1

1.1R41,7( F(33 }-"I.

DL

I

-.-1 R13 r-a .9E1V1O1-40_0171213. 111. 1

SKI

R23

026

132 "D

T112.3

TR 1.

Fig 8. Printed circuit board layout.

1.1;DU. 2' D15I:R4

f -is5

,t,I

51

1R 30

CJ7 X71

10..4 123

C35 10 P 1-.4[7E-4

12

I P12001TAKE

CARE NOTTO BEND

PINS

EXPANSION

110-1 Re }.-

}-0

R L A

RIB

OP10-COUPLER

I IL WHITT BLUE RED,

SELF C Till

PRACTICAL ELECTRONICS FEBRUARY 1990 I 7

The ring detector is quite simple: theringing voltage of around 65Vac will find itsIvay through the 470nF capacitor, C4, and aslong as it exceeds 62V it will turn on the optoisolator led. The zener prevents it fromtriggering falsely if a parallel phone is used todial, and the capacitor prevents it from goingoff during normal operation.

The isolator output is fed to a simple filterconsisting of C6 and R14 which removes the25Hz ac component from the ring signal.IC2a is used as a high impedance buffer toprevent filter loading. The output will be apulse of the same duration as the ringingsignal and is fed to the computer on the RIinput.

The computer then decides whether or notto go on line if the line switch S1 is closed.Si may be used to set the modem on or offline under operator control. Thus you can talkover the phone to the person at the far end andeither of you decide when to go online.

The power supply in Fig.7 is a simple andfamiliar unit which doesn't really need anyexplanation.

CONSTRUCTION

It is recommended that you use the pcblayout shown here, this is because there are

low level analogue signals (down to -40dB orso) with fairly high level digital signals.Consequently, ground loops MUST beavoided and this can only be done by carefullayout with earth `starpoints' etc. So a stripboard layout may not work satisfactorily.

There is not really much to be said aboutthe assembly except that I would recommenddil sockets for all ics, especially for the 7910.For which it should preferably be goldplated.

Note that there is provision on the layoutfor a mains transformer, but whatever placeyou decide to mount the transformer, ensurethat full safety considerations are adhered to.Mains voltages must not be allowed toconnect to a phone line, or to yourself! Themodem must be boxed for further safety.

An external centre tapped supply, eitherac or dc, up to 15Vdc or 12Vac may beconnected at the points marked X on theoverlay. Increase the size of the heatsink onthe +5V regulator in this instance, though.

When you have fully assembled the unit,and before plugging in the 7910, check the 5volt supply rails. If they are above 5 volts(or below in the case of the -5V) then theregulators could be oscillating, in which caseputting a 1µF tantalum capacitor from theoutput to ground (observing the polarity)should cure it.

There is no setting up to do so switch offand insert the 7910.

Then connect the modem to yourcomputer's RS232 port, and set the terminalprogram to 300 Baud. When DTR is raised,the Carrier lamp should come on andwhatever you type in should be printed on thescreen. Also, the TD (transmit data) and RD(receive data) leds should flash. If all is well,the unit is now ready for use.

USING THE MODEM

The 7910 is an excellent chip but it doeshave one really annoying bug. If you changespeeds without resetting the chip (by eitherswitching off or raising DTR) then it willprobably end up in a strange operating modewith the filters and transmitters set to thewrong frequencies. So always remember toeither set the baud rate switch before going online or before dialling.

The 7910 has the capability of operatingusing the BELL tones (Table 2) which are usedin North America and Canada. So, if youintend to call American Bulletin Boards direct,you can modify the unit by disconnecting MC2from +5V and connecting a changeover switchso it is connected to OV for BELL or to + v forCCITT as used in Europe.

Fig 9. PCB track layout. When copying ensure that IC pin spacing is 0.1 inch.

las

1

o0

LA

7(i

r

°I ellbrOMO 0 0 00 0 0 0 0 0 0 0 0 0

UI ' I

0

I 8 PRACTICAL ELECTRONICS FEBRUARY 1990

COMMUNICATIONS PROJECT

COMPONENTS

RESISTORS

RI 620R

R2, R3 22k (2 off)

R4, R5, R13, R14, R16,

R18, R21, R27 -R29,

R34, R35 100k (12 off)

R6, R9, R10, R12 100R (4 off)

R7

R8

R11, R20, R32

R15, R23, R24, R25

R17, R19, R22

R26

R30

R31

R33

CAPACITORS

CI

C2, C3, C10

C4, C5

C6

C7

C8

C9

C35

C37

8k2

10k

2k2 (3 off)

330R (4 off)

4k7 (3 off)

IM

47k

I OR

330k

2n2 polyester

1011 (3 off) elect

470n (2 off) polyester

100n polyester

47n polyester

10p polystyrene

22p polystyrene

lµ 16V elect

47n polyester

SEMICONDUCTORS

DI, D2

D3 -D15

Z1

Z2

TR I , TR2, TR3

TR2

ICI

IC2

IC3

IC4

IC5

IC7

REC2

3V zener (2 off)

IN4148 (13 off)

5Vzener

62V zener

ZTX300 (3 off)

ZTX 653

7910

4050

4001

741

4N26 opto coupler

1488

W004 Bridge rectifier

MISCELLANEOUS

LEDS (CD, OL, RD, TD) (4 off)

RLA, RLB, 2PCO relays (PCB mounting)(2 off)

2.45 MHz crystal

S 1 SPST toggle switch

S2 2p6w rotary switch

BLUE

ORANGE

RED

YELLOW

GREEN

BLACK

SCREW DOWN TIGHT

A(ORANGE I C (BLUE ITO RUN ONLY TO LEFTHAND SIDE.ALL OTHER LEADS FIT

INTO 1.2. SLEEVE.

BROWN

ICI,811601

REGULATOR

HEATSINK

6 BA PCB

;.z"-%.,

WHITE

Fig 10. Switch wiring details, andheatsink construction and mountingfor regulator.

The ring input on the 7910 will enable theunit to produce the V25 ring if pulled low.However, for flexibility and being able toanswer when switched to originate, I haven'tused it here. There is nothing to stop youfrom using it, though; experimentation is thename of the game! Try connecting it to pin 2of IC2 (after disconnecting it from +5V first)so that when DTR is raised by the computerit should answer correctly after ringing. Itwill only work in answer mode, though.

If you want to use the unit to drive a 4 -wire circuit (a ham radio, cb or carphone),omit the circuit from the 741 onwards. They

(x 1DIGITRELAY ON REPEAT UP 0 10x

750mS33mS 66mS

750mS NEXT

RELAYON

INTERDIGITPAUSE

DIGIT

RELAYOFF

I DJ 17591

POWER SUPPLYCOMPONENTS

Cl, C2

C3, C4

REC 1

REG 1

REG 2

T1

100011 25V elect (2 off)

470n polyester (2 off)

Bridge rectifier W001

7805

7905

Mains transformer,

9V -0-9V 1 amp secondary.

connect directly to pins 5,8 and 9 on the 7910.A word of caution if you are using the

SGS Thomson version of the chip. It is veryeasily upset (for some reason) by dc offsetson its receive inputs so I would recommendleaving the decoupling capacitors and the47k resistor in circuit. If DTR (pin 16) istied low, then the relay can be used withDTR from the computer to toggle thetransmit key on a ham transmitter. Again,experiment with it.

If you want to use the unit simply with apair of wires slung across a fence to yourfriend's computer, then you will need toprovide a dc supply to forward bias thebridge rectifier. Connected it via a 2k2 1Wresistor directly across the line to eachmodem, isolating each dc -wise with a 2112polyester capacitator, as in Fig. 11.

Alternatively, leave out the gyrator etc,and connect directly to the line transformervia the lOpf capacitor. You may find withthis latter arrangement that it may need someattenuation, so try experimenting with a fewdifferent value attenuators.

The unit can be made to dial by togglingthe DTR input and hence the relay. First ofall put the relay on for about 750mS thenturn it off for 33mS, and then on again for66mS for each digit followed by the interdigit pause of 750mS (see Fig. 12) and thenstay on at the end. It is that easy. Mostsoftware packages mentioned in Table 1 willdo this for you. Plus a load of other cleverstuff!

Kevin Kirk is a lecturer in electronicsat the Aberystwyth College of FurtherEducation. PE

Fig 11. (right)connecting twomodems back toback.Fig 12. (above)dialling timingwaveforms.

MODEM 1

'D16175131

MODEM 2

PRACTICAL ELECTRONICS FEBRUARY 1990 I 9

THE DREAM MACHINEAdjust the controls to suit your mood and let the gentle, relaxing sound drift over you. At first youmight hear soft rain, sea surf, or the wind through distant trees. Almost hypnotic, the sound drawsyou irresistably into a deep, refreshing sleep.

For many the thought of waking refreshed and alert from perhaps the first truly restful sleep inyears is exciting enough in itself. For more adventurous souls there are strange and mysteriousdream experiences waiting. Take lucid dreams, for instance. Imagine being in control of yourdreams and able to act out your most secret wishes and fantasies. With the Dream Machineanything is possible.

DREAM MACHINE PARTS SET £22.77

POWER CONDITIONERYou don't have to be a hi-fi nut to notice the huge improvement in sound quality when you runyour music system from a clean mains supply. How do you describe the difference? It's as if allthis time your favourite artists have been playing and singing inside a wardrobe ... and someonejust opened the door and let them out. You could spend twenty times as much on special cables,plugs, mats and stuff without achieving a tenth of the difference. But don't take my word for it.Try it for yourself.

The sophisticated circuitry of the Power Conditioner begins with a bank of six VDRs to eliminateimpulsive spikes. Then comes a massive filter with thirteen capacitors and two current -balancedinductors to smooth away every trace of noise and interference. A ten LED logarithmic displayflicks up and down as each spike is eliminated, and gives a second by second account of theinterference removed. Maximum load 1.5kW.

POWER CONDITIONER PARTS SET £34.27

MAINS CONDITIONERA general purpose workhorse for cleaning up the mains supply to computers, hi-fi, television sets,and any other electronic equipment that might be disturbed by mains interference. The solid, no-nonsense circuit will handle loads up to 1kW, and the PCB can be built into the equipment to beprotected or housed in the optional case.

MAINS CONDITIONER PARTS SET (no case) £6.21

RUGGED CASE (optional) £2.07

< AERIAL BOOSTERGood TV pictures from an indoor aerial, from a caravan aerial, or in areas of poor reception. This booster ranks withthe very best, giving wideband operation from 10MHz to 1.4GHz (it will boost FM radio signals too), mid -band gainof up to 26dB, and a wide supply voltage range of 9V to 26V. It will run from car batteries for caravanners, dry batteries

for campers, or a battery eliminator for use in the home.

There are two parts sets for the project. AA1 gives you the PCB, hybrid amplifier, components and instructions.AA2 is the optional case set: rugged screened box, front and rear panels, waterproofing gaskets, feet, sockets andhardware.

AA1 PARTS SET £14.72 AA2 PARTS SET £5.52

KNIGHT RAIDER I>The ultimate lighting effect for your car. Picture this: eight powerful lights in line along the frontand eight along the rear. You flick a switch on the dashboard control box and a point of lightmoves lazily from left to right leaving a comet's tail behind it. Flip another switch and the pointbecomes a bar, bouncing backwards and forwards along the row. Now try one of the other sixpatterns. An LED display on the console lets you see what the main lights are doing.

The Knight Raider parts set contains the case and all components for the control box. Thepattern generator and lamp driver is supplied as a PCB with all components for you to mountanywhere convenient. We supply the electronics, you supply the lamps (from any caraccessories shop).

KNIGHT RAIDER PARTS SET £28.52

'\/ QCM-DITUPi-DaTel: (0600) 3715 LIMITED

SALES DEPT., ROOM 108, FOUNDERS HOUSE, REDBROOK, MONMOUTH, GWENT.

ORDERINGAll prices include VAT

UK orders: please add £1.15 postage and packing.

Eire and overseas: please deduct VAT and add

£5.00 carriage and insurance.

ACCESSPhone 0600 3715 for immediate

I attention to your Access order.

COMPUTER FEATURE 0hat is a computer? That whichcomputes of course. So is a pocketcalculator a computer? No,

computers are accepted as being capable ofmore than just adding or multiplying twonumbers. Computers are expected to be ableto run programs with 'branch' or 'jump'instructions, displaying greater intelligencethan a calculator. However, there areprogrammable pocket calculators and thesemay be classed as computers.

With a whole range of computersspringing up in the last ten years it is difficultto distinguish between microcomputers,minicomputers and computers. Going by theword size they can handle, minis and microsuse word sizes of 8, 12 and 16 bits. Mediumsized computers use 16, 24 and 32 bits andlarge computers 64 bits. However, withpersonal computers (pcs) becoming more andmore powerful, the dividing line is becomingless noticeable.

Another interesting difference betweencomputers is the difference between theHarvard class of computers and the VonNeuman (or Princeton) class. Both haveinput/output peripherals, memory, arithmeticunits and control devices. But whereas theHarvard class allocates separate portions ofmemory for instructions and data, thePrinceton (built at Princeton University)stores instructions and data in the same formand in the same portion of memory. This

Does it all add up?Yes, and the rest aswell. From Ascii to

Zilog, this is thecomputer story,

told by MikeSanders.

respectively and in 1973 the 8080, animproved 8 -bit processor. Not to be outdone,Motorola introduced their 6800 the same yearand Zilog, the Z80 in 1975. Today, mosthome computers and business machines arebuilt around these popular processors: 8080,8085, Z80, 6502, 6800.

In 1978 the Commodore Pet appeared andaround that time all the popular machines thatwe are familiar with: the Sinclair range, theBBC micro, Acorn, Electron, etc. Knowledgeand technology increase on an exponentialscale. So, what next?

COMPUTERSmeans that instructions can be treated as dataand the machine can modify its owninstructions, which is a very powerful tool forcomplex operations like space missions.

Microprocessors are stripped downversions of microcomputers. Although theyhave memories and arithmetic units, they donot as a rule have an input device, eg akeyboard, or a separate output device, egprinter. They are usually programmed inmachine code (ones and noughts) as opposedto high level language (statements inEnglish). They usually have a dedicatedfunction like controlling the washprogrammes in a washing machine orcontrolling the central heating boiler.

HISTORY OFCOMPUTING

The abacus was probably invented inseveral countries around 1500 BC. All goesquiet after that for a long time while peoplewere happy to count beads. It took 3000years before John Napier invented thelogarithms in 1614 AD and the slide ruleappeared soon after.

Babbage's analytical engine was designedin 1832 but never completed because thetechnology could not cope with the ideas.The famous Collosus designed by Newman,Flowers et al used 1500 valves and was built

in 1943 at Bletchley Park for decodingGerman messages. The US military built theUNIAC in 1946 and its 18000 valves usedabout 150kW of power.

Manchester and Cambridge universitieswere both developing computers and so werethe American universities. In 1952 the VonNeuman machine appeared and in 1963 IBMintroduced its famous 360 series which is stillgoing strong.

Then microprocessors started arriving. In1971 Intel produced their 4004 and 8008which were 4 -bit and 8 -bit microprocessors

COMPUTERARCHITECTURE

In order to function satisfactorily, acomputer needs four parts, broadly speaking,as shown in Fig. 1. Basically a centralprocessing unit (cpu) is responsible for all thearithmetic operations and needs a memory forstoring results, partial results, raw data andeven the program to be run. An inputinterface is required for loading the program

C1=31

17=1 I ImPuT/ CuTFUT 'NIERFACE

Fig 1 (left). Computer basic block diagram.Fig 2 (below). A computer in more detail.

ACCUMULATOR FLAGS

DATA- BUSCONTROLBUS

BUS

INSTRUCTIONREDiSTE

INSTRUCTIONDECODE

CONTROL

10.11.471

REGISTER A

REG B REG C

REG D REG E

REG F REG G

STACKPOINTER

PROGRAMMECOUNTER

- RAM - INPUT

OUTPUT


WRITE

Hf

WIREY

WIRENSEY FCT

READ/WRITEX S LECT

1010144111

FLUX DENSITYB FOR BINARY 0

111

'NFLUX DENSITYFOR BINARY I

0.1614491

HREAD

Fig 3 (top). Ferrite core.Fig 4 (bottom). Hysteris loop.

and data, and an output interface for obtainingthe results. Co-ordinating all these activitiesis the control unit which opens and shutsgates as required so that data reaches itsintended destination.

Fig. 2 shows a more detailed arrangementof the processes within a typical smallcomputer. All the parts will be dealt with indetail and any that do not fit into a particularcategory will be discussed subsequently inComputer Architecture II.

MEMORY

A computer's physical size is taken up bymemory mainly and it is interesting tocompare the different kinds: old and new,volatile and non-volatile, internal andexternal. Memory can be external like discs,punched cards, magnetic tapes, etc., but here,only internal memories will be examined.

Modern memories are semiconductortypes which can be volatile or non-volatile.Diodes are passive and non-volatile. On theother hand capacitors are volatile since thecharge leaks away and needs to be refreshed.In large scale integration (lsi), thousands oftransistors can be formed on a chip and thecapacitance associated with each can beregarded as a memory device. This capacitorholds a logic 1 when charged up and logic 0when discharged.

Silicon sapphire cmos (complementarysymmetry mos) is the fastest of the mosfets.Dynamic and static memories are alternativeterms for volatile and non-volatilerespectively. Dynamic mos costs less and hasa lower heat dissipation than other memoriesbut it needs to be refreshed every 150 - 2000gs. This is achieved by a time sharedamplifier.

Memories can also be serial access orrandom access. Magnetic tapes are anexample of serial access, ie, the tape has to

run from the beginning till the required pointis reached. A matrix would be an example ofrandom access since only the x and y co-ordinates need to be specified in order toaccess a particular element (cell) of memory.

Old memory systems used rotatingmagnetic drums and there are still computersaround with ferrite core memories. Readingthe information in memory is destructive, ie,reduces it to zero, so a write after readfunction needs to be built in. Fig. 3 shows aferrite core with the read/write wires and Fig.4 a hysteresis loop. A wire carrying currentand surrounded by magnetic metal willgenerate a loop such as in Fig. 4 where B isflux density and H is magnetomotive force.This loop is read by the sense wire and a largeflux is sensed in reading a 1, typically thekind of flux generated by 50mV. A 0 will besensed by the kind of flux generated by10mV.

Since the x and y wires thread every core,an inhibit wire is required to cancel the readcurrent through the unwanted cores. Writeafter read is accomplished by reversing thecurrents in the x and y wires which reducesthe number of wires and amplifiers that wouldotherwise be required.

READ ONLY MEMORY(ROM)

As the name implies, reading is allowedbut writing into this position of memory is notallowed. Roms used as look up tables, likeconverting Hex codes to binary or for readingvalues of contents like II. Memory size is interms of bits or bytes where 8 bits = one byte(and 4 bits = a nibble). Often memory sizesare stated in terms of kilobytes where kilodoes not have the usual 1000 equivalent but1024 (210) and has the symbol K.

Roms can easily be implemented usingdiodes as in Fig. 5. Table 1 shows the effectof earthing any of the word lines: the diodesconduct giving a 0 output. Only where a diodeis absent is the supply voltage available whichrepresents a logic 1.

It follows from the above that informationcannot be written into this device fortemporary storage, hence read only facilities.

Fig 5. ROM matrix.

RANDOM ACCESSMEMORY (RAM)

Random access memories are faster thansequential access and are therefore used inmodern computers. A typical home computerhas a memory between 1K and 64K words(bytes) with cycle times between 300ns and3000ns. Cycle time is the time that it takes toread a word from memory and rewrite it backinto memory. Some operations require threeor four cycles and cycle time is a useful unitfor calculating how long a series of operationswill take.

Rams used to be synonymous with ferritecore stores but semiconductors have takenover. However the method of selecting aparticular memory location using x and y co-ordinates is still the same (Fig. 6). Inaddition, there is an arrangement to specifywhether reading or writing to memory isrequired.

Fig 6. Accessing a RAM.

A 1K memory arranged as 4 bit wordswould have 256 different locations (256 x 4 =1024). For each reading and writing, fourlines are required. A chip select line isrequired for selecting the particular chip ifthere are several. Together with powersupplies and read/write select this is a total ofabout 20 which can be met by a standard dualin line (dil) package of 22 pins.

Fig. 7 shows the memory map of a smallmemory where 2 Kbytes is devoted to romand 1 Kbytes to ram. The input port is ataddress 2000 and the output port at address4000. The monitor program scans thekeyboard for inputs and also serves the vdu.In hex code the monitor occupies address0000 to 07FF and the user area on the ram isfrom 0800 to OBFF.

PROGRAMMABLE ROM(PROM)

Standard roms manufactured in a factoryuse a mask for producing the right pattern.Also fets can be used instead of diodes. If themetalisation is left out on the source, drainand gate, this produces a high impedancegiving a logic 1.


COMPUTER FEATURE

0000

07F F080 0

OAF F 1K

0800 RAMDATA STORE

+ STACK

MONITORPROGRAMME

2KROM

OBFF

20 00

4000

USER'SPROGRAMME

INPUT PORT

OUTPUT PORT

[DIG 11.511

INPUTSCS

lo,sicss

Fig 7 (above). Memory map.Fig 8 (bottom). Programmable ROMFig 9 (bottom right). Memory cell.Fig 10 (top middle). PLA.Fig 11 (top right). ULA cell.

+VE SUPPLY

FUSIBLELINK \

1= A.B.02= A D

A.0B.D

DJG14531

OUTPUTS

SILICON DIOXIDE

SOURCE GATE DRAIN

SUBSTRATE 191514.561

0-W-0

[11,61,561

VC

Roms can also be prepared forprogramming or 'burning' in the field. Fig. 8shows the arrangement. A fusible link isburned by a pulse of current between 50 µAand 10OµA for 2µs. In this way the requiredpattern can be created but it is tedious and if amistake is made the chip is spoilt.

In addition to programmable roms there areerasable roms which can be reprogrammed.These are of two kinds, those that useultraviolet light and those that use electricalpulses - electrically alterable rom (earom).

The ultraviolet type (sometimes calleduvprom) can be fabricated from an array of A-mos with floating gates, all in the nonconducting state, logic 1. Applying a voltagebetween source and drain removes positivecarriers from the gate leaving a negativecharge which makes the source drainconductive. Applying a reverse voltage tocancel this would damage the device. Instead,ultra -violet light is applied for 10-20 minutesand for this operation, the chip needs to beremoved from the computer or any otherequipment it serves. The ultraviolet in sunlightor fluorescent lighting has no effect on thesechips.

In order to respond to ultraviolet light thechip is equipped with a transparent quartzwindow. The ultraviolet ionises the gas insidethe seal making the silicon dioxide slightlyconductive and the charge leaks away (Fig. 9).

Early designs of memory arrays usedmemory cells containing all zeros in theunprogrammed state and the charge was usedto turn the cell ON (logic I). Present daymemories contain logic 1 and the gate chargescauses 'pinch off', logic 0 in the programmedstate.

The ultraviolet type is inconvenient for

several reasons. Ultraviolet can harm the eyesand skin, and ozone is also produced which isexplosive. The main drawback of the uvpromis the fact that it needs to be removed from theequipment while the electrical type can bealtered without removal. These are of twokinds: nitride insulated gates or fieldavalanche mos (famos) and require 30-40 voltpulses.

The nitride type has a read time of 2-5 µswhich is not particularly fast but, since thecharge leaks away so slowly, it can be classedas non-volatile and therefore offers almostinfinite storage.

PROGRAMMABLELOGIC ARRAY (PLA)

In some instances not all 2n words of a rommay be required, converting a 12 bit Hollerithcode into an 8 bit ASCII. In this case only 96combinations are required out of 212 = 4096since the Hollerith code has a high content ofredundancy.

The pla is an arrangement of programmableAND gates driving programmable OR gatesand the above code conversion requires a plaof 768 bits (96 words x 8 bits per word). Anaddress decoder is also required whichincreases this to 3072 bits. Compared to thisan equivalent rom would require 32,768 bits.For a pla to be economical, it needs to be ableto reduce the rom equivalent by ten times andthis application does exactly that.

Fig. 10 shows a simple example of plawhich is available as both mask programmableor field programmable. Fig. 10 shows ANDgates but OR gates are equally possible if a

negative supply is used and the resistors areplaced in the cathodes.

UNCOMMITTEDLOGIC ARRAY (ULA)

In the pla, the memory array nodes arealtered but if it is possible to alter the arrayinterconnection, then an uncommitted logicarray is created. This achieved as in Fig. 11 bycreating the resistors and transistors but not theinterconnections.

Since modem integrated circuits aremanufactured by diffusion into a silicon slicewith metalisation on top for interconnection,the metalisation can easily be left out until thecustomer specifies his requirements.

MATERIALS FORMEMORIES

We have looked at old materials like ferritecores and magnetic drums which have beenreplaced by newer types: semiconductor ttland mos. In addition to these there are othermaterials of interest: plated wires andmagnetic bubbles.

Plated wires have a thick film ofmagnetisable material deposited on wires. Onthis film, small zones of memory are created.As memory devices, plated wires are goodsince they are non-volatile and therefore nondestructive. However, it is difficult to obtainmaterials of a uniform quality and productionmethods need improvement.

Magnetic memories went out with theferrite core but scientific principles rarely dieand quite often crop up as a differentvegetable, in this case magnetic bubbles.Garnet crystals as thin as 10 pm can support1500 bubbles per square millimetre and this isachieved by applying a magnetic fieldperpendicular to the crystal.

At a certain critical value of field strengththe usual domains of a magnetic material breakdown into bubbles, each of which can bethought of as a separate magnet with north andsouth pole, Fig. 12. Since like poles repel,each bubble will repel its neighbour so therewill be a limit to the packing density.


0 0

0JC714581

MAGNETICFIELD WITHZERO BIAS

RESULT OFLARGE FIELD

101014.57i

Fig 12 (above).Magneticbubbles.Fig 13 (left).ALU symbol.

CENTRAL PROCESSINGUNIT (CPU)

Basic to the central processing unit is thearithmetic and logic unit (alu) whose symbolis shown in Fig. 13. The cpu consists of aluplus registers. As the name indicates, the aluis capable of both arithmetic operations inbinary as well as the logic functions AND.OR, etc.

The four arithmetic operations can all becarried out by a single circuit, an adder. Ahalf adder using NAND gates is shown in Fig.14 and the outputs in Table 2. From this tableit can bee seen that it is similar to an exclusive- OR circuit, S = A + B and C = A.B.

This is called a half adder since two suchcircuits make a full adder, Fig. 15, in whichthe carry digit is taken into account from theprevious stage of addition. A single full adderof this kind could add two digits at a time and

Fig 14. Half adder.

is called a serial adder. If parallel addition isrequired, then a full adder is employed foreach pair of digits and the result of eachaddition stored in pipo (parallel in, parallelout) registers, Fig. 16.

A drawback of the parallel adder is thedelay produced by successive carry digits andcomputers use more elaborate, more expensivecircuits for addition and multiplication.

The logic circuits mentioned also allowsubtraction to be implemented using acomplement of the number to be subtracted.Fig. 17 shows how an adder is used togetherwith an accumulator to store the results. Theaccumulator status CZNO will be dealt withlater.

INPUT

TWO'SCOMPLEMENT

ADDER

ACCUMULATOR

CZNO10.1Ch21

OUTPUT

Fig 17. Subtraction using an adder

With reference to Fig. 2, it can be seen thatseveral registers are associated with the cpu.Registers A to G can be used singly or inpairs, both for storing data temporarily or inconjunction with the memory for referring to aparticular address in memory location. Noticethe accumulator for temporary storage in theinput to the alu.

PROGRAM COUNTER

The program counter has an obviousapplication and as with most obviousapplications it is taken very much for granted.However, it cannot be dispensed with sincesome method is required to keep track of theprogram steps and the end detected by meansof an END statement. What is even moreimportant is the need to point to the nextprogram step when the program is interruptedin order to go into a subroutine. The programcounter (pc) points to the step to return towhen the subroutine is completed.

If a program can be considered as a big jobthen a subroutine is a small job, complete initself, within that big job. For instance whilebuilding a house, there will come a time whenall the electrical wiring needs to be run beforereturning to the main task of building the house.

STACK REGISTER

A stack is a group of registers or an area ofmemory reserved for storing informationtemporarily while the program goes into asubroutine. A stack pointer (sp) is a registerwhich holds the address of the top of the stackin binary. Access to the stack is on a last -in,first -out basis (lifo). Think of the stack as apile of tins each with its lid on top. The toptin needs to be removed before one can lookinside the next tin. So when a program isinterrupted in order to jump to anotherlocation and perform another calculation, theregisters need to be cleared. The stack is usedfor such dumping.

If one is programming using mnemonics,the instruction is PUSH, which pushesinformation onto the stack. To retrieve theinformation, the instruction is POP and ofcourse the stack needs to be popped in reverseorder. The stack pointer merely tells how fullthe stack is, as shown in Fig. 18.

STATUS REGISTER

In performing calculations, certainimportant limits need to be observed. Forinstance, the operator will want to know whena calculation results in a carry (C) digit,negative (N) value, zero (Z) or overflow (0).Carries often occur in additions, a negativevalue in monitoring a condition for less than aset value, zero in counting down. Forinstance if a calculation needs to beperformed ten times, a counter is set to tenand the zero condition detected when thecounter runs down.

Overflows often occur if two largenumbers are multiplied and some indicationneeds to be given to the human eye that theanswer is not correct.

All these conditions can be flagged by flip-flops in a register, Fig. 19. The flag goes up ifany of the zeroes changes to a one.

INSTRUCTIONS

Each step of a program is called aninstruction. These instructions are read frommemory or any other device, sequentially intoan instruction register.

An instruction cycle consists of one ormore machine cycles and each machine cycleconsists of two sub -cycles: 1) Fetch cycle 2)Execute cycle. During the fetch cycle, the cpuprovides the address of an instruction via the

Fig 15 (below). Full adder.Fig 16 (right). PIPO registers.

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COMPUTER FEATURE 0=Er

Fig 18 (left).Stack pointer.Fig 19 (above).Status Register.

address bus. The address is decoded by thememory and the instruction read from memoryinto the cpc via the memory data bus.

During the execute cycle, the instruction isdecoded by the cpu and the required operationis performed. So it can be seen that during theinstruction cycle, two kinds of words are beingprocessed: instruction and data.

There are five different kinds of instructionwhich are found in programs: 1) data transfer;2) control and branch; 3) subroutine linking;4) operations; 5) input/output.1. Data Transfer: This may involveinstructions like Move, Load, Store andExchange, either to a register or memory.2. Control and branch: These involve Halt,Jump or conditional Jump. The jumpinstructions may involve the C,Z,N,O flags.At the Jump instruction the program willalways jump to the subroutine before returningto the main program but the Conditional Jumpdepends on a condition pre-set by theprogramme, if the sum is greater than 100 orthe remainder less than 0.001.3. Subroutine linking: Call subroutine andreturn to main program at the end of it.4. Operation: Arithmetic and logicoperations. Clearing accumulators,incrementing/decrementing counters, shiftingregister contents to left or right. Alsoperforming 1 's complement and resentingC,Z,N,0 flags.5. Input/output operations: are selfexplanatory. They involve taking in oroutputting information to the world outside thecomputer. The outside world, or peripherals,are those companions friendly to thecomputer: the vdu, line printer, tape, floppydisk, etc.

PROCESSORS INGENERAL TERMS

It is useful to gain an appreciation ofmicroprocessor chips in order to compare theirperformance against each other, for thepurpose of selecting a computer that uses aparticular chip or selecting a particular chip toperform a specific task.

The Intel 8008 was the first 8 bitcommercial microprocessor in 1971. Twoyears later the 8080 appeared and the 8085 is"therefore the third generation. The 8085 isfaster than the 8080 and requires fewersupport chips.

The 8080 requires a clock generator andsystem controller which are built in features ofthe 8085.

The Motorola 6800 was the first competitorof the 8080 and its followers. Sometimes

comparing one processor against another canget complicated. For instance, a processorwith a faster clock may be expected to befaster than one with a shower clock. Notnecessarily so. The 6800 with 1MHz clock isactually faster than the 8085 with 3MHz clocksimply because the 6800 needs only one clockcycle for a memory operation while the 8085needs three to six clock cycles to open andshut the gates.

The 6800 has other interesting features. Ithas relative and indexed addressing in additionto direct addressing. Addressing will be dealtwith later, but briefly: direct addressing refersto a register or memory location by name, RegA or 0800H (memory location in hex code).

For indexed addressing, the 6800 has a 17 -bit register called an index register which isuseful for transferring blocks of data. Inrelative addressing, an address relative to thecurrent instruction is stated enabling theoperator to jump forward or backward byseveral addresses.

As mentioned before, processors using 4bit, 8 bit, 12 bit and 16 bit words have been onthe market. Many control applications do notrequire 8 bit processors; 4 bit is adequate. Thebig difference between 4 bit and 8 bitprocessors is that 4 bit processors use separatememory for data and instructions (program).The data memory is usually 4 bits wide but theprogram rom is 8 bits wide. The instructionsrequire 8 bits since 4 bits can provide only 16different instructions. The 4 bit processors arelimited, then, by instructions.

handhave been limited by number of pins in apackage and the chip layout. The 8 bitprocessor is a happy medium except where 16bit precision is required in calculations.

Finally, we come to bit slice processorswhich are chips of small words (say 4 bit).These can be stacked together to give therequired length of word; three will give 12bits. The disadvantage of bit slice processorsis that the programmer must write theinstruction set, and this is calledmicroprogramming. Such an investment intime and effort can only be justified for specialapplications like high speed control or digitalsignal processing. In general, processors arebuilt from mos but bit slice processors arebipolar transistors, which means they arefaster and consume more power.

CHOOSING APROCESSOR

In 1971 there were only two kinds ofmicroprocessor; today there are 50. Ratherthan list the merits and demerits of each, it isuseful to bear in mind certain essentialfeatures. This is useful both in purchasing amicroprocessor for a specific task or selectinga computer depending on the cpu it uses.

In general, the larger the cpu, the greaterthe processing power. If the cpu has fewregisters, then memory space will have to beused by functions normally handled byregisters. A stack will be limited if integral

with a processor and a stack is better off aspart of a ram. A clock may or may not be partof a microprocessor and if it is not then aseparate chip has to be purchased. All clocksuse an external piezo-electric crystal forderiving the clock frequency.

Having dealt with the cpu, one needs toconsider access to it. How many bits wide arethe data and address buses? Are theymultiplexed, do they share the same highway?How many different kinds of address mode areavailable?

The number of ics to make a completesystem needs to be considered. For instance, asalesman can state a minimum configurationof two chips but the fan out could be so pooras to require another two chips as buffers.Once again, if the processor is required todrive ttl circuits, then it needs to be ttlcompatible.

A microprocessor may not have all theflags. Also, the number of interrupts availablemay be limited. Interrupts will be dealt withlater.

Last but not least, the speed of amicroprocessor is around 100µs so if a speedof faster than 100µs is required, thenhardwired logic is required. This saves timeover a processor which needs to read data andinstructions in and out of registers andmemories thus making a computer controlledarrangement slower.

INPUTS

Inputs to a computer can be from akeyboard, magnetic tape, paper tape, punchedcard or disk, or a plethora of specialisedelectronic control devices. Paper tapes andpunched cards are similar in that they haveholes of varying sizes to represent a code.This code is read by pins protruding from aread head and the mechanical action isconverted to an electrical signal. These holescan also be scanned by a beam of light and theoptical signal converted to an electrical one.Either way some form of transducer isrequired to convert from one form signal toanother.

DISKS

Whereas paper tapes and magnetic tapesare accessed serially, from the start andmoving until the required point is reached,disks can be accessed at random. The disk hasa drive hole, where the drive mechanismconnects and revolves the disk until a beam oflight shines through the optical reference hole.This is the reference point at which the floppydisk starts. In response to coded instructions,the disk revolves the required distance forwardand the read head aligns on the correct trackoutwards from the centre, without having toread every track.

Next month we'll continuelooking at disks then move on toarchitectural matters.


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26 PRACTICAL ELEC fRONICS FEBRUARY1990

COMPUTING PROJECT

Last month I looked at how somestandard 2048 words by 8 bits epromdevices might be replaced by eeprom

and lithium battery -backed devices that do notrequire an additional erasing unit. I went on todescribe a simple keypad operated eepromprogramming unit with alphanumeric readoutdisplays.

This month I shall now describe an evensimpler programming unit that uses hex -

switched address and data coding, and a binarycoded led strip for the read out. Following thatunit, I shall show how either of the twoprogrammers can be converted forprogramming standard eproms of up to 4096words by 8 bits.

As I explained last month, in order toprogram an eeprom or similar, we needfacilities which will allow us to specify anaddress code, a data code, and the read or writemode, together with a visual readout.

SWITCHING

In this month's circuit, as diagrammaticallyshown in Fig.1, we have two switches, S1 andS2, which define the data code to be enteredinto the eeprom. Three more switches, S3 -S5define the address within the eeprom at whichthe data will be stored.

At last - MalcolmHarvey presents a

low costprogrammer for the

occasionalconstructor!

data or address port. However, since the switchpins unconnected to the common pin areeffectively left floating, they need to be biasedto ground so that the eeprom sees logic 0 fromthose lines. The biasing is performed byconnecting a resistor from each switch output tothe ground line.

A second consideration to be overcome onthe data input-output lines is that when theeeprom is in read mode, the switches areshorting all relevant pins together, which ofcourse will affect the code seen on the eepromlines. The use of the diodes DI -D8 between, theswitch pins and the eprom port lines preventsthis mutual feedback.

When programming the eeprom, S6 switchesthe common pins of S1 and S2 to the +5V line.It also disables the eeprom output lines,configuring them instead as input lines. Theeeprom is normally held in read mode by

READOUT BUFFER

The validity of the eeprom data could, verylaboriously, be read by a voltmeter connected inturn to each data line. The better method is toread off the data by means of some form ofvisual display.

You could, if you wanted, use thehexadecimal decoding readout leds shown withthe programmer I described last month. Cost isthe only reason I have not used them in thissimpler programmer. The cheaper solution is touse eight separate leds, each displaying thelogic status of the data lines. Unfortunately, wecannot connect the leds directly to the eepromsince it does not have enough output power todrive them while retaining its correct logiclevel. Consequently, we have to use a bufferbetween the eeprom and the leds, in thisinstance using a non -inverting octal device, IC2.The leds are connected to IC2's respectiveoutput pins, and then taken via buffer resistorsto ground.

MODULAR PARTS

For convenience, I used resistor modulesfor most of the biasing resistors. They take upfar less space than conventional resistors, and

SWITCHED EEPROMPROGRAMMER

Since this circuit is intended as a very cheapsolutionfor those who may only occasionallywish to program an eeprom device, I havechosen the lowest cost switches that I couldfind. These are printed circuit mountingswitches with which a small screwdriver is usedto turn the switch to the correct position. Theswitches have a single common pin and fouradditional pins which are connected to thecommon pin in a fashion which allows them tobe arranged to produce a binary relationship forthe numbers 0 to 15. Instead of being codedfrom 0-15, the face of the switches is markedwith the hexadecimal coding of 0-9 and A -F,consequently the hex codes required for easyprogramming can be readily selected.

DATA PORT

In Fig.l, the eeprom is designated as ICI,and you will see that the data switches areconnected to the eeprom's eight data input-output lines. Likewise, and not unexpectedly (!)the three address switches are connected to theeeprom's address lines. Internally, the switchesare basically on -off types which means that anyoutput line which is not switched to thecommon pin is completely open circuit. Thecommon pin of each switch is taken to the +5Vpower supply line, consequently any output pinconnected to the common pin will also have+5V on it, so presenting logic 1 to the eeprom

holding its read-write connection, pin 21, at+5V via R5. Pressing S7 sets the eeprom intowrite mode, whereupon the data present fromSI and S2 is written into the memory at theaddress location set by S3 -S5. When S7 isreleased, the eeprom reverts to read mode withthe data now stored in it. In order to check thatthe data is indeed stored correctly, S6 isswitched to ground, so disabling Si and S2, andenabling the eeprom output.

Top: last month's simple keypad oper-ated programmer. Bottom: this month'seven simpler switch operated unit.

require fewer solder connections. Internally,the modules simply consist of a given numberof resistors, one end of which each comes to anoutput pin. The other ends of the resistors areinternally joined together with a single outputpin. Hence an 8 -resistor module has nine pins.On the modules, the common pin has painteddot alongside it. If you are really penny-pinching, you could use standard resistors inplace of the modules, mounting them verticallyin the positions on the pcb, then hard wiringthe remaining leads together into a commonconnection.

Again for convenience, I used a multi -ledmodule for the displays. You could equallywell use individual leds instead. The moduleactually has ten leds within it, and iscommonly known as an led bargraph module,but only eight of the leds are used.

BINARY DISPLAYS

When programming the eeprom, the factthat the readout display is in binary format isof no great significance since the programmedoutput data should be identical to the data fromthe switches. When setting the switches, youwill see the equivalent binary code displayedon the leds then, after writing the data into theeeprom and switching S6 to eeprom displaymode, you should still see the same binarycode displayed. Should you simply want to


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check the contents of the eeprom on some lateroccasion then it will be necessary to use abinary to hex conversion chart; only a minorproblem considering the cost of this unit. Ifyou find it is a problem, you can use the hexdisplays I mentioned earlier, wiring them on aseparate board, such as a piece of Vero, drivingthem direct from the eeprom lines, so

eliminating the need for IC2, LM I and RM3.Because eeproms and lithium battery -

backed equivalents can be instantlyreprogrammed at any address, if you make amistake you can simply rewrite the correctdata into that address point. You do not need tocompletely erase the device.

The unit must only be run from a 5V powersupply; it draws about 90mA under maximumcurrent drain conditions.

ASSEMBLY

As with last month's programmer, theswitches are mounted directly on the pcb, thetrack and component layouts for which areshown in Fig.2. You should use a dil socket forthe led module and IC2, and preferably use azif socket for the eeprom. For my ownconvenience, I also used dil sockets for the hexswitches and resistor modules, cutting them tosuit the pin configurations needed.

A note of caution on the assembly: if youare in any doubt about the orientation of theled or resistor modules, check them out with ameter before soldering them in place!

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COMPUTING PROJECT

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1.JFig 3. Switched eeprom PCB tracks.

It is quite likely that some PE advertisersmay stock the hex switches and the modules,though I have not checked this out; my own,plus IC2, were bought from Electromail.

VPP CONVERSION

I have to confess that providing the facilityfor using either of the two units as standardeprom programmers was an after -thought!Consequently, the facility has been designed asa separate module. Its circuit diagram is shown

SWITCHED PROGRAMMERCOMPONENTS

RESISTORSR1 -R5 10k 0.25W 5% carbon film

(5 off)RM1, RM2 8 x 10k sil resistor module

(2 off)RM3 8 x 470R sil resistor module

CAPACITORCl 100n polyester

SEMICONDUCTORSD1 -D8 1N4148 (8 off)IC1 see textIC2 74HC541LM I 10 x led dil module

SWITCHESSI-S5 hex coded pcb mounting (5 off)S6 min spco toggleS7 min push -make

MISCELLANEOUS20 -pin dil sockets (2 off), 24 -pin dil socket,printed circuit board.

in Fig.4. Regular readers will no doubt noticethat it is a variation on part of the circuit usedin my computer controlled 4K EpromProgrammer published in PE October 1987.

When programming standard eproms, thewrite control pin, usually marked as Vpp, isnormally held at +5V, then toggled upwards tohigher level voltage to 'burn in' the data, andthen to be returned to +5V. The Vpp voltagerequired can vary with the type of eprom used,and I am aware of types requiring Vpp levelsranging from 12.5V to 25V. A general purposeeprom programmer must therefore be able tocater for a broad Vpp range.

Whereas in the Oct 87 circuit a high dcvoltage was stepped down to the required Vpplevel, and then down to +5V, for these twosimpler programmers I am taking a basic +5Vsupply and stepping it up to the higher level.

VOLTAGE STEP-UP

Those of you who read George Kerridge'sBattery to HT Converters articles in PE July andAugust 88 (and, I am sure, many more of you),will know that by using a frequency generatorand a tuned inductive circuit, quite modest dclevels can raised to significantly higher voltages.

In Fig.4, a standard 555 timer is configuredas the frequency generator. Cl sets the basicfrequency, which is then adjustable by VR1. Thesquarewave output feeds into R3 and to theinductor Ll and its associated tuning capacitorC3. The ac voltage at the output of Ll isreferenced to ground by D2, and then rectifiedby D3 and stored by C5.

The peak to peak output from Ll, and thusthe rectified dc level on C5, will depend upontwo factors, the load seen by the inductor, andthe frequency feeding into it. The closer theinput frequency is to the tuned frequency of Lland C3, so more power is transferred and agreater p -p output voltage will result. The tuningof the oscillator circuit can thus be used to setthe final raised output dc voltage.Experimentally, with the circuit in Fig.3 andwith the load following C5 omitted, I haveachieved voltages well over 50Vdc. Themaximum output required, though, is not likelyto be over 25V, and so I have put in a safetylimiter, R4 and zener diode D4, to restrict theoutput to no more than the value of D4. Youcould if you prefer, substitute a different valuefor D4 to suit your own needs.

OPTOCOUPLING

The control of the Vpp pulse is achieved byusing an optocoupled device, IC2. Inside IC2 arean led and a photosensitive transistor. Whencurrent passes through the led, thephototransistor is turned on and conducts.Between them, R6 and D5 ensure that thetransistor's emitter, which is fed to the epromVpp pin, is normally held at around +5V. WhenSI is pressed, current flows through the led viaR5, the transistor is turned on and the voltagestored on C5 is conducted to the Vpp pin, so

Fig 4. Circuit diagram for the VPP generator

10k

R2

10k

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VPP GENERATORCOMPONENTS

RESISTORSRI, R2 10k (2 off)

lkR4 470RR5 270RR6 100kAll 0.25W 5% carbon film

CAPACITORSC1 1n8 polystyreneC2, C3 15n polyester (2 off)C4, C5 100n polyester (2 off)

SEMICONDUCTORSIC1 555IC2 307-979 (Electromail)

MISCELLANEOUSVR1 50k skeleton presetLl 10mH inductorS1 min push -make switchPrinted circuit board8 -pin dil sockets (2 off)

causing data to be written into the eprom.Since the output from LI depends on load as

well as on frequency tuning, the charge on C5will fall once the optodevice conducts.However, the charge will be at a high enoughvoltage sufficiently long to ensure that theeprom is programmed. As soon as S1 isreleased, so the charge on C5 will begin to buildup again, and by the time you've manually setthe address and data codes for the next byte, thevoltage will be back up to its required level. In

view of the delay required between each Vpppulse, this is not a circuit suitable for puttingunder high speed automatic control.

VPP USING

To use the Vpp generating circuit with thesimple switched programmer, delete S7 and R5in that circuit, and take the Vpp output to thepoint at which S7 was connected, ICI pin 21.To use the circuit with last month'sprogrammer, delete S4 and R6 in that circuitand take the Vpp output to the point at which S4was connected, IC3 pin 21.

(You could, alternatively, choose not todelete the resistors and switches just referred to,but instead change the resistor values to 100k,and then either delete R6 of the Vpp generatoror increase it to about 1M.)

In order to use 4096 x 8 eprom devices ineither of the programmers it is necessary toallow for an additional address line, All, to betaken to the eprom. For eproms of the type

2532, 2732 etc, pin 18 is the All pin. As thepcbs for the two programmers stand, pin 18 iscurrently taken to OV, it will therefore benecessary to cut the track at pin 18. With lastmonth's unit, then connect IC3 pin 18 to IC4pin 1. For this month's programmer, connectIC 1 pin 18 to pin 4 of S5.

CHIP DATA CHECK

Before attempting to program any epromwith either of the two programmers you shouldascertain that the pin configurations are correct,and also ensure that the correct Vpp voltagesare set on the Vpp generator. The required Vppvoltage and pin connections will be on themanufacturer's data sheet, and a highimpedance voltmeter should be used to checkthe voltage setting.

May you find that these programmers openup a whole new world of possibilities for yourelectronic interest.

*-1

1R2

1 R5 1-

05

Fig 5. VPP PCB layouts.

PR a 6-11)-01

OUTNOW!

C210188000ELECTRONICS

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Electronics Handbookby Jorge de Sousa PiresIt's'not easy to be an expert on every aspect of electronics -but this 800 page integrated reference book will make sureyou can always find that vital information.Well illustrated and detailed, it covers Digital andAnalogue Electronics, Tables and Standards, Com-munications technology, Personal Computers, andrelevant Mathematics. The author has created anenvironment where the reader will find answers to notonly those problems directly related to Electronics, butalso to those in related areas. The text includes all themajor definitions, theories, facts, equations, andstandards you need to use.Very enjoyable to read, with numerous illustrations andcircuit diagrams, and a detailed index. Many of theequations are accompanied by programs to ease other-wise time-consuming calculations. Answering a widerange of electronics -related problems, this is an invaluablereference source for anyone involved in electronics.p9:93, 1989, 800 pages, ISBN 0-86238-061-8£29.95, SPECIAL OFFER TO PRACTICAL ELECTRONICS READERS

TWO OTHER GREAT VALUE BOOKS

Analogue and DigitalSignal Processing and Coding

by P M Grant, C F N Cowan, B Mulgrew, J H DrippsExtensive coverage of signal processing and coding, bothanalogue and digital, with a distinct practical flavour. Itis based on an industrial course given by the ElectricalEngineering Department at Edinburgh University.Chapters 1-4 cover basic techniques such as Laplace and z -transforms, Fourier transforms, sampling theory and timedomain analysis. Chapters 5-7 cover frequency filterdesign - both Finite and Infinite Impulse Response types.Optimal time domain estimation techniques such asWiener, Kalman and adaptive filters are covered inchapters 8-10. The next major section, chapters 11-13covers spectral estimation - including material on thedesign of discrete and fast Fourier transforms and modernspectral analysis. This is followed in chapters 14-17 by aconsideration of coding techniques, including forward errorcorrecting codes and data reduction techniques for voiceand image data transmission. Finally, chapters 18-20cover some practical aspects of signal processing - includ-ing analogue and digital implementations and the use ofmatched filters in radar and communications.£11.95, 1989, 450 pages,ISBN 0-86238-206-8

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Fundamentals of Microprocessor Systemsby Philip A Witting

This complete guide for designing microprocessor -based equipmentconsiders all the hardware and software issues to be resolved duringdesign. Theory is explained in a manner not specific to any particularprocessor, however two chapters are dedicated to the Motorola 6800family of devices. These chapters are paralleled by two appendices whichconsider the Intel 8080/8085 family.Microprocessors and Microsystems Review:"It is almost certainlythe best of the books on microprocessor systems design currently on themarket. I would strongly recommend it for all second and final -yearcourses in microprocessor systems design. Owing to its size, it covers allthe topics likely to be found in such courses."£11.95, 1984, 525 pages, ISBN 0-86238-030-8

TO: CHARTWELL-BRATT, OLD ORCHARD, BICKLEY ROAD, BROMLEY, KENT, BR1 2NE, TEL 01-467 1956

ORDER FORM Please send me

I enclose my cheque for £Please charge my ACCESS/VISA

Electronics Handbook (£29.95)Analogue & Digital Signal Processing &Coding (£11.95)Fundamentals of Microprocessor Systems (£11.95)

Expiry

Name Address

Date

PRACTICAL ELECTRONICS FEBRUARY1990 31

EAST -BUILD PROJECT

T wo for the price of one this month!(But, of course, PE always gives yougreat value for money!) What I show

you here is a common filter circuit into whichyou feed a mono signal, and then wire up thethree separate frequency outputs in one of twoways, resulting in either a simulated stereocircuit, or a three channel equaliser.

FILTERING

The filter circuit is shown in Fig.1 and isbased upon one of my favourite chips, theLM13600. (You could also use the LM13700without any circuit modifications if youwish.) The mono signal is brought into VR1,which enables you to preset the signal level.From there it goes into the opamp mixerstage, IC1 a, on through two separate filterstages, and then fed back to IC 1 a. You mightmarvel that two filters can split the signalinto three frequency bands, low, middle andhigh, but in fact it's not such a strange resultbecause, of course, feeding the signal backon itself results in the third frequency bandselection.

TCA

The LM13600 is a dual transconductanceopamp (tca), of which the gain of each stageis controllable by the current feeding into the

1.

R33

117 +IIIHR31A

0-1 632 110-i R18 }-401 R17 1- it -f]

11.-{ R20 }-4.-f R21 I--j R22 }-4,

118-1 R23 1-40

-i R26 I-41

R16 1-4

11.4 R 25 1.-

R27

19

29

21

22

-1 P29 I-

41.11 clo

-

R9 1-0

81 R13 }-1-

-(2ZD--1 931 HO

Fig 2. Printedcircuit boardlayout.

MOCK STEREOrespective control node. In Fig.1 the twoseparate tca stages are IC2a and IC2c. Eachtca stage also has a following highimpedance buffer stage, IC2b and IC2d.

In the application here we can vary thecurrent going into the control nodes via VR6and R9. The controlling current has theeffect of modifying the rate at which thecapacitors on each tca output, C4 and C5, areallowed to charge up. As everyone knows (!)the rat e at which a capacitor charges anddischarges determines how much of a given

John Becker showshow to split yourmono mods intothree and double

your audio listeningperspective.

frequency will be mopped -up by thecapacitor.

In the feedback configuration shown, thenet result is that the high frequency end ofthe input signal is available at the output ofIC1 a, the middle (band pass) frequenciesappear at IC2b's output, the low end of thespectrum emerges at output IC2d. There's noroom to explain the maths, but it's atechnique that works! By varying VR6 wecan shift the entire frequency split up ordown the scale.

Fig 1. The common frequency filter circuit diagram for both the mock stereoand equaliser units.

C2

931A 1004

R9 20k 696z500k

VR1

10k

R1 100

R2 20k

R31

+VE

C

R12 100

3

IC2c

12 10

R17

4k7

,12

33p

R6 100kIC2d

424

'Cid

R30 10kC2bIC2aICI 324 PIN 4 +VE

PIN 11 06IC? LM13600 PIN 6 DV

ICla

3 R7 R8 minmo1808

Cl.

R11

ok13

k

R14 ONES1k ow 180p

100k kk7 224

C9

PIN 11 +vE 100k 1k I F.POWER LINE a 9V DC 100n

1016.522/11

8 (HIGH PASS) C (BAND PASS) (LOW PASS)

3-) PRACTICAL ELECTRONICS FEBRUARY 1990

0101p

MOCKSTEREOOUTPUTS

C12

VRA 100k

A

vR2 10 Ok

R21 1203A 00k100k

loJGns12/71--J

Fig 3. Circuits for the alternative output interfacingfor use with the common filter circuit.

R5

500k

101410 24 I

CIO 51. yR2 R16 ,7k

3- CHANNELEQUALISEROUTPUT

TRIPLEXING

What we have achieved, of course, is

three signals instead of one. What we do nowis recombine them, as in Fig.3! Taking themock st ereo option first, we feed the trebleonly to one output buffer amp, IC1b, and thebass only to buffer IC 1 c. So, left = treble,righ t = bass (or vice versa if you swap theconnections). What about the mid range, do Ihear you cry through the hole in the middle?No problem, we mix the bandpass signalequally into both left and right channels. I'vealso given you level and pan controls so thatyou can vary the relative signal strengths toeach channel.

EQUALISED

Recombining the signals for equalisationpurposes is even simpler. We just feed theminto a common opamp mixer, varying thelevel of each signal to suit our tonal tastes. Inthis instance, I've nominated IC1c as themixer, with VR2 to VR5 offering the panelcontrol options.

All very nice and simple. Just build up the

Fig 4. PCBtrack layout,common toboth modesof use.

pcb, choose which wiringfrom Fig.5, and join it!

option you want diagrams. Resistors are 0.25W 5% cf, 'p'caps are polystyrene, 'n' caps polyester, `11'caps electrolytic and all pots are linear. (PS,ICId simply provides a split level referencevoltage.)

Happy listening!PARTING

Sorry, there's no space for a separate partslist, but all the values are given in the circuit

POINT IC SOLDEREDDIRECTLY TO JUNCTIONOF R25 AND R27 ON PCB

12

1-

1 11 N. 25 17 53

[W]VR5 vR6GA N FREQUENCY

vcRi.LOW

2

0-1.23OUT

i.UVR3MID

4

_CU

13

BATTERYVE

IN SOCKET ALSOWIRED ASBATTERY ON/OFF)

VTRO2, II:J.1°1,121M

Fig 5. Wiring details for theequaliser (left), and the mockstereo (right).

N. 25 17

VR6

FREQUENCY

12 S

RU

LOW

OUT

23

C114

VR3A

18Li

20 1 22 24 7

10

VR3

iC) B LANCE 27

Ek4c'E R1-Va .:EZY.srtE

mONC . 40IrtRL05 RJScBE cSEC

IF&


AltNew Beginnings...

Program Now- the magazine for professional software applications developers hasundergone a spring cleaning.

Hard-hitting and up the minute news balanced with in-depth and uncompromisingeditorial come together - possibly for the first time in a computer magazine - to bring

you, up to date in the non-stop world of software technology.

In the January issue - out on 20th December, Program Now examines the subject ofdisaster recovery, for the major hardware platforms and major software applications, it

also puts two major Pascal implementations in a head -to -head challenge and tears apartthe latest Watcom C compiler in a magazine you shouldn't miss if you are interested in

today's world of high-technology in software.

January issue out now - available from all good newsagents- price £1.50

Published by Infra Press - Publishers of Practical Electronics

34 PRACTICAL ELECTRONICS FEBRUARY1990

LOGIC TUTORIAL M*

resh from last month'ssuccessful investigations,I expect you're keen to

try some more: see whatanswers you can give to thenext few!

RESISTANCES INSERIES

Investigation 6Resistances in series

You need the same equipment as inInvestigation 5. Fig. 13 shows two resistorsconnected in series. The current flowsthrough one resistor and then through theother.

Repeat Investigation 5 replacing the singleresistor of Fig. 11 (last month) with tworesistors connected in series. Measure thecombined resistance of the two resistors andset out your results in a table:

Resistors (ohms) Combined Resistance100 and 150220 and 470etc.Can you work out a rule for calculating the

combined resistance of two resistors in series?

Volts and currentsexplained and

illustrated - OwenBishop offers moreexcellent advice on

controlling yourpotential!

Repeat Investigation 5 replacing thesingle resistor of Fig. 11 with two resistorsconnected in parallel. Measure the combinedresistance of the two resistors and set outyour results in a table:

Resistors (ohms) Combined resistance100 and 100100 and 150220 and 220220 and 470etc.Can you work out a rule for calculating

the combined resistance of two resistors inparallel?

Since there is no other resistance present,there is nowhere else that voltage can dropand we can say that:

V = VI + V2Now to consider the relationship between

VI and V2. In this circuit, current flowsthrough both resistors. For each resistor, thefollowing equation applies, as explainedearlier:

I = Vl/R1 and also I = V2/R2Since I is the same in both equations, we

can write:VI /121 = V2/R2This can be rearranged to give:V1/V2 = RI/R2In words, the voltage drop across each

resistor is proportional to its resistance. Thebigger resistance has the bigger voltage dropacross it. If the resistances are equal, thevoltage drops across them are equal (both areV/2).

This explains why the circuit of Fig. 12 isnot suitable for measuring low resistances.The ammeter has low resistance and R haslow resistance. The voltage drops acrossthem will be about the same size. Thevoltmeter measures the total voltage drop,which includes the appreciable drop acrossthe ammeter. This leads to an error inmeasuring the drop across R. In the circuitthat we actually used for measuring lowresistance (Fig. 11), the voltmeter measures

BASIC ELECTRONICSPART TWO - MORE RESISTANCE

Repeat the investigation with threeresistors in series and try to work out a rulefor calculating their combined resistance.

R1 R2

R1 -I

R 2-w-

DJG17361

101517371

0

Fig 13. (Top). Resistors in series.Fig 14. (Bottom). Resistors in parallel.

RESISTANCES INPARALLEL

Investigation 7Resistances in parallel

You need the same equipment as inInvestigation 5. Fig. 14 shows two resistorsconnected in parallel. The current splits andpart of it flows through each resistor.

VOLTAGES ACROSSSERIES RESISTORS

The rules about series and parallelresistors crop up often in electronics. Theycan be useful if you do not have a resistor ofthe correct value. By wiring two, possiblythree, resistors together in series or inparallel, you can obtain the resistance yourequire.

Let us look at series resistors more closely(Fig. 15). The voltage drop across bothresistors is V. This is in two parts, VI thedrop across R1, and V2 the drop across R2.

Fig 15. Voltage and current withresistors in series.

-I.- R 2Ft 1

I V2vi

FINGI7381

only the drop across the resistor, so it givesan accurate result.

USING RESISTORS INSERIES

Investigation 8Dividing the potential

You need:a battery box with 4 cells (6V);470 ohm resistor;220 ohm resistor;testmeter or voltmeter reading to 10V;breadboard (optional).Connect the circuit of Fig. 16. Measure

and record these voltages:Voltage Result (V)VV1V2Confirm that V = VI + V2. If you find

that V I + V2 comes to much less than V,there is a likely explanation, which we shallgive later.

Calculate V1/V2. Calculate R1/R2. Dothese calculations confirm the equation givenabove, that V1/V2 = R1/R2?

The simple circuit above is one that isused time and time again as part of larger


Fig 16. Investigation 8. (Left) circuit diagram for measuring V1; (right)breadboarded version. Connect voltmeter to B and C to measure V2, and to Aand C to measure V.electronic circuits. It is known as a potentialdivider. Often we need to obtain a voltagethat is smaller than the supply voltage of thecircuit. A potential divider is used for thispurpose. Ths supply voltage V is connectedacross the divider (both resistors) and thesmaller required voltage VOUT is obtainedacross one of the resistors (Fig. 17). Thevalues of the output voltage from the divideris given by:

VOUT = V x R2R2 + R2

Fig 17. Potential divider circuit.

+VE

ov

R2 BOUT

[DJG174.01

Fig 18. Using a potentiometer as avariable potential divider.

Fig. 18 shows another way of making apotential divider. It uses a variable resistor(often known as a potentiometer). As the knobof the resistor is turned, the output voltageranges from zero to V. This is because, ineffect, the variable resistor consists of tworesistors in series. As we turn the knob, oneresistor increases and the other decreases. Theresulting change in the ratio between them(R1/R2) gives a variable output voltage.

CURRENTS THROUGHPARALLEL RESISTORS

If we apply a voltage V to two resistors inparallel, the current splits into two (notnecessarily equal) currents that pass along theresistors (Fig. 19). Since V is the same forboth resistors we can say that:

V = I1R1 and also V = I2R2Since V is the same in both equations, we

can write:I1R1 = I2R2This can be arranged to give:11/12 = R2/R1In words, the current through each resistor

is inversely proportional to its resistance. Thebigger resistance has the smaller currentflowing through it. If the resistances areequal, the currents are equal (both are 1/2).

MORE ABOUTPOTENTIAL DIVIDERS

Investigation 9A high -resistance potential divider

You need: a battery box with 4 cells (6V);470 kilohm resistor;220 kilohm resistor;testmeter or voltmeter reading to 10V;breadboard (optional).Connect the circuit of Fig. 16, but using

the 470k and 220k resistors in place of the470 ohm and 220 ohm resistors. Measure andrecord these voltages:

Voltage Result (V)VVIV2

Confirm that V = V1 + V2. If you find thatVI + V2 comes to much less than V, there is alikely explanation, given later. CalculateV1/V2. Calculate R1/R2. Do thesecalculations confirm the equation givenabove, that V1/V2 = R 1/R2?

Since the ratio 470000/220000 equals theratio 470/220, you expect to get the sameresults in this investigation as you got inInvestigation 8 (allowing a little leeway fortolerance differences). Whether or not you getthe expected results depends on the meter youare using. If you use a solid state digitalmeter, you probably will get the same result,since meters of this type require virtually nocurrent to drive them. But if you use aninexpensive moving -coil meter (in which aneedle moves across a scale), this requires anappreciable current to drive it. When you aremeasuring V2, for example, R2 and the coil ofthe meter are in parallel. If R2 has a lowresistance, most of the current flows throughR2. Connecting the meter to the divider haslittle effect.

In this investigation R2 has a highresistance, possibly more than the resistanceof the meter coil. If so, the current through themeter is larger than the current through R2.Connecting the meter to the divider has, ineffect, reduced the value of R2 by putting thecoil in parallel with it. This reduces V2.

This investigation shows that, when usingan inexpensive meter to measure voltages incircuits, we must beware when resistances inthe circuits are high. Connecting the meter tothe circuit drains current away and alters thevoltage we are trying to measure.

There is another consequence of thiseffect. If we are using a potential divider toprovide a given voltage to part of a circuit,and that part takes a large amount of current,the voltage obtained from the divider is lessthan expected. Worse still, if the current takenvaries from time to time, the voltage from thedivider varies too. As a practical rule, it isbest if the current drawn from a potentialdivider is never more than a tenth of thecurrent flowing through the divider. Forexample, if we want to provide a current of upto 5mA, the current flowing through thedivider must be at least 50mA. Given a supplyvoltage of 6V, for example, the total RI + R2must not be more than 6/0.05 = 120 ohms.

Question AA potential divider is needed to supply a

current of 10mA. The supply voltage V is 8V.What is the maximum allowable total for RI+ R2?

Fig 19. Voltage and current withresistors in parallel.

0I I

,o

IDJG,74.21


LOGIC TUTORIAL

Fig 20. Module 1, stripboard layout. Note the track cuts and solder blob.

DESIGNINGPOTENTIAL DIVIDERS

The first step in design, to establish themaximum total resistance, was describedabove. Call this RMAX.

The second step is to decide on the valuesof the two resistors. Assuming that the outputvoltage is that across R2, we can adapt theequation given earlier:

R2 + VOUT.RMAXV

For example we saw that if V = 6, and thecurrent supplied is to be 5mA, then RMAX =120 ohms. If VOUT is to be 2V, then:

R2 = 2 x 120 =40 ohms6

RI would therefore be 80 ohms. The E 12series does not include these values, but wecould approximate by using 39 ohms and 75ohms (from the E24 series). Alternatively wecould use a variable resistor as in Fig. 18 andset it to give the exact VOUT required.

Question BIf VOUT of the potential divider of

Question A is to be 3.3V, suggest suitablevalues for the resistors.

We shall look at more examples ofpotential dividers later in the series.

MODULES OF THEMONTH

Most months we shall describe simplecircuit units that you can build, and later jointogether into electronic systems. Most of thedesigns use a standard -sized stripboard andhave screw -terminals for connecting onemodule with another:Module 1: Press -buttons (Fig. 20)

Parts required: Si, S2 press -to -make click -switch, pcb mounting (2 off).

SKT1 4 -way printed circuit terminal block.Stripboard 65mm x 25mm (Vero 10401).

Module 2: Indicator lampParts required: 6V 0.1A filament lamp,

round MES type, and socket. Attach single -stranded wire, about 10cm long, to eachterminal of the socket and strip about 7mm ofinsulation from the free ends.Module 3: Potential Divider

Parts required: 10k carbon rotarypotentiometer. Plastic knob to fit. Solder asingle -stranded wire about 10cm long to eachof its 3 terminals and strip about 7mm ofinsulation from the free ends.

SYSTEM OF THE MONTH

Lamp dimmer:Use the potential divider module to

provide a varying VOUT for dimming thelamp (Fig. 21a). Why does this not work verywell? Why does the circuit of Fig. 21b, inwhich the module is used as a straightforwardvariable resistor, work better?

RESULTS AND ANSWERS

Investigation 6: R = RI + R2 + R3 + etc.Investigation 7: R = (RI x R2)/(R1 + R2).Potential dividers: (A) To supply up to

10mA, the divider current must be at least100mA (=0.1A). Total resistance is 8/0.1 = 80ohms (or less). (B) R2 = (3.3 x 80)/8 = 33ohms. So RI = 80-33 = 47 ohms.

Next month we look at capacitance andinductance.

Fig 21. Lampdimmersystems. 6V BATTERY

6V BATTERY

POTENTIOMETER(MODULE 3)

POTENTIOMETER(MODULE 3)

LAMP(MODULE 2)

IDJG t74.41

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KNIGHT HORRORS

Dear Mr BeckerI have been reading PE for a fair

number of years (I started as aschoolboy in Nov 68) and I think itis fair to say that the solidgrounding given me by PE in thedays of Fred Bennett has stood mein good stead in my chosen careerof electronics and computing.

As I have said before in previouscorrespondence, I am delighted bythe way that you have restored PEto its former glory, and I waslooking forward to the SilverJubilee issue. (This letter waswritten before the Nov 89 issue waspublished. Ed)

Imagine my horror when I foundthat PE was to present an award toClive Sinclair (I never couldstomach the 'SW). Clive's namewas, and is, synonymous with cheapand tacky gadgets, foisted on theunsuspecting public by misleadingadvertising. His adverts have beenwithdrawn by the AdvertisingStandards Authority on more thanone occasion. Those of us whoactually work in the industry heaveda sigh of relief when he fmally quitthe scene.

The fact that the readership ofPE have voted him an award issimply another example of how hehas successfully hoodwinked even aknowledgeable audience. I suspectthat a poll of those of us whobought his products in the latesixties and seventies might havereturned a different result.

That said, (and I don't expectyou to publish this letter), carry onthe good work. You are doing awonderful job, and I hope to be stillbuying PE in 2014.

Nigel Titley MIEE, CEng, BA(Oxon), Hollesley, Suffolk.

You're too modest in expectingthat we wouldn't publish yourpraising and provocative letter - Iam pleased to do so with regard toboth aspects of it.

First, many thanks for your kindcomments and good wishes. I look

forward to inviting you to PE's 50thAnniversary Party!

Quite possibly had we polledthose who bought Sinclair productsin the years you quote we may wellhave had a different result. That,though, would have been a totallybiased and unjust survey. Our aimin carrying out the survey was toseek the opinions of a randomcross-section of PE readers.Additionally, we asked the opinionsof several of our regularcontributors, a majority of whomalso nominated Sinclair.

There is no doubt that Sinclairproducts suffered from problems inthe early years, but I believe thatthe high regard in which Sir Clive isheld by many people is derived notfrom his marketing achievements,

TRACKFEEDBACK

but rather from the deeperperception the public have of hisremarkable inventive capabilities. Itseems unfair (though I guess it'shuman nature) that he should stillbe judged in some quarters by hisfailures rather than hisachievements. He certainly wouldnot have been knighted had hisachievements not been recognisedat a very high level. As I am sureyou will know, Sir Clive is currentlyvery much involved with severalcompanies who are at the fore ofmajor research intotelecommunications, wafer scaleintegration and personal computertechnology, namely ShayeCommunications, Anamartic andCambridge Computers.

You may be interested to knowthat our Publisher has recentlybought the Cambridge Z88 for usein conjunction with oursophisticated DTP system. Thefacilities the Z88 offers, and at aremarkably low price, make it anideal machine for occasional use bycontributors to our sister magazineAstronomy Now. They borrow themachine fora few days, type intheir article text, and return themachine to the office with the scriptheld in memory. Direct transfer canthen be made from the Z88 to theDTP, saving a lot of hassle!

Contrary to what may appearfrom this reply, I have no personalreason for defending Sir Clive'snomination other than the fact thatI try to take a balanced view of anysituation that deserves a fairappraisal. Though other nameswere put forward by readers, nonecame near to Sir Clive's popularity.

Just out of interest, who wouldyou have nominated as the personwho has done most for technologyin the last 25 years? (I throw openthis question to the readers at large,as well as to Mr Tilley.) Ed

CAREERING AHEAD

Congratulations PE!I remember the first issue in

1964, and as a spotty -faced teenagerI used to pour over the articles andthen save my pocket money (£..s.d.in those days) and rush off to mylocal electrical shop to buy the bitsfor the project of my fancy. In Nov68 editor Fred Bennett introduced aseries of articles on 'Bionics',which had me fascinated and myMeccano set was never the sameafter that!

Over the years I havethoroughly enjoyed reading PEand found series such as On theFringe by Gerry Brown (1970)and First Steps in Circuit Designby A.P. Stephenson absorbing andinformative. I am currently in myfinal year of a B.Ed in CDT (craft,design and technology), and theknowledge I have gained over theyears, thanks to PE, now standsme in good stead as the need forteachers in electronics is verygreat. Back in 1964 as a pupilmyself I never guessed that theenthusiasm for electronics whichPE fostered would, 25 years later,be the key to a new career.

Thank you for all the articlesand inspiration!

Malcolm D. Cook, Hexham,Northumberland.

It's good to have confirmationyet again that PE has played arole in shaping readers' futures.That is an aspect of PE's purposeof which I am extremelyconscious.

Your comment on the need forelectronics teachers reminds meof the following recent incidentwhich emphasises the point:

SHORT CIRCUITTEACHING

Someone rang me seekingclarification on a few pointsregarding a PE project. From thenature of the questioning and theresponses to my answers, itbecame increasingly apparent thatthe caller knew virtually nothingabout electronics, and couldbarely read a circuit diagram.Jumping to the conclusion that thecaller was asking on behalf ofsomeone else, I suggested that theoriginator of the questions shouldcall me personally and we coulddiscuss the matter more readily. Tomy amazement, the caller said thatapart herself, only her pupil wasinvolved. Her pupil? I queried.Yes, she said, she was teachingelectronics to pupils at her school!

For once, I found myself shorton words! What a profoundcriticism of technology educationas it is being performed in someschools. Can There be any excuse,even when teachers may be inshort supply, for electronics to betaught by someone who does not

know how to read a circuitdiagram? And what other othersubjects are being taught by thosewith no knowledge of them? Ed.

TEACHER CRITICAL

Criticism of the educationalsystem also comes from at leastone of the affected participants, asthe following extract from anotherletter shows.

Dear Mr BeckerI am 15 and am taking CDT for

GCSE. Because I have decided touse electronics in my final project,I started getting. PE in June 89 andhave got a lot of useful informationfrom it. If I send a copy of mycircuit diagram for the electronicsin my project would you be able tocheck the diagram and give meyour expert advice on it?

The reason I ask is that I havelost confidence in my CDTteacher after many failures inpast projects.

For obvious reasons I withholdthe pupil's name.

I try to communicateenthusiasm for electronicsamongst my readers, but it'sblatantly obvious that I am notbeing backed up by those whohave the responsibility foreducating today's youngsters. Thelast statement in this pupil's letter,and the phone call quoted above,are not the only examples I've hadof inadequate attention totechnology education. I'm notgoing to air political points inthese pages, but the onlyconclusion I can come to is thatboth Government and the teachingprofession should have their headsknocked together and resolve asituation that is increasingly indanger of depriving this country oftomorrow's technologists. It'simperative that the situationshould change.

To answer the reader'squestion, though, I much regret todisappoint him by saying that Icannot undertake to check hiswork for him - I'd never get PE topress if I were to offer such in-depth help to all readers who ask!I would also comment that one ofthe things the CDT examiners willbe looking for is how much thestudent has shown initiative insolving the problem for him orherself. As frustrating as it may beto not achieve a working success,the method by which the projecthas been tackled will also countheavily towards the exam results.

Lastly, if this pupil trulybelieves he has no trust in histeacher, he should persuade hisparents to complain to theheadteacher at his school. Lack ofattention to technology teachingmust not be tolerated. Ed


FOUNDATION TOEMPIRE

Dear John,I must express my thanks to you all

for the PE celebratory party. I wasdelighted to have the opportunity ofmeeting you again after all these years,and to meet for the first time Angelo,and other members of Intra Press.Thanks to you I also met up with manyold friends and colleagues from myNewnes and IPC days.

It is perfectly obvious that youthoroughly enjoy your work as Editor.This I well understand, for it is a mostsatisfying job. I certainly enjoyedevery moment I occupied that position.Since taking over, you have markedthe pages of PE with your irrepressibleenergy, enthusiasm and humour. Thatthis will prove infectious andencourage a host of new recruits tothis hobby, I have little doubt .

Here's wishing you and all yourassociates good fortune in the future -and every success to PE as this journalenters upon its next 25 -year era.

Fred Bennett, Hayes, Middx.(PE's Founding Editor)

Many thanks Fred. My sinceresthope is that I may encourage'readers toenjoy electronics as much as youencouraged me through PE's pages.

1, too, was delighted to again meetyou and everyone else who has beenassociated with PE across the years..Ed.

INTERAKTIVF

Dear Ed,Congrats on your 25th year; I

enjoyed your special issue just like allthe rest. Sir Clive Sinclair is a man Iadmire very much, so I was very keento read the interview. Yes, what willhappen next?

I was also very interested to seefrom the Catalogues section thatGreenbank had sent you some info! Ibuilt my first Interak computer abouteight years ago, and it's still goingstrong, in fact I'm writing this letter onit. It is a very versatile machine, tapeor disc based; software galore!

The hours of pleasure thiscomputer has given me areunmeasurable, both in constructionand computing itself.

Mel Saunders, Thurnby Lodge,Leicester.

It always gives me great pleasure tohear from people who enjoy the true'hands-on' aspect of electronics, deli-ghting in building things for construct-ion's sake, irrespective of the availa-bility of ready-made equivalents. Ed.

VIVE LA NOSTALGIA

Dear John,Congratulations on a splendid

anniversary edition!

MOREFEEDBACK

I thoroughly enjoyed reading allthe articles, especially those whichlooked back on the history of PE. Iam pleased to say that I recognisedmany past editions mentioned, eventhough they were published manyyears ago. Excellent, nostalgic stuff!

Incidentally, I thought you mightbe interested to know that while onholiday in France I came across aFrench magazine calledElectronique Practique - soundsvaguely familiar!

Tony Smith, Blackburn, Lancs.

Are you suggesting we have animitator? The French may beunsurpassable for their vintagewines, but nothing matches up tovintage PE, in any language!

(Tony is, of course, the author ofthe current Time and Frequencyseries, lately discussing UCTs.) Ed.

MARKING TIME

At our 25th anniversary party inthe Kew Bridge Steam MuseumWere many personalities who havebeen connected with PE in variousways . One of the was a kindly,elderly gentleman, Alfred Marks,who had for some time been SirClive Sinclair's advertising agent.It was a pleasure to see the warmthof greeting between Sir Clive andAlfred. Below is an extract fromAlfred's delightful letter to DavidBonner, PE's advertising executive,following the party. I should addthat the clocks referred to wereelectronic clocks inscribed tocommemorate the event and givenas gifts to all who came to theparty.

Dear David,Thank you very much indeed for

the clocks. They are muchappreciated, I assure you. Formyself, I am not keen on markingtime's inexorable march onwards.On the other hand, it was well worthsurviving to take part in that verypleasant evening with Sir Clive et alamidst those formidable Victorianmonsters. If only they could havehissed and blown steam! They werebeautiful.

Meanwhile, good luck to youngPE - a mere quarter of a century old.Should I be around when the next 25years have elapsed (that means mybeing 104) might not a grandfatherclock be appropriate?

Alfred Marks, London.

NUMBER CRUNCHING

Dear Sir,I do hope that you get your DTP

system sorted out in the nexttwentyfive years, in your anniversaryissue, you have constantly referred to"twenty five years". This means thatyou should be celebrating yourcentenary.

Also, in one advert one kit is listedas a "Community Tester"; I think thatthis should be "Continuity Tester", oris it a new measuring device for thePoll Tax?

Colin Long, Chigwell, Essex.

Wow! Does your first commentreopen Pandora's Box? I understandwhat you are saying, that "twenty fiveyears" implies 20 x 5 (years) = 100.Have I never been taught to writeEnglish correctly? Perhaps so! Yet,when writing cheques, I would write25 as two words, and would not expectthe'Bank to debit me with f100, or toreject my cheque on the grounds thatwords and figures differ. I do notrecall ever running the two words intoone, though my wife assures me shealways hyphenates them.

Intrigued by this question, Ichecked several sources. Among mydictionaries, only Chambers gives aclue, hyphenating the words, which tome is more pleasing than runningthem together. The next research I didwas into Fowler. To my surprise, hedoes not appear to tackle the questionat all, although his publishers havehyphenated similar numeric forms inthe texts. Fowler himself makes twocomments that might validate the useof all three forms : "No twodictionaries and no two sets of stylerules would be found to giveconsistently the same advice" (onhyphens); and, "Sir Winston Churchillwrote that 'One must regard thehyphen as a blemish to be avoidedwherever possible.'"

Until I know better, I shall in futureonly use the figured form! As an aside,one written numeric form I find hardto accept is the dropping of 'and'following 'hundred', as in 'onehundred twenty' (120), for example.

Regarding the advert, yes, it was aContinuity Tester. Nonetheless,perhaps an enterprising PE designercould well come up with the public-spirited (?) project as listed!

P.S. Running this text through thespellcheck program, "twentyfive" hasbeen rejected as a recognised form!Ed

SPELLBOUND!

Dear John,Your DTPed magazine shows signs

of having been run through an Englishspellcheck program. In my "Ask PE"number two I observe a caption saying"Circuit for a mosfet poser amplifier".I have lost the photocopy of myoriginal text for this article, but I feelsure it should have read "mosfetposeur amplifier".

Andrew Armstrong,Leighton Buzzard, Beds.

Andy, your wit leaves mespellbound! Ed

REPRIZAL

Dear Editor,Of course I accept your decision as

to the draw result for the Sept 89Maplinoscopy competition but amdisappointed to learn that two of myreasoned answers were invalid.

I had briefly considered the CRT ascommon to all except for radio butremembered that CR Tuning indicatorswere also once common and CRTSpectrum displays were incorporatedin a number of radios. Conversely,large numbers of computers, cameras,tvs, etc, do not use crts. So, that's notthe common link in Q6, thus leavingRADAR as the logical answer since itis the only acronym in the list.

Thanks for reading this; I expectyou get a number of differentviewpoints and I just felt I had to airmine.

J.Councell, Crowborough,Sussex.

Yes, you're reasoning is quitevalid, radar could well be regarded asthe odd -one out. You'd be amazed,though, at the variety of answers 1received regarding Q5 and Q6, eachof which was perfectly logical in itsown right. But, they weren't the'answers I was after, nor did they tallywith the majority of entrants who diddeduce correctly the answers I wanted.

What you also needed to considerin answering the questions was, if youcould see several possible choices,which one was most likely to be themost relevant. To be fair to themajority of contestants in anycompetition, the acceptable answersmust be those which tally with theQuestioner's and Adjudicator'sopinion; both roles are played by yourKindly Editor, so naturally hisdeciiion is final!

Now for the astonishment! l' mwriting this reply some time after thedraw for the 25th Anniversarycompetition took place. Incredibly, Inow note that the Mr J. Councell whowrote the above letter, is the samegentleman who has won first prize inthat competition.

Total coincidence! Ed.


For simplicity the positive and negative peaksof the sawtooth are assumed to be equal.

This results in a high output when theaudio is greater than the sawtooth voltage.The output of the comparator is applied to amultivibrator which switches at each fallingedge and the output of the multivibrator is arectangular pulse whose phase isproportionate to the instantaneous value of theaudio voltage.

Such modulators are available in is formfor operation up to a few MHz. The linearityof the sawtooth determines the linearity of themodulator. The maximum deviation of 180degrees is not usually achieved because ofswitching delays and fall time.

Frequency modulated feedback (fmfb) andautomatic frequency control (afc) can be usedto improve the linearity and stability offrequency modulators. The methods aresimilar to feedback in good quality audioamplifiers and available as ics.

The circuits detect the frequency of theoscillator or modulator output, compare it tothe audio input or detector output referenceand generate a correction voltage. Fmfb actsat the audio frequency rate while afc acts at alower rate.

SSB TRANSMITTERS

Single sideband transmitters are used inaircraft, marine, military and amateurcommunication. Here the communicationspectrum could occupy slots over severaloctaves whereas that for cw, am and fm isconfined to discrete bands.

The modulating signal is usually speechand therefore speech compression is usuallyemployed. Transmitters and receivers may beclassed as direct conversion, multipleconversion or broadband. In multipleconversion, several stages of frequencytranslation are employed.

DIRECT CONVERSION

Direct conversion is used in low costtransmitters when a single band offrequencies needs to be transmitted. Fig. 26shows one possible configuration.

Fig 25. (right) FMmodulator using acomparator.Fig 26 (below). Directconversion.

OSC.

AUDIOIN

SAWTOOTHGENERATOR

INTEGRATOR

V, COMPARATOR

MuLTIVIERATOREFL.M

OUT

6cP1016.6191

-t

DISCRETE BANDMULTIPLE CONVERSION

HF communication in the range 1.6MHz to30MHz using the ionosphere is subject torapid fading depending on the frequency used,condition of ionosphere, time of day, latitude,season of year, etc. Therefore several bandswithin the spectrum are allocated. In order toswitch bands the stages need to be gangedtogether, (Fig. 27) so that they may beswitched simultaneously.

Discrete band frequency translation startedduring the vacuum tube era. At 3.5MHz andduring daytime one would expect a range of300km, and 1000km to 2000km at 14MHz onthe first hop.

In Fig. 27, the variable frequency oscillator5MHz to 5.5MHz translates the 3MHz ssbinput into the 8MHz to 8.5MHz band. Acrystal oscillator then translates this band into

a discrete 0.5MHz band depending on thecrystal and bandpass filter selected. Fig. 27shows some possibilities but the harmonicsrequire careful investigation. For instance thethird harmonic of the vfo is in the range15MHz to 16.5MHz.

When mixed with the crystal oscillatoroutput of 12MHz, the output is in the 3MHzto 4.5MHz band. Therefore if the thirdharmonic of the vfo reaches the second mixerwhen operating in the 3.5MHz to 4.0MHzrange, spurious signals will be heard.

BROADBANDCONVERSION

Newer transmitters and receivers usebroadband conversion since it is beneficial tostandardise a design. Also militaryapplications require transmitters to operateanywhere in the hf band.

3MHz05C.

558MODULATOR

f2

I 6:4 1.1

05C.F IN

rin17i MHz f2 MHz

BAND SELECT12 3.5 - 4.015.5 7.0 - 7-5 [DJ oms, I

22-5 14, 0 -14.5

Lo -Y

10MHz

SSI3

MODULATOR

10.'4,682162MHz

L _L

_ OUTPUT16

If -7? 30MHzI

I

II

L I

SYNTHESISER73- 6 -102MHz

BAND SELECT

OUTPUT

J

Fig 27. (Above) Discrete andmultiple conversionFig 28. (Left) Broadbandconversion.

40 PRACTICAL ELECTRONICS JANUARY 1990

ASK PE Arr

starting point for experimentation. This'frequency is set by R2, R3 and C2.

A degree of motor speed stabilisation isachieved by measuring the current drawnby the motor, and applying positive currentfeedback to the circuit. Thus, if the currentin the motor rises due to an increase in theload, the mark:space ratio is increased toprovide more power. It is necessary tomeasure the average current rather than thepeak current in the motor, so a filter circuitconsisting of R5, C3, R6 and C4 isprovided. The gain of the feedback isadjusted by VR1, which should be set toprovide reasonable speed stability withoutcausing oscillation due to too much

feedback. The feedback signal is added tothe basic speed control signal, and fed tothe 555's control input via IC2a and Rl.C1 provides decoupling to give noiseimmunity. R4, which measures the motorcurrent when the hexfet is switched on,may be made from several strands ofresistance wire (eg old fire element) inparallel.

The 555 is notoriously susceptible tonoise problems, so a decoupling capacitormust be fitted close to the ic. It is alsoimportant to route the wiring sensibly toavoid imposing spikes and switchingtransients on the ic's power supply. Theonly other point to make is that D1 needs

to be rated at at least 15A and 15V, and tobe a fairly fast switching device. There areno doubt a number of suitable devices, butthe one chosen here is a BYW31. Thisdiode must be mounted on a heat sink, andthe heat sink must be isolated from the restof the metal work, because the stud of thediode is one of the terminals.

This circuit has been designed without adetailed knowledge of the motorcharacteristics, including leakagereactance, so some experimentation withcomponent values is likely to be necessary.

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PRACTICAL ELECTRONICS FEBRUARY 1990 4 I

HEX KEYBOARD

R eady-made hexadecimal keyboards arewidely used for data entry to many

digital circuits. The alternative keyboarddesign described here is much simpler,cheaper and can be used in more applicationsthan the customary hexadecimal keyboards. Italso has fewer components and can be used inmany projects. The 4 -Bit output is ttlcompatible.

A normal hex keyboard has 16 keys, but inthis circuit only 10 keys are used. Keys 2-7also serve as keys a,b,c,d,e,f when pressedalong with the SHIFT key.

The circuit diagram is given in Fig. 1. Itsheart is IC1, a ttl 74LS147, 10 -to -4 priorityencoder. Operation is straightforward anduses a decimal to binary conversion methodrather than the more common keyboardscanning technique. The latter technique isideal for computer keyboards, but for smallerapplications it seems unnecessarily expensive.

ICI assigns the binary equivalent value toeach depressed key's decimal value. Forinstance, if key 6 is pressed then the outputobtained will be binary 0110. If the same keyis pressed along with the SHIFT key, the

R1-9

100n

Fig 1. Simplehexadecimalkeyboardcircuit.

51 10k

4=6 rivv`0

S2 10k

o11

53 10k

o- ij\At-

54 10k

o

55 10k

o- /AA/L

56 10k- 1-1.\/\/LO

5, 10k

58 10k

omllso /ANL.

59 10k

SHIFT

0

16

+VE I +5V

3' ICI74L5147

R74HOC147

7B

3 D

0 A I LSB)

oS

D1

101G,7051

D (M5B1

0v

INGENUITY UNLIMITEDoutput bit 'D' will go high. Consequently thenumber obtained will be binary 1110,becoming 14, or E. D1 is used to preventshorting ICI 's output 'D' to ground. RI to R9positively bias the chip inputs when the keysare open. The 74LS147 can be replaced bythe high speed CMOS equivalent 74HC147.

Masroor HS BukhariPakistan

A selection of novelideas from

enthusiasticreaders

CAR BURGLAR ALARM

©he circuit may be separated into twostages: IC1 and its surrounding circuitry

forms a 40 second timer with its time periodcontrolled by R3 and C2. Upon receipt of anegative going edge at the trigger input (byconnecting this via a switch to ground) the

R1

10k

TRIGGERC5INPUT

1nE

R2

100k

I C555

8-to

R3

330k

RL A

Rt.

100k

57,°1417.8

It:06358i

C1- C210n

TR2BC182 L

Op

TR1BC182

R6 10k

LED1 R7

R6

470k2

3

7011

IIIMMWMIF

1C2555

R8

1000

Fig 2. Car burglar alarm circuit

C4 +C3 Nim o o c10nF 1T

-f-12 V

R L A 1

41

HORN

OV


BRIGHT IDEAS sli)

timer is activated, which switches TRI on,thus grounding the emitter of TR2.

IC2 is configured as an astable the outputof which pulses TR2 on and off, with afrequency of approximately 1Hz. With bothTR1 and TR2 on, the horn will sound on andoff at one second intervals.

LED1 should be mounted in a prominentposition to let any prospective intruders knowthat an alarm has been installed.

To avoid the possibility of false triggeringdue to ignition noise RI, R2, and C6 havebeen added. It is also advisable to mount thecircuit in an earthed metal case.

M.S. McNich, Bangor

RF OPERATED RELAY

nihis simple circuit was designed out ofnecessity, after I had built two separate

modules, ie a rf amplifier and a pre -amplifier,to boost both transmit and receiveperformance of an ageing mobile two -metrestransceiver. The idea of the circuit is to senseradio frequency power to switch ancillaryequipment on or off when the transmitter ispowered up. Possible uses are many, forexample you may wish to perform an aerialchange -over function for transmit/receiveoperation, crossband working, receivermuting capability requirement, or how aboutcontrolling a studio type ON AIR sign outsidethe shack door?

The basic circuit is shown in Fig. 3.Operation is relatively straightforward.

The rf is picked up and coupled by Cl to thedemodulator and rf decoupling circuitcomprising of DI, D2 and C2. This dc istaken to the base of TR1, which along withTR2 forms a Darlington pair high gainamplifier. The signal causes the Darlingtonamplifier to switch on, so turning on the relay.D3 protects the transistors from beingdamaged by back emf produced every timethe relay is turned off.

The combination of C3 and VR I producewhat is known as a "hang -time" which isrequired during cw or ssb transmission, sincethe pauses between information wouldotherwise produce chopped speech, and anannoying relay contact chatter.

To increase the hang -time, increase thevalue of C3. It may be prudent to include aswitch so as to switch the hang -time in andout, eg for use in ssb or fin mode of operationrespectively (see Fig. 4).

Fig 4. SSB/FM switch

02

C2

SSEi OFkl

C.3

VR1

IDJG,7081

As the circuit stands, it will switch whenminute quantities of radio frequency power arearound. With the prototype, it was found thatwith 18 inches of hook-up wire attached to theinput at Cl, a small child's walkie-talkiecaused switching when it was a foot or soaway. When the pick-up wire was connecteddirectly to the aerial, the relay switched overstraight away when the walkie-talkie was onreceive! The rf switch was being activated bythe receiver's local oscillator. Therefore, if youfind that this circuit is a little too sensitive foryour purpose, then it may be necessary todampen it a little by installing a sensitivitycontrol in the form of a preset potentiometer atthe front of the circuit (Fig. 5).

Another consideration is that it may berequired to operate this circuit remotely, ie atsome distance away from the actual transmitteritself. This may be achieved with varyingdegrees of success, dependant upon frequency,

INPUT

10k

Fig 5. Sensitivitycontrol

C1 D2

[016170

D2

IDAA7,111

Fig 6. Remote operation

FROMRIG

BOX

SWG WIRE

STAND-OFFINSULATORS

Fig 7. Method of coupling

IDJ5i7til

output power of the control transmitter, etc.The requirement is a parallel tuned circuit atthe front end of the circuit, resonant at thesame frequency of the transmitter (Fig. 6).

If you wish to have "positive" control overthe rf operated switch, and have reservationsabout connecting an expensive black box to anunknown quantity, then you may like toconsider a type of parallel lines pick-up similarin idea to those used in some rf swr powermeters. The general idea is shown in Fig. 7.

A further modification that may be requiredis a manual overide. This can achieved simplyby means of a dpdt switch (as in Fig. 8).

Fig. 9 shows how the rf operated relaycould be used to switch in either a linearamplifier or a receiver preamplifier.Paul Benton, Staffs PE

Fig 8. Manual overide

C4W

ING17121

A

B

N.C.

+VE

o 0

o 0

O 0

DPDT

Dv

D

N.C.

At AL

T

IDJG17131L

° J

PRE -AN'

POWER

AMP.

Fig 9. PA and preamp controlswitching

I°

0,

DOUBLE POLE CONTACTS OF RELAY

0

TINPUT TO R.F.OPERATED RELAY

R1C3


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PHOTOGRAPHIC PROJECT

M any people who take flash picturessimply plonk the gun on top of thecamera, and fire away. With some

cameras, you don't get much option about wherethe flashgun sits as it's already incorporated intothe camera body! This will invariably lead topictures with nice black shadows thrown againstthe wall, and if you're really lucky, your subjectswill also get red glowing pupils into the bargain!Better results can be obtained either by movingthe first gun and/or using a second gun tobrighten up the shadows, so giving somemodelling form to the subject.

The device described here will fire a

second, (or third, fourth and fifth) flashgunwithout any direct connection to the mainflashgun. It doesn't require any additionalpower; it derives this directly from the gun.

FLASHY CIRCUIT

Light from the main flashgun causes lightsensitive transistor TR I to conduct, pullingTR2's base down to the negative line. SinceTR2 is a pnp device, current will flow inthrough its emitter, and out via its collector.Attached to the collector is a differentiator.When TR2 conducts suddenly, a pulse ofcurrent reaches C1. The opposite plate of thecapacitor pushes out a similar current pulse,which fires the thyristor via its gate. Thiseffectively shorts the two power lines together,

Fig 1. Circuit diagram for the Slave Flash.

Chris Brown'sgenii of the light

disperses shadowsin milliseconds.

Find it othertasks too!

shielded wire connecting to the positive. Theunit is in fact so small, with the photocellmounted aloft the thyristor, the whole caboodlecan be housed in a Tic Tac box!

Plug the unit into the flashgun, and chargethe latter. In a normally lit room, with thesensor pointing away from bright light sources,nothing should happen. Now, using a torch,shine the beam at, or across the photo -transistor. The gun should fire. If not, checkyour wiring. It's so simple, not a lot can gowrong. Now try the same experiment, butusing a flashgun in place of the torch.

SLAVE FLASHwhich acts like a closed switch over the flashcontacts, and so the gun fires. All this happensvery, very fast, so the second gun appears toflash at exactly the same instance as the maingun. The gun now starts charging again, andwill flash again on the next burst of light.

The differentiator, ie the capacitor and theresistors which join it to the neg line, areneeded to avoid the unit firing when theambient or surrounding light changes. Withoutthe differentiator, if you were workingoutdoors on an overcast cloudy day and thesun came from behind a cloud, the gun couldfire as conditions brightened. However, withthe circuit, these slow changes of light chargeand discharge the capacitor too slowly to firethe thyristor. However, when the flash fires,

there's a sudden increase in current, and thecapacitor passes a largish current. It's like aslow waterfall emptying into a stream; thestream below gently ripples. But drop a largestone into the stream, and you get a mini tidalwave due to the sudden surge!

CONSTRUCTION

Nothing too complicated here, so I'm sureyou can find an odd bit of veroboard and figureout your own layout. Be sure to mount thethyristor the correct way around. The flashleads must also be connected the correct wayaround, usually with the outer screened braidconnecting to the negative line, and the inner

OTHER USES

Of course, the unit need not be limited toflashguns. It'll serve purpose wherever a briefflash of light indicates a certain event hasoccurred, such as when a reel to reel tape hasbroken, or come to the end. Normally, the cellsees no light, but when the break occurs, it seeslight from a nearby source, and the circuit istriggered. Similarly, in a blacked out room, theunit could fire an alarm if a burglar's light fallson the cell. Or a cupboard could be made todraw attention whenever it's opened, anddaylight falls onto the cell. The uses areendless with a bit of imagination.

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O ne of Britain's important smallobservatories, the Norman/LockyerObservatory at Sidmouth in Devon,

has been through a bad period; following theretirement of its last Director, D.R. Barber, itfell into the hands of Exeter University, who -to be honest - neglected it badly. One of itsthree telescopes, the Mond, was so damagedthat it could not be repaired. However, theother two telescopes, the McClean and theKensington - both medium-sized refractors -survived, and when the Observatory wasacquired by the East Devon District Councilrenovations were put in hand. The McCleanrefractor has been completely restored, andwork on the Kensington telescope shouldbegin soon. It is hoped to turn theObservatory into an educational centre as wellas making the instruments available forresearch. The official re -opening took place inOctober; I was honoured to be invited toofficiate at the ceremony!

Efforts continue to be made to salvagesomething from the Hipparcos disaster.Hipparcos, the astrometric satellite, wasexpected to provide the best -evermeasurements of the positions and parallaxesof stars, but the failure of the on -board rocketmotor put the satellite into the wrong orbitSince then, technicians have managed to raisethe orbit to the extent that its lowest point isnow at an altitude of 600 km, but whether ornot Hipparcos will provide really good resultsis by no means certain; for one thing, its solarcells have been damaged by repeatedpassages through the Van Allen zones. At allevents, it is clear that Hipparcos will not beable to carry out more than a part of itsplanned programme. If it is a total loss from atechnological point of view, there will be acampaign to build a replacement - but this,like Many other proposals, will be a slowprocess.

Traces of activity on the Moon have oftenbeen reported; they are generaly known as

BY DR PATRICK MOORE CBE

The damage atSidmouth is

being repaired.Will space

"advertising" bethe next threat?

TLP or Transient Lunar Phenomena. Thelatest case to be recorded photographicallywas that on 23 May 1985, when the Greekastronomer G. Kolovos secured a picture of aslash near the lunar crater Proclus C, andsuggested, very reasonably, that it was due toan electrical charge in gas rising from cracksin the surface at the time of sunrise.

Proclus is an area in which TLP have beenrecorded on various occasions. However, twoinvestigators, H. Rast and P. Maley, ofHouston in Texas, suggest that the flash couldhave been due to reflection from an artificialsatellite; they claim that a large meterologicalsatellite was passing some 800 km above theEarth, and was directly between the Earth andthe Moon, at the moment of Kolovos'photograph.

This seems very unlikely. The agreement isby no means good, and in any case the outlineof the flash was confined by features on theMoon's surface, indicating that it really waslunar in origin. Decisive proof is lacking, butthe earth satellite theory seems to be veryunconvincing indeed.

BRIGHT LIGHTSIN THE SKY

Many years ago, one irritated Germanastronomer described asteroids or minorplanets as 'vermin of the skies', becausephotographic plates exposed for quitedifferent reasons were often found to becrawling with asteroid trails. But an asteroidis a faint object; a much brighter moving bodycan do real damage, and it is true that someartificial satellites have produced effectswhich astronomers find unwelcome.

However, the idea of sending up a 'lightring' to celebrate the centenary of the EiffelTower was much more dangerous. A brilliantobject of this type, passing across the field of

THE JANUARY SKYT he start of 1990 is not very encouraging for planetary

observers inasmuch as Mercury, Mars and Saturn are allvery badly placed, while Venus passes through inferior

conjunction on January 18 and is visible only for a brief period asan evening object in the south-west (for the start of the month) thenas a morning object in the south-east (at the very end). However,Jupiter at least is on view for most of the hours of darkness. It lies inGemini, the Twins, and is very brilliant, so that it cannot bemistaken. At the moment it is of special interest for telescopicobservers; the Red Spot, which had been obscure for some years upto the autumn of 1989, has now reappeared, while the SouthEquatorial Belt has almost vanished. The 'weather' on Jupiter isgrange at the moment; the planet is worth monitoring.

The Moon is at First Quarterlon January 4, full on January 11,Last Quarter on January 18 and new on January 26. On the 26ththere is an annular eclipse of the Sun, but unfortunately anyone whowants to have a really good view of it will have to go to Antarctica;not even the partial phase can he seen from Britain.

The evening of January 3 sees the maximum of the annualQuadrantid meteor shower. The Quadrantids can be spectacular, but

the maximum is always very short and sharp. This year there will nothe a great deal of interference from moonlight, so the Quadrantidsmay well put on a good display.

The glorious winter constellations are now at their best, withOrion high in the south after dark; the three stars of the belt pointdownward to Sirius, upward to Aldebaran, the orange 'Eye of theBull'. Sirius, 26 times as luminous as the Sun, is a pure white star,but it seems to twinkle, and to show various colours of the rainbow -not because it is actually doing so, but because its light is coming tous via a thick layer of the Earth's turbulent atmosphere. It is strangeto recall that many ancient astronomers described Sirius as 'red'. Itis certainly not red now, and it is not the kind of star to show anymajor short-term changes, so that presumably there must be an errorin interpretation; all the same, it is decidedly odd.

Ursa Major, the Great Bear or Plough, is in the north-east; the Wof Cassiopeia, on the opposite side of the Pole Star, is in the north-west. Overhead, the brilliant yellow Capella is very prominent. TheSquare of Pegasus has now almost set in the west, and much of thesouthern aspect is filled by the large but very faint and formlessconstellation of Cetus (the Whale).


view of a telescope using sensitive electronicequipment, would cause immense damage.

" Scientists all over the world were so volublein their protests that the entire project wasdropped.

Now there is a rather different idea. Thistime it is proposed to launch a 'clock', orrather two hands of a clock, which will crossthe sky several times a day and provide GMTfor the entire world. Financial backing for thisplan has been sought. And again there will beprotests if there is any fear that the projectwill be followed up.

It is not easy to see what use such a clockwould be - after all, it is much easier to lookat one's wrist -watch. And suppose that theclock went wrong? Presumably it would haveto be regularly adjusted, which would not beeasy.

However, the main objection is the same: abright moving object can damage delicateequipment, even if its orbit is well known.The fewer bright artificial objects we launch,the better. Of course, not everyone wouldagree, and one can picture advertisers lickingtheir lips; no doubt they would be delighted tosee sunlit space -probes urging the merits ofindigestion tablets, double glazing, or cat -food! But it would indeed be the thin end ofthe cosmical wedge; and we can only hopethat the space clock, like the Eiffel Towerlight ring, will never 'get off the ground'either literally or metaphorically.

(Hear, hear! I'll he among the first to signa petition against such outrageous ideasif anyone starts one. Ed.) PE

ASTRONOMY FEATURE

Jupiter: on view for most of the hours of darkness during January. You'll see itwith the naked eye, but not as clearly as in this photograph taken by JeanDragesco at the Pic -du -Midi observatory in France.

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Thermal Images,f the Moon

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TIME AND MEASUREMENT

reviously we saw how the resolutionand accuracy of a low frequencymeasurement can be improved by

measuring the period and then finding thereciprocal. For counters not fitted with thecircuitry required to compute the reciprocal, itis necessary to determine it manually, whichcan be a tedious business, especially if we aremaking many measurements, or if the inputfrequency is gradually changing.

FREQUENCYMULTIPLIERS

An alternative approach is to multiply thelow frequency before feeding it to the maingate. Consider, for example, an inputfrequency of 23.456 Hz: measuring this in theconventional manner with a one second gatetime will yield a reading of 23Hz (±1Hz), ie,a resolution of 1Hz, and an inaccuracy of±1/23 = ±4.3 per cent (neglecting time baseerrors).

If, however, we multiply the frequency by1000, the counter will accumulate 23,456 (±1)counts during the one second gate time; withsuitable decimal point selection, the readingcan be displayed as 23.456Hz, ie, a resolution

CLOSE

CONTACT CIDSURE TIME, T

INPUTSIGNAL -

FROMCONTACTS _

OPEN

\,__ - - YILI I. INPUTHYSTERESIS_ __ _ _ _ - _ ____ VII THRESHOLDS

...."----- CONTACT BOUNCE

CONDITIONEDINPUT uu

TRIGGER DELAYPULSE

CORRECTMEASUREMENT

tD I

10,61641

Fig. 11.Avoidingcontactbounce errors.

also increase, such that fO, and thus f0/N,increase, too.

Consequently, because fin = f0/N, itfollows that to = Nfin, and so the countedsignal is actually fin multiplied by N. Theactual value of N (usually a multiple of 10) islimited by the maximum frequency of the vco.For example, if f0(MAX) = 1MHz, a value ofN = 1000 will limit the maximum inputfrequency to lkHz:(fin(MAX) = fO(MAX)/N= 1MHz/1000).

For input signals with varying frequency,the rate of change must not be too great,otherwise the pll will 'loose lock', thusinvalidating the readings. On some units, anindicator is illuminated when the loop is notlocked, or the circuit may even blank thedisplay to avoid false readings.

Despite these restrictions, the pll multiplieris insensitive to high frequency noise, and canbe added to the basic counter withoutrequiring any extra computing circuitry.

INCREASING UCTPERFORMANCE

of 0.001Hz, and a quantisation inaccuracy ofonly ±1/23,456 = ±0.0043 per cent.Obviously, both resolution and accuracy areincreased by the multiplication factor, N (inthis case, by 1000).

The multiplication can be carried out by aphase -locked loop - see Fig. 10. The phasedetector compares its two input signals, finand to/N, and generates an error signal Se,which is low pass filtered to provide an errorvoltage Ve, proportional to the phasedifference between the inputs. The voltage Vecontrols the vco, whose output frequency, 10,is divided by N and fed back to the phasedetector, thus completing the loop.

The effect of the closed loop is to maintaina constant phase relationship between fin andf0/N, ie, fO/N is kept at the same requency asfin. If, for example, fin increases, Se and Ve

PART TWO

Time and TDwill wait for anyman, concludesAnthony Smith,summarising his

Time andFrequency series.

SPECIAL TECHNIQUES

As well as methods for increasing therange, accuracy and resolution ofmeasurements, many ucts feature specialtechniques to simplify operation, or to allowmeasurements on signals which wouldusually cause false readings.

A good example is the 'Trigger Delay'control, sometimes called 'Trigger Hold -Off'. This feature is especially useful inmeasuring signals output from relays andother types of switches, where contactbounce would normally cause gross errors.

For example, assume we wish to measurethe contact closure time of the signal in Fig.11. The presence of contact bounce results in

Fig 10. Frequencymultiplier scheme.

PHASE -LOCKED LOOPLOW I(PLL) LOW PASS

FREQUENCYINPUT CONDITIONING f IN I

FILTERt a

FIN

PHASEDETECTOR

ex_o VOLTAGECONTROLLED

OSCILLATOR 0/C01

CIRCUITRY se VQ (="IN)MAIN _11.11ILIULGATE COUNTED SIGNAL

TO Dal

I 0/N DIVIDER

( - N

L _J

TIME

BASE

PRACTICAL ELECTRONICS FEBRUARY 1990 4')

three short pulses occuring at the start of theconditioned input signal. Normally, thecounter would measure the duration of thefirst of these 'bounce' pulses, resulting in atotally erroneous reading.

The trigger delay feature, however,generates a delay pulse, tD, starting at theleading edge of the input signal. Byadjusting the length of tD to be longer thanthe bounce duration, but shorter than theclosure time, the operator can delay anyfurther triggering until the trailing edge ofthe input signal, such that the displayedreading is the true closure time.

Trigger delay is found on counters suchas the Philips PM6652 and PM6671, and onthe Hewlett Packard HP5335A; the delay isadjusted by a front panel knob, and typicallycovers a range from a few microseconds to afew hundred milliseconds.

As well as eliminating the effects ofcontact bounce, trigger delay can be used tomake pulse train measurements; forexample, in Fig. 12, tD is adjusted to 'blankout' the individual input pulses, allowing usto measure the duration of the pulse trainitself.

INPUTPULSE TRAIN

TRIGGERDELAY PULSE

MEASUREDTIME INTERVAL

-.r PULSE TRAIN DURATION

ID

loromcsj

Fig 12. Pulse train measurements

ARMING

'Arming' is a versatile feature, usuallyfound only on the more sophisticated ucts,and, like trigger delay, is used to preventtriggering on unwanted pulses. Unliketrigger delay, however, it relies on anexternal arming signal to determine whichpulses are ignored.

Fig. 13 shows how the counter iseffectively disabled while the arming signalis high: as soon as the signal goes low,however, the counter is armed and ready toinitiate measurement on the next transitionof the input signal.

In this way, the arming signal can be usedto select which input transition opens themain gate, and the trigger delay feature candetermine which transition closes the gate,such that we can 'pick out' and measurevirtually any portion of a complex inputsignal.

ADDITIONALFEATURES

In addition to the techniques discussed sofar, several uct manufacturers offer specialfeatures intended to extend the performanceand measuring power of their products.

For example, many modern counters cancommunicate with - and be controlled by - amicrocomputer via a standard interface bus,typically the IEEE 488/IEC-625. This allowsthe computer to set up all the relevantmeasurement parameters - such asmeasuring mode, averaging range, triggerlevels, attenuation, etc - using only digitalcodes sent via the bus. On completion of themeasurement, the results are sent back to thecomputer in the same way. Because there isno need for user intervention, the countercan easily be incorporated into an ate(automatic test equipment) system.

Many ucts are now fitted with front -panelkeypads which enable the operator to set upanalogue parameters with digital simplicityand precision. The Philips PM652, forexample, has keyboard trigger levelselection over the range -5V to +5V in10mV steps. The same keyboard is also usedfor mathematics operations, such asmeasurement scaling, display off -setting,and units conversions. Also, this model -like several others on the market - can makevoltage measurements, such that themaximum, minimum, peak -peak, or rmsvalues of the input signal can be read fromthe digital display.

Fig 13. Arming permits selectivemeasurements.

CONCLUSIONS

Over the past few months we've looked atmany aspects of time and frequencymeasurement, concentrating mainly on theUniversal Counter Timer, both from anoperational and a technical point of view.

The primary aim was to examine all themajor topics in detail, and in the articleswe've dealt with the basic measurementconfigurations; the requirements of the inputconditioning circuits; the time base and thedifferent frequency standards available;sources of error and how to avoid them; andsome of the more specialised techniquesavailable to improve performance.

Although we've covered quite a lot ofground, the series is not a totallycomprehensive treatment of the subject, andmany areas have had to be left unexplored.Nevertheless, I hope the fundamentals havebeen dealt with in enough detail to enable youto get optimum performance from yourcounter, or - if you're thinking of buying anew one - to allow you to choose the modelbest suited to your budget and your intendedrange of applications.

Alternatively, I hope that any inspiredconstructors will now be in a good position toattempt to build their own Universal CounterTimers!

mmWe shall soon he encouraging them to do

so when we publish Tony Smith's UCT project.Don't miss it! Ed.

-J No inflation at PE. - Still at the same price for over four years!Yes, you can still save money! For a limited period thesubscription to your favourite magazine is being held

at the old price of £15.00 ( overseas £18.00).Don'twait! So, raid your piggy bank for hard cash (cheques or

payments by credit cards accepted also) and send itto us. For full details see page 9!

iSs if if mtif ititf if it ttof if 414ftf hitt50 PRACTICAL ELECTRONICS FEBRUARY 1990

BAKERS DOZEN PACKSAll packs are £1 each, it you order 12 then you are entitled toanother free Please state which one you want Note the figure onthe extreme left of the pack ref number and the next figure is thequantity of items in the pack, finally a shat description.

BD2 5 13A spurs provide a fused outlet to a ring main wheredevices such as a dock must not be switched off.

BD9 2 6V 1A mains transfamers upright mounting with hxingdamps.

BD11 1 6.5 -in speaker cabinet ideal fa extensions, takes curspeaker. Ref. BD137.

BD13 12 30 watt reed switches, its surprising what you can makewith these - burglar alarms, secret switches, relay, etc., etc.

BD22 225 watt loudspeaker two unit crossovers.BD30 2 Nicad constant current chargers adapt to charge almost

any nicad battery.BD32 2 Humidity switches, as the air becomes damper the

membrane stretches and operates a miaoswitch.BD42 5 13A rocker switch three tags so aVoff, or change over with

centre off.BD45 1 24hr time switch, ex -Electricity Board, automatically adjust

for lengthening and shortening day, original cost £40 each.9049 5 Neon valves, with series resistor, these make good night

lights.BD56 1 Mini uniselecta, one use is for an electric jigsaw puzzle,

we give circuit diagram for this. One pulse into motor, movesswitch through one pole.

BD67 1 Suck or trow operated pressure switch, or it can beoperated by any low pressure variation such as water level inwater tanks.

BD103 A 1 6V 750mA power supply, nicely cased with mains inputand 6V output leads.

00120 2 Stripper boards, each contains a 400V 2A bridge rectifierand 14 other diodes arid rectifiers as well as dozens ofcondensers, etc.

BD128 10 Very fine drills for pcb boards etc. Normal cost about 80peach.

BD132 2 Plastic boxes approx Sin cube with squats hole through topso ideal fa Interrupted beam switch.

BD134 10 Motors for model aeroplanes, spin to start so needs noswitch.

BD139 6 Miaophale inserts - magnetic 400 ohm also act asspeakers.

BD148 4 Reed relay kits, you get 16 reed switches and 4 cot setswith notes on making do relays and other gadgets.

EID149 6 Safety cover for 13A sockets - prevent those inquisitivelittle fingers getting nasty shocks.

BDIBO 6 Neon indicates in panel mounting holders with lens.BD193 6 5 amp 3 pin flush mounting sockets make a low cost disco

panel.BD199 1 Mains solenoid, very powerful, has lin pull or could push if

modified.BD201 8 Keyboard switches - made for computers but have many

other applications.BD211 1 Electric dock, mains operated, put this Ina box and you

need never be late.BD221 512V alarms, make a noise about as loud as a car horn.

Sightly soiled but OKBD242 2 On x 4in speakers, 4 ohm made from Radianobile so very

good quality.BD252 1 Panostat, controls output of boiling ring from simmer up

boil.BD259 50 Leads with push -on 1/4in tags - a must for hook-ups -

mains connections etc.BD263 2 Oblong push switches for bell or chimes, these can mains

up to 5 amps so could be foot switch if fitted into pattess.BD268 1 Mini 1 watt amp for record player. Will also change speed

of record player motor.EID283 3 Mild steel boxes appox 3in x 3in x lin deep - standard

electrical.BD305 1 Tubular dynamic mic with optional table restBD400 4 Books, useful for beginners, describes amplifiers

equipment and kit sets.BD653 2 Miniatire driver transformers. Ref. LT44, 20k to 1k centre

tapped.BD548 2 3.5V relays each with 2 pairs changeover contacts.13D667 2 4.7 of non -polarised block capacitors, pcb mounting.There are over 1,000 items in our Bakers Dozen List If you want acomplete copy please request this when ordering.

TOASTERS 2 SLICE toasters - may need slight attention only £3 each ref3P84.PERSONAL STEREOS Again customer returns but complete and withstereo head phones a bargain at only £3.00 each. Our ref 3P83.MAINS OPERATED MICROWAVE CONTROL PANEL with Touchswitches. This unit has a 4 digit display with a built in dock and 2 relayoutputs - one for power and 1 for pulsed power level. Could be used forall sorts of timer control applications. Only £6.00. Our ref 6P18.EQUIPMENT WALL MOUNT It is a multi -adjustable metal bracket thatcould be used for mounting lood hght loudspeaker, TV camera, even afan and on almost any sort of wall or ceiling even between wall andceiling. The main fixing brackets rotate such that an inward or an outwardcoma can be accomodated. Front panel also tilts upward or downwardsto a reasonable angle and can be easily removed separately fa wiring. Avery useful bracket. Regular price would be around £6 each. Our priceonly £3. Our ref 3P72. Or 2 for £5. Our ref 5P152.SUB -MIN TOGGLE SWITCH Body size 8mm x 4mm x 7mm SBDT withchrome dolly sizing nuts. 3 for £1. Order ref BD649.COPPER CLAD PANEL for making PCB. Size approx 12in long x 8.5inwide. Double -sided on fibreglass middle which is quite thick (about1/16in) so this would support quite heavy components and could evenform a chassis to hold a mains transformer, etc. Price £1 each. Our refBD683.REAL POWER AMPLIFIER for your car, it has 150 watts output.Frequency response 20hz to 20Khz and signal to noise ratio better than60413. Has built in short droolt protection and adjustable input level to suit

POWERFUL IONISERGenerates approx. 10 times more IONS than the ET1 and similarcircuits. Will rehesh your home, office, workroom etc. Makes you feelbetter and work harder - a complete mains operated kit, case included£12.50 . £2 p&p. Our ref 12P5/1.

your existing car stereo, so needs no pre -amp. Works Into speakers ref.30P7 described below. A real bargain at only £57.50. Order ref: 57P1.REAL POWER CAR SPEAKERS. Stereo pair output 100W each. 4 -Ohmimpedance and consisting of 6.5in woofer, On mid range and tin sweeter.Each set in a compact purpose built shelf mounting unit. Ideal to workwith the amplifier described above. Price per pair £29.96. Order ref: 30P7.STEREO CAR SPEAKERS. Not quite so powerful - 71kv per channel. 3inwoofer, 2in mid range and tin tweeter. Again, in a super purpose builtshelf mounting unit. Price per pair: £27.95. Order ret. 28P1.VIDEO TAPES These are three hour tapes of superior quality, madeunder ICW1C8 from the famous NC Company. Offered at only £3 each.Our ref 3P63. Or 51a £11. Our ref 11P3. Or for he really big user 10 for£20. Our ref 20P20.

ELECTRONIC SPACESHIP. Soundand impact controlled, responds toclaps and shouts and reverses when ithits anything. Kits with really detailedinstructions. Ideal present fa buddingyoug electrician. A youngster shouldbe able to assemble but you may

have to help wtht the soldering of the components on the pcb. Completekit E10. Our ref. 10P81.

12" HIGH RESOLUTION MONITOR Black and white screen,beautifully cased for free standing, needs only a 12v 1.5 ampsupply. Technical data is on its way but we understand these areTTL input. Brand new in makers' cartons. Price: £22.00Free delivery. Order ref: 25P10.

14" COLOUR MONITOR made by the American Display Tek Company.Uses high resolution tube made by the famous Japanese Toshiba company.Beautifully made unit intended for console mounting, but top and sidesadequately covered by plated metal panels. Full tediniad spec, on its wayto us. We have a limited number of these. Al brand new stills makerscartons. Price: £89 each plus £6 insured carriage. ()der ret 89P/1.COMPOSITE VIDEO KITS. These convert composite video into separateH sync, Vsync and video. Price £8.00. Our ref 8P39.BUSH RADIO MIDI SPEAKERS Stereo pair. BASS reflex system, usinga full range 4in driver of 4 ohms impedance. Mounted in very nicely madeblack fronted walnut finish cabinet. Cabinet size approx 8.5in wide, 14inhigh and 3.5in deep. Fitted with a good length of speaker flex andterminating will a normal audio plug. Price £5 the pair plus £1 post Ourref 5P141.3.5in FLOPPY DRIVES We still have two models in stock: Single sided,80 tack, by Chinon. This is in the manufacturers metal case with leadsand IDC connectors. Price £40, reference 40P1. Also a double sided, 80track, by NEC. This is uncased. Price £59.50, reference 60P2. Both areband new. Insured delivery £3 on each a both.

1 ATAFt1 65XE COMPUTERsAut it6a4bKlethtiosr ihsommoes tanpdowbuerskiinl easnsd

Brand new, complete with PSU, TVlead, owner's manual and sixgames. Can be yours for only £45plus £3 deivoy.

REMOTE CONTROL FOR YOUR 65XE COMPUTER With this outfit youcan be as much as 20 feet away as you will have a joystick that cantamsmit and a receiver to plug into and operate your computer and TV.This is also just right if you want to use it with a big screen TV. Thejoystick has two fire buttons and is of a really superior quality, with foursuction cups fa additional control and one handed play. Price £15 for theradio controlled pair. Our ref 1 5P27.ASTEC PSU. Mains operated switch mode, so very compact Outputs+12v 2.5A, +5v 6A, +-5v .5A, ..-12v 5A. Size: 7.5in long s 4.75in wide x2.25in high. Cased ready for use. Brand new. Normal price £30+, ourprice only £12.95. Order ref 13P2.VERY POWERFUL 12 VOLT MOTORS. 1/3rd Horsepower. Made to dive theSinclair C5 electric car bul adaptable to power a go-kart, a mower, a rail car,model railway, et. Brand new. Price £20 £2 postage. Our rel. 20P22.

PHILIPS LASERThis is helium -neon and has a power raing of 2mW. Completely safeas long as you do not look directly into the beam when eye damagecould result. Brand new, full spec. £30 plus £3 insured delivery. Mainsoperated power supply fa this tube gives Sty stiffing and 1.25kv at5mA running. Complete kit with case £15. As above for 12V battery.Also £15. Our ref 15P22.

ORGAN MASTER is a three octave musical keyboard. It is beautifullymade, has lull size (piano size) keys, has gold plated contacts and iscomplete with ribbon cable and edge connector. Can be used with manycomputers, request information sheet. Brand new, only £15 plus £3postage. Our ref 15P15.

FULL RANGE OF COMPONENTS at very keen prices are availablefrom our associate company SCS COMPONENTS. You may alreadyhave their catalogue, if not request one and we will send it FOC withyour goods.

HIGH RESOLUTION MONITOR. On black and white, used Philips tubeM24/306W. Made up in a lacquered frame and has open sides. Made foruse with OPD computer but suitable for most others. Brand new. £16 plus£5 post. Our ref 16P1.12 VOLT BRUSHLESS FAN. Japanese made. The popular square shape(4.5in x 4.5in x 1.75n). The electronically run fans not only consume verylittle current but also they do not cause interference as the brush typemotors do. Ideal for coding computers, etc., Of fa a caravan. £8 each.Our ref 8P26.MINI MONO AMP on p.c.b. size 4in x 2in (app.)Fitted Volume control and a hole lor a tonecontrol should you require it. The amplifierhas three transistors and we estimate thecutout to be 3W rms. More technical data ,will be included with amp. Brand new,perfect condition, offered at the very low priceof £1.15 each, or 13 for £12.00.

J&N BULL ELECTRICALDept. PE 250 PORTLAND ROAD, HOVE,

BRIGHTON, SUSSEX BN3 50T.MAIL ORDER TERMS: Cash, PO or cheque with order. Orders under£20 add £2.50 service charge. Monthly account ceders accepted fromschools and public companies. Access and B/Card orders accepted -minimum £5. Phone (0273) 734648 a 203500.

POPULAR ITEMS - MANY NEW THIS MONTH ItJOYSTICKS for BBC, Atari, Dragon Commodore, etc. All £5.00 each. Allbrand new, state which required.TELEPHONE TYPE KEYPAD. Really first dass rear mounting unit Whitelettering on black buttons. Has conductive rubber contacts with soft cickoperation. Circuit arranged in telephone type array. Requires 70mm by55mm cutout and has a 10 IDC connector. Price £2.00. Ref. 2P251.SUB -MIN PUSH SWITCHES Not much bigger than a plastic transistorbut double pole PCB mounting. 3 for £1.00. Our ref 130688.AA CELLS Probably the most popular of the rechargeable NICAD types.4 fa £4.00. Our ref. 4P44.20 WATT 4 OHM SPEAKER With built in tweeter. Really well made unitwhich has the power and the quality for hifi 6.5in dia. Price £5.00. Our ref;5P155 a 10 for £40.00 ref. 40P7.MIN RADIO MODULE Only 2in square with ferrite aerial and solid dia.tuner with own knob. It is superhet and operates from a PP3 battery andwould drive a crystal headphone. Price £1.00. Our ref. BD716.BULGIN MAINS PLUG AND SOCKET The old and faithful 3 pin withscrew terminals. The plug is panel mounted and the socket is cablemounted. 2 pairs for £1.00 a 4 plugs or 4 sockets for £1.00. Our ref.80715,130715P, a BD715S.MICROPHONE Low cost hand held dynamic microphone with on/offswitch in handle. Lead terminates In 1 3.5mm and 1.25mm plug. Only£1.00. Ref. 8D711.MOSFETS FOR POWER AMPLIFIERS AND HIGH CURRENT DEVICES140v t00watt pair made by Hitachi. Ref 25K413 and its complement2S.11113. Only £4.00 a pair. Our Ref. 4P42.Also available in H pads Ref 2SJ99 and 25K343 £4.00 a pair. Ref 4P51.TIME AND TEMPERATURE LCD MODULE A 12 hour dock a Celsiusand Fahrenheit thermometer a too hot alarm and a too odd alarm. Approx50x20mm with 12.7mm digits. Requires IAA battery and a few switches.Comes wit full data and diagram. Price £6.00. Our ref. 6P12.REMOTE TEMPERATURE PROBE FOR ABOVE. £3.00. Our ref. 3P60.A REAL AIR MOVER Circular axial fan moves 205 cubic foot per minwhich is about twice as much as our standard 4.5in fans. Low noisemains operated 6.5in dia, brand new. Regular price over £30.00. Ourprice only £10.00. Our ref 10P71.600 WATT AIR OR LIQUID MAINS HEATER Small coil heater made forhealing air a liquids. Will not corrode, lasts lor years. Coil size 3in x 2nmounted on a metal plate for easy fixing. 4in dia. Price £3.00. Ref. 3P78or 4 for £10.0. Our rel. 10P76.EX -EQUIPMENT SWITCHED MODE POWER SUPPLIES Various makesand specs but gem* +-5, +-I2V ideal bench supply. Only £8.00. Ourref 8P36.ACORN DATA RECORDER Made fa the Electron or BBC computer butsuitable for others. ridudes mains adapts, leads and book £12.00. Ref.12P15.PTFE COATED SILVER PLATED CABLE 19 strands of .2inm copper willcarry up to 30A and is virtually indestructible. Avalable in red or black.Regular price is over £120 per reel. Cur price only £20.00 for 100rn reel.Ref. 20P21 a 1 of each for £35.00. Ref 35P2. Makes absolutely superbspeaker cadetNEW RR SENSORS Infra red movement sensors will switch up to 500wmains, UK made, 12 month manufacturers warranty, 15-20m range with a0-10min timer, achustable wall bracket. Only £20.00. Ref 20P24.MITSUBISHI 3.5IN DISC DRIVES Brand new drives, 1/2 height doublesided, double density warranted. Our price £60.00. Ref. 60P5.10 MEMORY PUSHBUTTON TELEPHONES These are customer returnsand sold as seen. They are complete and may need slight attention. Price£6.00 Ref 6P16 a 2 for £10.00 Ref 10P77 BT apfrovedNON -MEMORY PUSHBUTTON TELEPHONES. Same condition asabove with redal £3.00. Our ref. 3P79. BT approved.DEHUMIDIFIERS Domestic mains powered dehumidifiers these arecustomer returns and sold as seen. Price £30.00. Our ref 30P9. Callersonly please. Also working dehumidifiers at £99.00 each.SPECTRUM PRINTER INTERFACE Add a centronics interface to yourSpectrum complete with printer cable for only £4.00. Our ref. 4P52.SPECTRUM SOUND BOX Add sound to your Spectrum with this device.Just plug in. Complete with speaker, volume control and nicely boxed. Asnip at only £4.00. Our ref. 4P53.BBC JOYSTICK INTERFACE Converts a BBC joystick port to an Ataritype port Price £2.00. Our ref. 2P261.TELEPHONE EXTENSION LEAD 5m phone extension lead with plug onone end, socket an the other. White. Price £3.00. Our ref. 3P70 or 10leads for only £19.05 Ref. 19P2.LCD DISPLAY 4.5in digits supplied with connection data £3.00. Ref.3P77 or 5 for £10. Ref. 10P78.CROSS OVER NETWORK 8 Ohm 3 way for tweeter midrange andwoofer nicely cased will connections marked. Only £2.00. Our ref. 2P255a 10 fa £15.00. Ref. 15P32.REVERSING LIGHT ALARM Fits to car reversing light and sounds whenreversing. Only £2.00. Our ref. 2P248.BASE STATION MICROPHONE Top quality uni-directional electretcondenser mic 600r impedance senrativIty 16-18KHz - 68db built in chimecomplete with mic stand bracket. £15.00. Ref. 15P28.MICROPHONE STAND Very heavy dammed mic stand, magnetic base4in high. £3.00 if ordered with above mic. Our ref. 3P80.SOLAR POWERED NICAD CHARGER 4 Nicad AA battery charger.Charges 4 batteries in 8 hours. Price £6.00. Our ref. 6P3.MAINS SOLDERING IRON Price £3.00. Our ref. 3P65.SOLDERING IRON STAND Price £3.00. Our ref. 3P66.RR SENSORS Suitable or alarm systems etc. Nicely boxed. Priced atonly £10.03. Our rel. I0P79.SHARP PLOTTER PRINTER New 4 odour printer originally Intended forSharp computers but may be adaptable fa other machines. Completewith pens, paper etc. Price £16.00. Our ref. 16P3.CAR IONIZER KIT Improve the air in your car, dears smoke and helpsprevent fatigue. Case req. Price £12.00. Our ref. 12P8.NEW FM BUG KIT New design with PCB embedded coil 9v operation.Priced at £5.00. Our ref. 5P158.NEW PANEL METERS 50UA movement with tree different scales thatare brought nto view with a lever. Price only £3.00. Ref. 3P81.STROBE LIGHTS Fit a standard edison screw fight fitting 240V 40/min.lash rate avatable in yellow, blue, green and red. Complete with socket.Price £10 each. Ret 10P80 (state colour required).ELECTRONIC SPEED CONTROL KIT Suitable for controlling ourpowerful 12v motors. Price £17.00. Ref. 17P3 (heatsink required).EXTENSION CABLE WITH A DIFFERENCE It is fiat on one side makingit easy to fix and look tidy. 4 core, suitable fa alarms, phones etc. Ourprice ally £5.00 fa 50m reel. Ref. 5P153.METAL PROJECT BOX Ideal for battery charger, power supply etc.Sprayed grey size 8in x 4in x 4.5n. Lamed lor ventilation. Price £3.00.Ref. 3P75.

PRACTICAL ELECTRONICS FEBRUARY1990 5 I

PE PCB SERVICEIT IS EASY TO BUILD MAY 88

RF SPEECH PROCESSOR 172 £6.26

PRACTICAL ELECTRONICS PROJECTS! JUNE 88AMSTRAD ROM EXPANSION 173 £10.80

Simplify your project assembly - use a ready-made printed circuit MAINS MODEM 174 £4.27

board. All are fully drilled and roller tinned. Just slot in the components JULY 88

as shown in the project texts, and solder them. PCBs are the VOCALS ELIMINATOR 175 £4.31

professional route to project perfection. AUG 88SPEAKING CLOCK 176 £16.75

MAIL ORDERING SEPT 88

Select the boards you want, and send your order to: PE PCB BBC MULTIPLEXER 177 £4.50

SERVICE, PRACTICAL ELECTRONICS, 193 UXBRIDGE ROAD, OCT 88

LONDON W12 9RA. METAL DETECTOR 178 £6.50DEC 88

Prices include VAT and postage and packing. Add £2 per board for PANNING MIXER 181 £7.80overseas airmail. Cheques should be made payable to Intra Press JAN 89(Payments by Access and Visa also accepted). Quote the project name RUDOLPH'S NOSE 182 £6.25and PCB Code Number, and print your name and address in Block ANGELS HALO 183A/B £9.40Capitals. Do not send any other correspondence with your order. CANDLE FLICKER 184 £6.25

MAR 89CAMERA SHUTTER TIMER 187 £9.95

Use your Access or Visa card and phone your order to 01-743-8888 APR 89clearly stating your name and address, card number and order details. PC MULTIPORT 188A/B £20.55

All orders receive priority attention. Many PCB's are held in stock, so MAY 89

they are dispatched within few days, but please still allow 28 days for KIRLIAN CAMERA 189A/C £10.50

delivery in case a PCB is temporarily out of stock. JUNE 89SOLAR HEATING CONTROLLER 197 £7.20

We can only supply the PCB's listed here! Please always check DELUXE METRONOME 198 £10.95the latest issue of PE before ordering. JULY 89

We can also supply the photocopies of the text at £1.50 for each PROJECTOR SYNCHRONISER 190A £9.50

project part inclusive of postage and packing ( overseas £2.00). AUG 89EASI-BUILD - VODALEK 191 £4.90

Please note that we do not supply components - they can ordered from HAND CLAPPER 192 £6.50our advertisers! SEP 89

TELEPHONE ORDERS

APRIL CodeVIGILANTE CAR ALARM - keeps cars alert 139 £5.90MAY 87BRIGHT FUZZ - Foot operated overdrive 145 £3.90JUNE 87AUDIO SIGNAL GENERATOR 146 £10.20JUL 87WORD GENERATOR - 16 bit binary words 147 £13.42SCOPE STORE oscilloscope add-on data storage148 £11.94SEP 87SPEECH PROCESSOR - clarifies speech 150 £5.86GCSE TIMER UNIT - versatile variable delay 151 £5.18FUNGEN - triple waveform signal generator 152/153 £9.69OCT 87TEACHER LOCKER - digital lock control 155 £7.50POWER SUPPLY - stabilised ±15V 156 £7.50GUITAR TO SYNTH - music interface 157A/B £9.95NOV 87DUAL POWER SUPPLY - GCSE 158 £6.20MIDI EXPANDER - Music Interface 159 £5.04DEC 87RS 232C TO MIDI 160 £6.43TEACHER RADIO - GCSE 161 £5.58JAN 88LEGO BUGGY DRIVER 163 £6. 42FEB 88TEACHER TALKBACK - GCSE 164 £6.36DC MOTOR SERVO 165 £7.53MAR 88APPLIANCE TIMER 166A/B £9.38TEACHER LIGHTSHOW - GCSE 167A/B £9.09LOGIC ANALYSER - Double -sided 168 £20.65APR 88LIGHT METAL EFFECTS 169 £7.10

EASI-BUILD - COMPRESSOR 193 £4.90FREQUENCY COUNTER -GENERATOR 194A/B £12.50OCT 89EASI-BUILD VOICE -OP -SWITCH 195 £4.90HOME SECURITY CONTROLLER 196A/C £19.50DEC 89VIDEO AGC STABILISER 199 £6.50ECHO STATION 200A/B £11.50MINI METRONOME 201 £4.90JAN 90BARGRAPH TACHOMETER 202 £5.90EEPROM PROGRAMMER (KEYBOARD VER) 203 £14.50FEB 90EEPROM PROGRAMMER 9SWITCH VERSION) 204 £4.90MODEM 205 £11.50MOCK STEREO 206 £4.90

PE PCB's are the professional routeto project perfection!


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pages: Rates 20p per word or £8.50 per single column cm (plus VAT). All classifiedadvertisements must be pre -paid. Send your copy with the remittance (payable to Intra Pressor payment by Visa or Access accepted) to: Practical Electronics, Intra House, 193 Uxbridge

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LONDON ELECTRONICSCOLLEGE

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Start training now forthe following courses.Telecomms Tech C&G 271

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We stock a large range of electroniccomponents, test eguipment,telephone

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PRACTICAL ELECTRONICS FEBRUARY199053

CLASSIFIEDSResistors 1/4 W 59 carbon (E12) Ip metal film 1% .......................3pResistor Pack 85 different E12 values + zero ohm link total content1000 resistors .....................£8.95

LEDs red/ green 3/5mm 6op each. Yellow IlpCable ties 75mm ......................Ip each £5.9511,00 £49.50 per 10,000Stepping motor 4 phase 12v 7.5' step 50 ohms 18.95SAA1027 stepping motor driver chip £3.95FM transmitter Kit good quality sound £7.94High quality photo resist toper clad epoxy glass boardsDimensions single sided double sided

3x4 inches £0.95 £1.074x8 inches £2.40 £2.68

6x12 inches £5.3712x12 inches £10.66

Special OffersComputer Grade Capacitors with screw terminals 38000uf 20v £2.508700uf 10v £1.95, 68000uf 15v f2.95, 10000uf 16v £1.507 segment Commom anode led display 12mm £0.45LM2931AT5.0 Low drop out 5V regulator T0220 package £0.85BS250 P channel MOSFET £0.45, BC559 transistor f3.95 per 10074LS05 hex inverter £10.00 per 103, used 8748 Micrecantroller £3.50Stereo LW/MW/FM Tuner pm -amp assembly complete withvolumeltone controls and tuning scale Brand new in makers carton

£5.95, faulty £2.50Circuit diagram description and setting up procedure for tunerassembly described above £0.50. 5 digit. 6v electromagnetic counter ...

£1.95£0.95£2.50£5.00

Hour counter (used) 7 digit mains 240V AC 50HzLCD display 16 digit 7 dots dot matrixQuerty keyboard 58 key good quality switches

95p for P&P per order ( VAT inc )wide range of CMOS TTL 74HC 74F Linear transistors kits

capacitors, resistors tools etc always in stockJPG Electronics 276 Chatsworth Road Chesterfield S40 2B11

Access orders (0246) 211202. Callers welcome

Carbon Film Resistors '4W E24 series 0.51R to10M0 - 1p100 oh per value - 75p 1000 off in even hundreds per value - £7Metal Film IAW 1060 to IMO 5% E12 series - 2p 1% E24 series - 3p'hWatt metal/carbon film E24 series 160 to 10M0 - 11hp

1 Watt metal/carbon film E12 series 4R7 to 1DMO - 5p

BC107/8/9 - 12p BC 547,P9 - 8p BC 182L 184L - 10p13E1'50/5152 - 20p 263055 - 50p TIP31A,32A - 25p TIP,41,42, - 40p

Tantalum head subminiature electrolytic (Mids/Volts)0-1,35, 0.2235, 047/35, 33/16, 4.7/16 - 14p 47/35 - 15p22/35, 47/25, 10/5 - ISp 4.7/35, 6/8/16 - 16p 10/16,22/6 - 20p72/16 - 30p 33/10- 30p 47'10 - 35p 100/6 -40p

Aluminium Electrolytics IMidsNolts)1/50, 2.2/50, 4.725, 47/50, 10.16, 10/25, 10/50 - 5p 22/16, 22/25 - 6p22/50, 47/16, 47.25, 47/50 - 6p 100/16, 100/25 - 7p 100/50 - 12p100/100 - 14p 220/16 - 8p 220/25, 220/50 - 10p 470/16, 470/25 - II p1000/25 - 18p 100035, 220/25 - 22p 4700/25 - 70p

Miniature Polyester Capacitors 250V Wkg. Vertical Mounting.01, .015, .022, .033..047 068 - Op 0-1 - 5p 0-15, 22 - 6p 0.47 - Bp

Mylar Capacitors 100V Wkg. Vertical Mounting E12 Series1000p to 8200p - 3p -01 to .068 - 4p 0.1 - 5p 0.15, 0.22 - 6p

Subminiature Ceramic Plate 100V Wkg. E12 Series Vertical Mounting2%, 1P8 to 47P - 3p 56P to 330P - 4p 10% 390P to 4700P - 4pCeramic plate disc E6 Series 50V 22P to .047 - 2p

Polystyrene Capacitors 63V Wkg. E12 Series Axial Mounting10P to 820P - 3p 1000P to 10,000 - 4p 12,0000 - 5p164148 - 2p 1N4002 -4p 165404 - 14p WO1 bridge - 25p0A91 - 6p A.A143 - 8p W005- 20p 164006 - 6pZener diodes E24 series 3V3 to 33V 400mW - Bp 1 wan - 12pL.E.D's Red, Green & Yellow 3mm & 5mm - 10p 8mm - 35p20mm fuse 0,1A to 5A quick blow - 5p Anti Surge - 8pHigh Speed drills 0.8mm, 1.0mm, 13mm, 1.5mm, 2mm - 30pExpo Reliant drilling machines 12Vd.c. with improved 3-jawchuck 6.50Nicads AA - 80p HP11 -f2 PP3 -E4.20 Universal Chargers -£6.50Glass reed switches single pole make contacts -8p Magnets -12p

VAT inclusive. Return postage 25p (free over E51 Lists free.

THE C.R. SUPPLY CO.,127 Chesterfield Road,

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* Series X Mixer Kits* up to 1.000 inputs* 60,100 mm faders, pots,panels and audio switches* 6-9 echo sends* versions for recording PA, 111111A.radio, disco* circuit diagrams* From £9.92

Send 40p for catalogue to:: K. Tek, P.O. Box 172a,Surbiton, Surrey KT8 61IN. Te1:01-399-3990

TURN YOUR SURPLUSICS transistors etc into cash, immediate

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ESTABLISHED 15 YEARSTe1:0945 584188 - Fax: 0945 588844

CAMBRIDGE COMPUTER SCIENCE LTD10MByte Winchesters, used, 3 months Wty,

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5.25' Disks, DSDD, 48tpi boxes of 10

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Gould PSU 0-30V @5A Limited quantity only at

Dual Data lead (BBC Micro to 2 Disk Drives)

Power lead (BBC Micro to Disk Drive)

Dual power lead (BBC Micro to 2 Disk Drives)

68000 CPUs (The first orders get 10MHz chips)

z80A CPU, CTC, P10 £1.20 each; DMA £2.00 £4.50 all 4

74LS TTL , pick and mix, buy 10 or more for £0.12 each

Types available: '00 '02 '04 '08 '11 '13 15' 20 21 26 27 '30.32 '38 '42 '75 '83 '96 '107 '109 '122 '132 '136 '139 '145 '151'153 '157 '158 '163 '164 '166 '191 '193 '298 '365 '670

EPROMS 27128 used £1.50 each; new 27128-25 £2.50 each

6264 8K Byte SRAM -15 £3.00 each -12 £3.80 each

65256 32K Byte rams £7.00 each

8K Byte NV ram chips £3.00 each £10.00 four

20 pin dil lox profile IC sockets £0.50 (ten) £ 4.00 (100)

40 pin dil low profile IC sockets £0.60 (ten) - £ 5.00 (100)

Keyboard, 100 keys on board LCD & micro Vt £8.00 each

Toroidal mains transformer 12V 4A & 0.4A, 12-0-12 @0.1A & 2A ,

9-0-9 @0.2A £4.00 each - £6.00 for 2. £8.00 for 3

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430496 or 0831 430552(Mail order only, no facilities foe callers)

£42.00 each

£34.00 each

£15.00 each

64.00 set

£ 10.00 each

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£ 4.00 each

12.00 each

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Tel: 0243 545111, Fax: 0243 542457Wide range of items available. Send for lists.

74HC series ICs74HCOO 19p 74HCO2 19p 74HCO3 19p74HC04 19p 74HC08 19p 74HC10 19p74HC11 26p 74HC14 39p 74HC20 19p74HC27 31p 74HC30 35p 74HC74 38p74HC86 29p 74HC107 36p 74HC138 41p

74HCt61 48p 74HC175 52p 74HC393 69p

74LS series ICs74LSOO 20p 74LSO2 20p 74LSO5 20p74LS30 20p 74LS32 20p 74LS51 20p74LS90 30p 74LS93 30p 74L5107 23p

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74F series ICs74F00 32p 74F02 32p 74F64 32p

74E174 77p 74F251 64p 74F353 64p74F373 91p 74F398 96p 74F399 96p

ECL series ICs10101 1.15 10102 1.15 10103 1.1510104 1.15 10124 3.68 10125 3.68

A wide range of components and TTL, CMOS andECL ICs are available. We also supply PC, XT andAT spares at LOW PRICES. Write or phone for ourFREE catalogue.

Resistors at bulk prices Your MixCarbon film, 0.25W, 51', 1p each or 45p for 50Metal film, 0.25W. 15. 3p each or £1.35 for 50

UC sockets, solder type (pin:price)14:10p, 16'11p. 20:14p, 24(0.6 or 0.3):16p.

I/C sockets with 0.1uF decoupler14:60p, 16.65p.

Prototyping boards (:'arse)IBM XT £45.00, IBM AT £52.00

ACCESS welcome by post or phone. Callers byappointment please. VAT INCLUDED. Please add50p for post and packing. Catalogue free. No min-imum order charge.

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Laboratory stockclearance equipment,

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NEW VHF MICROTRANSMITTER KIT,tuneable 80-115 MHz, 500 metre range,sensitive electret microphone, high quality PCB.SPECIAL OFFER complete kit ONLY £5,assembled and ready to use £8.95 post free.Access orders telephone 021 411 1821.Cheques/ P.O.'s to: Quaniek Electronics Ltd,(Dept P.E.), 45a Station Road, Northfield,Birmingham, B31 3TESurveillance devices, lasers, Tesla coils,scramblers, ultrasonic and many more,over 150 designs. Send SAE to:Plancentre, Old Wharf, Dynock Road,Ledbury HR8 2HSKITS MICROTRANSMITTER, VHF/FM,received on standard radio, 25x15mm, freemicrophone included - £4.50. Telephonetransmitter, amazingly uses no batteries -£5.49. Bleeper transmitter vhf - £8.59 incp&p. SAE list. Remittance to: A.C.E. (PE),99 Greenheath, Hednesford, Staffs. -Access orders 05438 71902 - same daydispatch.

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Digital Logic Gates and Flip -Flopsby Ian R. Sinclair, 200 pp, £ 8.95 An intro to the basic building blocks of alldigital circuits, for enthusiasts, students and

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Voice/Sound activated switches easyto follow diagrams and uses only £1.00.Components and P.C.B.s available fromHerrington, 63 Home Farm Road,Hanwell, London W7 1 NLFine hand made telescopes! Terrestrialand astronomical. Contact Ian Poyser, 15Dale Road, Rochester, Kent ME21 2PJTel: Medway 401220RM Nimbus Experimental Analogueport. Two analogue and two digitalchannels, using mouse socket. Circuit,layaout and listing £5.00 - PCB £3.50.From: Logical Answers, 24 Elmwood,Mersea, Colchester, Essex CO5 8RD

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S ome years ago I went to a specialexhibition at the Victoria and AlbertMuseum, London, called 'The Wireless

Show'. It was an historical survey of the externaldesign of radio sets, taking some account of theelectronic technology as well. Half -way along thechronological line from kitsch crystal -sets tosevere 'utility' boxes I was stopped in my tracksby a shock of recognition. There, complete withexplanatory label, sat an identical model to thelittle Bush table radio we were then using at home.

My surprise wasn't entirely the result ofdiscovering we owned a working museum -piece. Itwas the shock of seeing something familiar in acompletely new role. At home the old radio was adisregarded household object, inherited withoutenthusiasm from a previous generation. It did auseful job for us, from an ignoble position of beingwedged in a corner with domestic junk piled on topof it and generally treated rather roughly. But herein the V&A, it - or at least its clone - had suddenlybecome an object of appreciation and regard,something of aesthetic value and perhaps scholarlyinterest. It had acquired a place in history.

INDUSTRYobjectively true knowledge of ourselves this stilldoesn't tell us what to do with the result - in short,how to live. Only the realm of values, expressedthrough art, religion, ethics, philosophy, aestheticsand other such distillations of experience helps uswith the immediate question of what to do next.We can't avoid action.

Our knowledge seems to be dividedfundamentally between facts (the subject matter ofscience) and values (the subject matter of thehumanities). But some recent thinking may bebreaking down the dividing wall.

CONCEPTUAL MODELS

The approach is that perhaps we have been a bittoo arrogant in claiming that science has privilegedaccess to reality. After all, its method is to buildconceptual models (eg mathematical equations)which fit as closely as possible our empiricalobservations and measurements. All the time thesemodels are being improved. But the map is not theterritory. The model is not, and never can be, the

The Two CulturesRevisited

SCIENCE AND HUMANITIES

This little experience, of suddenly seeing atechnological object transformed into an objetd'art, set me thinking again about the old problemof the separation between the humanities and thesciences. C.P. Snow, the scientist turned novelist,identified it as "The two cultures" in a famouslecture and book a few decades ago.

He was right to use the word 'culture'.Educational specialisation in the past - when youhad to decide at an absurdly early age to becomeeither an 'arts' person or a 'sciences' person - hasfostered different ways of thinking, feeling andbelieving which amount to different ways of living.People have become polarised in outlook. Frommy own experience the two sides are often unableto communicate with each other on more than ahumdrum, superficial level.

It is a problem for two reasons. First,individuals who are culturally polarised in this wayare poorly educated, in terms of the developmentof the whole person. This means not only alimitation of their own personal enjoyment andfulfilment but also - the second reason - they bringa narrow, impoverished attitude to the (necessarilyspecialised) work they do in the world. All of usmust be the losers from this artificial restriction ofUnderstanding.

TECHNOLOGICAL BRIDGE

I see it as a spiritual deformity. Trying to thinkabout it therapeutically, I wondered initially iftechnology (of which electronics is a part) couldform a bridge between the arts and the sciences.

By Tom Ivall

The split between"arts" and

"sciences" createsa lack of true

understanding.After all many technologies are derived from both,if you consider the arts as embracing crafts.Modern chemistry, for example, didn't arrive fullyarmed as a science. It developed gradually out ofalchemy. And nowadays the artist is increasinglymaking use of technological tools. Musicianscompose with electronic synthesisers, painters andgraphics designers make pictures with computer'paint -boxes'.

But this 'technological bridge' is only atheoretical or superficial one. It doesn't go deepenough to be able to weld together twofundamentally different philosophies forunderstanding the world. At one extreme you havethe narrow scientific view which sees art merely asdecoration, not as a way of experiencing reality. Atthe other extreme you have the narrow artistic viewwhich sees science merely as instruments andmachines, not as a way of exploring reality.Something pretty powerful is needed to knockthese two heads together.

The scientific world view has become highlyinfluential partly because of technology but mainlybecause it strives for objectivity and uses contactwith the real as a criterion of truth. But when, aspart of scientific enquiry, we have obtained

actuality. Other creatures (say extra -terrestrialbeings) could have different mental models of thesame reality. Who is to say which picture in themind would be the truth?

Aristotle observed what he called 'naturaldownward motion'. After Newton and hisequation, we say in a superior tone "Of course,what Aristotle really meant was gravitation." Butdoes the quantitative description in Newton'sequation of universal gravitation get us any closerto reality? Apples still fall, regardless of how wedescribe the process.

Science, according to the American philosopherWillard Quine, is a way of coping with the world.As such it is like art, religion, politics, etc. -another way of adjusting ourselves to ourexperiences, using a mental framework of images,theories, intuitions, convictions, revelations.

On this basis Newton's theory and equation ofuniversal gravitation helped us to cope - with ourceaseless curiosity and desire to understand thenatural world - better than Aristotle's idea did,because it was a more satisfactory explanation tous as humans. Whether it was truer to reality wasan open question. Anyway, Einstein came alongand modified Newton.

The Quinean view is behaviouristic. It seems topresent the analytical search for objective truth as away of finding 'what works' in satisfying thehuman mind. I wonder if this approach to bridgingthe two cultures will ever have an influence on thenational curriculum.

UPDATE

In last month's column, the date of 1988 givenfor the sale of Inmos and Plessey should, of course,have been quoted as 1989!


Books

PRACTICAL ELECTRONICS

BOOK SERVICEHere is your Editor's choice of books he

thinks will be of interest toelectronics and

computer enthusiasts

BEGINNERS AND EARLY STARTERSMU Mini -Matrix Boar Projects.R.A.Penfold. 112 pages. £2.50.Order Code BP99Shows a selection of 20 useful and interesting circuitsthat can be built on a mini -matrix board of 24 holes by 10copper strips in size - an ideal book for earlyexperimenters.

HMV From Atoms to Amperes.F.A.Wilson. 160 pages. £2.95.Order Code BP254.For the absolute beginner, clearly explaining thefundamentals behind the whole subject of electricity andelectronics.

`jai Electronic Projects forBeginners.F.G.Rayer. 128 pages. £1.95.Order Code BP48Specially for the newcomer to electronics who is lookingfor a book containing a wide range of eas ly made projects.Some circuits need no soldering and many others showactual component and wiring layouts.

Electronics Build and LearnR.A.Penfold. 128 Pages. £5,95.Order Code PC 101Combining theory and practice, the book describes acircuit demonstrator unit that is used in subsequentchapters to introduce common electronic components andcircuit concepts, complete with practical experiments.

PRACTICAL

,CLECTRORICS

HANDBOOK

Practica E ectronic Bui ng BlocksR.A.Penfold. There are two books -Book 1 : 128 pages. £1.95.Order Code BP117Book 2 : 112 pages. £1.95.Order Code BP118Book I is about oscillators and gives circuits for a widerange, including sine, triangle, square, sawtooth andpulse waveforms and numerous others from voltagecontrolled to customised lc types.Book 2 looks at amplifiers, ranging from low level discreteand opamp types to ft power amps. A selection of mixers,alters and regulators is included.

30 Solderless Breadboard ProjectsR.A.Penfold. Two books each of 160pages. Book 1 : £2.25. Order CodeBP107. Book 2 : £2.25. Order CodeBP113.Each project is designed for building on a Veroblocbreadboard and is accompanied by a description, circuitand layout diagrams and relevant constructional notes.Many of the components are common to several projects.Book 1 covers linear devices, and Book 2 covers cmoslogic chips.

Beginners Guide to BuildingElectronic Projects R.A.Penfold. 112pages. £1.95. Order Code BP 227Shows the complete beginner how to tackle the practicalside of electronics and includes simple constructionalprojects.

Vpfzioavamoig:TzioLALtmei_Getting the Most from YourMultimeterR.A.Penfold. 112 pages. £2.95.Order Code BP239There's more to what you can do with a meter than meetsthe casual eye. The book covers the basics of what youcan do with analogue and digital meters and discussescomponent and circuit testing.

HMV Test Equipment ConstructionR.A. Penfold £2.95.Order Code BP248Describes in detail how to construct some simple andinexpensive, but extremely useful, pieces of testequipment.

A introdeetion toSatellite Television

OscilloscopesI.Hickman. £6.95.Order Code NT3Subtitled 'How to Use Them, llow They Work' the book isillustrated with diagrams and photographs and isessential reading for any one who wants to know aboutscopes, from first principles to practical applications.

How to Get Your Electronic ProjectsWorking.R.A.Penfold. 96 pages. £2.50.Order Code BP110.Essential reading for anyone who wants first-time successin project assembly. Covers tracing mechanical faults aswell as testing for failures of active and passivecomponents of most types.

"%V YDIRROVallit

*NEW* Satellite TV Insta ationGuide - 2nd edition John Breeds.£11.95. Order Code STV1Full of vital information for any competent diyer whowishes to install a satellite tv antenna and obtainoptimum reception quality.

An Introduction to SatelliteTelevisionF.A.Wilson. 112 pages. £5.95.Order Code BP195Informative answers to many of the questions aboutthis communications revolution. The information ispresented on two levels, one aimed at the completebeginner, the other at professional engineers andserious amateur enthusiasts

AUDIO AND MU IIntroducing Digital AudioI.Sinclair. 112 pages. £5.95.Order Code PC102A non -mathematical introduction to the new digitaltechnology, discussing the principles and methodsinvolved in devices such as cd, dat and sampling.

Electronic Music ProjectsR.A.Penfold. 112 pages. £2.50.Order Code BP7424 practical constructional projects covering fuzz, wah,sustain, reverb, phasing, tremolo etc. The text is split intofour sections covering guitar, general, sound generationand accessory projects.

More Advanced Electronic MusicProjectsR.A.Penfold. 96 pages. £2.95.Order Code BP174Complementing BP74 by covering more advanced andcomplex projects including flanging, chorus, ringmodulation, plus a selection of drum, cymbal and gongcircuits.

X -M' Computer Music ProjectsR.A.Penfold. 112 pages. £2.95.Order Code BP173Shows how home computers can produce electronic musicand covers sequencing, analogue and Midi interfacing,digital delay lines and sound generators.

Practical Midi HandbookR.A.Penfold. 160 pages. £5.95.Order Code PC103A practical how -to -do -it book for musicians andenthusiasts who want to exploit the capabilities of Midi.Covers keyboards, drums, sequencers, effects, mixers,guitars, and computer music software.

Midi ProjectsR.A.Penfold. 112 pages. £2.95.Order Code BP182Practical details of interfacing many popular homecomputers with Midi systems, and also covering Midiinterfacing to analogue and percussion synths.

l'iM\V Electronic SynthesiserConstruction.R.A.Penfold. 112 pages. £2.95.Order Code BP185.Even relative beginners should find the monophonicsynthesiser described here within their capabilities if thebook is thoroughly read. Individual aspects of the synthare dealt with separately and pcb designs are shown forthe main modules.


DIGITAL AND COMPUTINGMMTF A Concise Introduction to MS-DOS.N. Kantaris. 64 pages. £2.95.Order Code BP232A ready -reference guide for those who need a quick insightInto the essential command functions of this operatingsystem, but who don't have the time to learn it fully.

An Introduction to ComputerPeripheralsR.A. and J.W. Penfold. 80 pages.£2.50. Order Code BP170Covers such items as monitors, printers, disc drives,cassettes, modems, etc, explaining what they are and howto use them with your computer and with each other.

Microprocessing Systems andCircuitsF.A.Wilson. 256 pages. £2.95.Order Code BP77A comprehensive guide to the elements of microprocessingsystems, covering the fundamental principles behind thisimportant subject.

Introduction to 6800/6802Microprocessor SystemsR.J.Simpson and T.J.Terrell. 238pages. £10.95. Order Code NT9'rhe book covers systems hardware, programmingconcepts and practical experimental work that will assistin understanding the 6800/6802 microprocessor, withadditional information on the 680205E evaluationsystem.

An Introduction to 68000Assembly Language.R.A. and J.W.Penfold. 112 pages.£2.95. Order Code BP184Covers the fundamentals of writing programs that willvastly increase the speed of 68000 based machines suchas the Commodore Amiga, Atari ST range, AppleMackintosh, etc.

Gotang is,Most From YourPrinter

:RiatElectronic Science Projects.Owen Bishop. 144 pages. £2.95.Order Code BP104A bumper bundle of experimental projects ranging incomplexity and including a colour temperature meter,electronic clock, a solid state (led display) scope, an infra-red laser, a fascinating circuit for measuring the earth'selectrical field strength, and many more.

Electronic Security DevicesR.A.Penfold. 112 pages. £2.50. BP56Full of ideas for keeping your valuables safe. The circuitsinclude designs for light, infra -red, ultrasonic, gas, smoke,flood, door and baby sensors.

7M7 More Advanced ElectronicSecurity Projects. R.A.Penfold. 112pages. £2.95. Order Code BP190Follows on from where BP56 leaves off and describes anumber of more up-to-date and sophisticated projects,such as pyro-sensors, infra -red and doppler-shiftdetection, fibre -optic loops, and many others.

7T-',7 Electronic Projects for Carsand Boats.R.A.Penfold. 96 pages. £1.95.Order Code BP94

Getting t t e Most om Your PrinterJ.W.Penfold. 96 pages. £2.95.Order Code BP181How to use the features found on most dot-matrix printersfrom programs and popular wordprocessors, showingexamples of what must be typed to achieve a given effect.

Micro Interfacing CircuitsR.A.Penfold. Two books, each of 112pages.Book 1 : £2.25.Order Code BP130.Book 2 : £2.25. Order Code BP131Both books include practical circuits and usefulbackground information though peb layouts are notincluded. Book 1 mainly covers computer input-outputtechniques. Book 2 deals primarily with practicalapplication circuits.

RIMW An Introduction to 6502Machine Code.R.A. and R.W. Penfold. 112 pages.£2.50. Order Code BP147Covers the main principles of machine code programmingon 6502 -based machines such as the Vic -20, Oric-1/Atmos, Electron, BBC and Commodore 64. It assumesno previous knowledge of microprocessors or machinecode and gives illustrative programming examples.

JrAV A Z-80 Workshop Manual.E.A.Parr. 192 pages. £3.50.Order Code BP 112A book for those who already know Basic but wish toexplore machine code and assembly languageprogramming on Z80 based computers.

Practical Digital ElectronicsHandbookM.Tooley. 208 pages. £6.95.Order Code PC 104Nine constructional projects introduce digital circuits,logic gates, timers, microprocessors, memory and interfacecircuits - an essential book for anyone interested in digitaldevices.

15 fairly simple projects t at can with a carand/or boat. Stripboard constructional details areincluded, as are explanations of the circuit theory.

Power Supply ProjectsR.A.Penfold. 96 pages. £2.50.Order Code BP76A selection of power supply designs, including simpleunstabilised, fixed voltage regulated and variable voltagestabilised, ni-cad charger, voltage step-up, and inverter.

More Advanced Power SupplyProjectsR.A.Penfold. 96 pages. £2.95.Order Code BP192Covers more advanced topics than BP76 and includesprecision supplies, switch mode and computer controlledsupplies, plus a selection of miscellaneous circuits.

in7/ Popular Electronic Circuits.R.A.Penfold. 160 pages. £2.95.Order Code BP80Containing a wide range of circuit designs for experiencedconstructors who are capable of producing workingprojects direct from a circuit diagram without specificconstructional details.

Books11 ' '17 :11

Digital IC Equivalents and PinConnectionsA.Michaels. 320 pages. £5.95.Order Code BP140Linear IC Equivalents and PinConnectionsA.Michaels. 256 pages. £5.95.Order Code BP141Between them these two books show equivalents and pinconnections of a popular user -orientated selection ofEuropean, American and Japanese ics. They alsoinclude details of functions, manufacturer and countryof origin. The Digital ICs book also quotes details ofpackaging and families.

OpampsB.Dance. £6.50.Order CodeNT2Subtitled 'Their Principles and Applications' thisinteresting book is written in a simple non -mathematicalstyle and provides a source of practical circuits that useboth commonplace and more sophisticated opamps.

Electronic Hobbyists HandbookR.A.Penfold. 96 pages. £4.95. OrderCode BP233Provides a source of useful information that the amateurenthusiast is likely to need for day-to-day pursuance ofhobby electronics.

Practical Electronics HandbookI. Sinclair. £7.95.Order Code NT1A useful and carefully selected collection of standardcircuits, rules of thumb and design data for enthusiasts,students and engineers involved in radio, computing andgeneral electronics

Newnes Electronics Pocket BookI.E.Parr. £6.95.Order Code NTIOPresents all aspects of modem electronics in a readableand largely non -mathematical style, and is a good sourceof valuable information for enthusiasts and professionalengineers alike.

kMW Key Techniques for CircuitDesignG.C. Loveday. £6.95.Order Code BM 101Tackles the problems of designing circuits from scratch,introducing the concept of target specifications, thedesign sequence, device selection, rules of thumb, anduseful equivalent circuits.

HOW TO ORDERState your order code and your name

and address clearly. Enclose a

cheque, PO or international money

order (add 50p postage per book -

£1.00 for overseas surface mail), and

send to:

PE Book ServiceIntra House193 Uxbridge RoadLondon W12 9RABooks are normally deliveredwithin 10 days but please allow 28days for delivery.

PRACTICAL ELECTRONICS FEBRIJARY1990

PE COMPETITION RESULTS

THE 25 SILVERED WINNERSHAVE BEEN DRAWN IN OUR

25TH ANNIVERSARYCOMPETITION!

136 7@ MOM n,[616k1 VTAKIt?

MR J.COUNCELL OFCROWBOROUGH,SUSSEX, DOES!

He's won the fabulous AcornArchimedes A3000 computer! Over

£1000's worth of First Prize -geiuDaffledgP

WELL DRAWN TOO ARE:D. Crome of West Kingsdown, Kent;M.J. Ward of Canvey Island, Essex;

C.J. Winscom of Pinner, Middx;

who have each won 25 years' subscription to PE- worth at least £375 at the time of winning!

Vlo euvoc; fflfliggo oe@GOEY

THEN THE SCALES REGISTEREDHIGHLY FOR:

Michael Hammond of Newcastle-under-Lyne, Staffs;John McCloskey of Carrickfergus, Co Antrim; I. Pepperof Stevenage, Herts; David Wi lson of Sowerby Bridge,

W. Yorks; P.H. Tanner of Glasgow; K.J.Corbett of Didcot,Oxon; David Thompson of Callender, Perth; D.R. Good

all of Horsham, Sussex; Roger Merrick of Erdington,Birmingham; Nicholas Brown of Winscombe, Avon;

all of whom have each won a superb CirkitTM5365 digital multimeter, worth £40 apiece

but invaluable for any workshop!

I flit 1 6h.fiA:61 -1

7 7 I 7 I 7 T iTi1

I 1.1 \ A

a.

AND WELL SUBMITTED WERE:

Peter Haverty of Craughwell, Co. Galway;J.F.Long of Wimbledon, London;

J.G. Seal of North Allerton, N. Yorks;Robert Williamson of Crumlin, Co. Antrim;

Roger Barnes of Hendon, London;Douglas Hunt of Kilwinning (it didn't this week!), Ayrshire;

Desmond Kavanagh of Grenville, Dublin;P.J. Marsden of Maidstone, Kent;

Derek Gledding of Hathersage, Sheffield;Roy Gillies of Dagenham, Essex;

W. Borrmann of Westpoint, Co. Mayo,

these eleven lucky entrants were drawn to beawarded a full years subscOption to Practical

Electronics - amazing return for just the cost ofa postage stamp!

* * *NEXT MONTH WE'LL LOOK

AT THE ANSWERS!


ELMASET INSTRUMENT CASE300 x 133 x 217mm deep £10 ea (£2.20)

REGULATORSLM317T PLASTIC T0220 variableLM317 METAL7812 METAL 12V 1A7805/12/15/24V plastic7905/12/15/24 plasticCA3085 T099 variable regLM338 5A VARIABLE

ClC2.20

35p 100+20p 1000+ 15p35p 100+20p 1000+ 15p

2/£1CS

COMPUTER ICS80C31 MICRO £2P8749H MICRO £5BBC MICRO PARTSVIDEO ULA 201647 £10ea 10+ £86845 CRT £56522 PIA £3DM88LS120 £4.50AY31015D UART E2.508086 processor equipment £2USED 41256-15 £3.00USED 4164-15 ex -equipment £19 x41256.15 S1MM MODULE NEW £258 x 4164 SIP MODULE NEW £8HD 146818 CLOCK IC £22864 EEPROM £327128A 250nS EPROM USED £2 NEW £2.30FLOPPY DISC CONTROLLER CHIPS 1771 £10 eaFLOPPY DISC CONTROLLER CHIPS 1772 £16 ea68000-8 PROCESSOR NEW £5HD63484-8 £5ALL USED EPROMS ERASED AND BLANK CHECKEDCAN BE PROGRAMMED IF DESIRED.2716-45 USED £2 100/£12732-45 USED £2100/£12764-30 USED £2 100/£1.6027C256-30 USED £2270512 USED £3.501702 EPROM EX EQPT £52114 EX EOPR 60p 4116 EX EQPT 70p6264-15 8k static ram £3.504416 RAM £3.50USED 4416-15 RAM £2USED 41464-15 £7

CRYSTAL OSCILLATOR1.8342 MHz C1 each

CRYSTALS2.77 MHz/4.000 Wlz/4.9152MHz 20MHz 49.504MHz, 8M, 16.588M

TRANSISTORS8C107, BCY70 PREFORMED LEADS

full spec £1 £4/100 f30/1000BC557, BC548B 30/£1 £3.50100

POWER TRANSISTORSPOWER FET IRF9531 8A 60V 2/C12SC1520 sim BF259 3/C1 100/£22TIP141/2 £1 ea TIP112/125/42B 2/£1TIP35B TIP35C £1 .50SE9301 100V 10A DARL. SIM TIP121 2/£12N3055 EX EQPT TESTED 4/£1PLASTIC 3055 OR 2955 equiv 50p 100/C352N3773 NPN 25A 160V £1.80 10/C16

QUARTZ HALOGEN LAMPSA1/216 24V 150 WATTSH1 12V 50W (CAR SPOT)

£2.25£1.50

14 WAY AMP ZIF SOCKET 2/£1TEXTOOL single in line 32 way. Can be ganged (couplingsupplied) for use with any dual in line devices.28 WAY TEXTOOL ZIF SOCKET EX NEW EQUIPMENT £2

CAPACITORS COMPUTER GRADE3300uF 350V SIC SAFCO FELSIC 037 f6(£1.50)2200uF 160V SIC SAFCO FELSIC C038 C4(C1 .20)

TURNS COUNTING DIALS all for 0.25" shaft10 turn dial 21 mm dia. fits 3mm spindle £210 turn digital dial (3 digits) for 3mm or 6mm shaft £3.5010 turn clock face dial for 6mm spindle £4

MISCELLANEOUSSLOPING FRONT PLASTIC CASE 225x215 x 76mm76 mm WITH ALI FRONT PANEL 200 x 130mm ... C4.00(C1)HUMIDITY SWITCH ADJUSTABLE £2WIRE ENDED FUSES 0.25A 30/£1NEW ULTRASONIC TRANSDUCERS 40kHz C2/pair12 CORE CABLE 7/0.2mm OVERALL SCREEN -C1/3metresPOWERFUL SMALL CYLINDRICAL MAGNETS 3/C1OP AMP LM1OCLN £2.90BNC 50 OHM SCREENED CHASSIS SOCKET 3/C1BNC TO CROC CLIPS LEAD 1 metre £1MOULDED INDUCTOR 47001

size of a 1 watt film resistor 5/£1

TO -220 HEAT SINK aim RS 403-162 10/C2.50SMALL MICROWAVE DIODES AEI DC1028A 2/£1D.I.L. SWITCHES 10 WAY £1 8 WAY 80p 4/5/6 WAY 50p180 volt 1 watt ZENERS also 12v & 75v 20/£1PLASTIC EQUIPMENT CASE 9 x 6 x 1.25 in. WITH FRONT ANDREAR PANELS CONTAINING PCB WITH EPROM 2764-30 ANDICS 7417 LS30 LS32 LS74 LS367 LM311 7805 REG, 9 WAY DPLUG. PUSH BUTTON SWITCH, DIN SOCKET 01.90041 OLM 60v 1/zA 5ohm TO -92 mosfet 4/t1 100/C20MIN GLASS NEONS 10/C1RELAY 5v 2 pole changeover looks like RS 355-741 markedSTC 47WB05T C1 eaMINIATURE CO -AX FREE PLUG RS 456-071 2/£1MINIATURE CO -AX FREE SKT. RS 456-273 2/£1.50DIL REED RELAY 2 POLE n/o CONTACTS £1PCB WITH 2N2646 UNIJUNCTION with 12v 4 POLE RELAY C1400m 0.5w thick film resistors (yes four hundred megohms) 4/E1STRAIN GAUGES 40 ohm Foil type polyester backed balco gridalloy £1.50 ea 10+ C1ELECTRET MICROPHONE INSERT 00.90Linear Hall effect IC Micro Switch no 613 SS4 sim RS 304-267

£2.50100+ £1.50HALL EFFECT IC UGS3040 + MAGNET E1OSCILLOSCOPE PROBE SWITCHED X1 X10 £10CHEAP PHONO PLUGS 100/C21000/£181 pole 12 way rotary switch 4/£1AUDIO ICS LM380 LM386 C1 ea555 TIMER 5/£1 741 OP AMP 5/£1ZN414 AM RADIO CHIP 80pCOAX PLUGS nice ones 4/E1COAX BACK TO BACK JOINERS 3/C14 x 4 MEMBRANE KEYBOARD £1.5015.000uF 40V 02.50 (01.25)INDUCTOR 20uH 1.5A 5/C1NEW BT PLUG + LEAD C1.501.25" PANEL FUSEHOLDERS 3/C1CHROMED STEEL HINGES 14.5 x 1" OPEN C1 each12v 1.2w small wire ended lamps fit AUDI VW TR7 SAABVOLVO 10/C112V MES LAMPS 10/C1STEREO CASSETTE HEAD C2MONO CASS.HEAD £1 ERASE HEAD 50pTHERMAL CUT OUTS 50 77 85 120°C C1 eaTHERMAL FUSE 121°C 240V 15A 5/£1TRANSISTOR MOUNTING PADS TO-5/TO-18 £3/1000TO -3 TRANSISTOR COVERS 10/C1STICK ON CABINET FEET 30/£1PCB PINS FIT 0.1" VERO 200/£1TO -220 micas + bushes 10/50p 100/£2TO -3 micas + bushes 15/£1PTFE min screened cable 10m/£1Large heat shrink sleeving pack £2CERAMIC FILTERS 6M/9M/10.7M SOp 100/£20MAINS LEAD WITH MOULDED 13A PLUG AND IEC SOCKET

01.50IEC chassis plug rfi filter 10A C3Potentiomenters short spindles values 2k5 10k 25k lm2M5 lin 5/C1500k tin 500k log 4/C140Khz ULTRASONIC TRANSDUCERS EX-EQPT NO DATA

£1/prPLESSEY INVERTER TRANSFORMER SO CYCLES11.5-0-11.5V to 240v 200VA £6 (C3)

DIODES AND RECTIFIERS1N4148 100/£1.501N4004/SD4 1 A 300V 100/C31N5401 3A 100V 10/£1BA158 1 A 400V fast recovery 100/C3BA159 1A 1000V fast recovery 100/£4120V 35A STUD 65pBY127 1200V 1.2A 10/£1BY254 800V 3A 8/£1BY255 1300V 3A 6/C16A 100V SIMILAR MR751 4/C11 A 800V BRIDGE RECTIFIER 4/£14A 100V BRIDGE 3/£16A 100V BRIDGE 2/C18A 200V BRIDGE 2/£1.3510A 200V BRIDGE £1.5025A 200V BRIDGE £2 10/£1825A 400V BRIDGE £2.50 10/£22

SCRSPULSE TRANSFORMERS 1:1+1 £1.252P4M EQUIV C106D 3/C1MCR72-6 10A 600V SCR C135A 600V STUD SCR C2TICV106D 800mA 400V SCR 3/C1 100/C15MEU21 PROG. UNIJUNCTION 3/C1

TRIACS DIACS 4/C1BT137-600 8A TO -220 2/C18T138-600 12A TO -220 70pNEC TRIAC ACO8F 8A 600V T0220 5/£2 100/£30TXAL225 8A 400V 5mA GATE 2/£1 100/C35TRAL2230D 30A 400V ISOLATED STUD E4 each

KEYTRONICSTEL. 0279.505543FAX. 0279.757656

P 0 BOX 634BISHOPS STORTFORD

HERTFORDSHIRE CM23 2RX

CONNECTORSD25 IDC SOCKET FUJITSU £234 way card edge IDC CONNECTOR (disk drive type)

£1.25CENTRONICS 36 WAY IDC PLUG £2.50CENTRONICS 36 WAY IDC SKT £4.00BBC TO CENTRONICS PRINTER LEAD 1.5M £3.50CENTRONICS 36 WAY PLUG SOLDER TYPE £4USED CENTRONICS 36W PLUG+SKT £3

USED D CONNECTORS price per pair0960p, D15 01.50, D25 E2, D37 E2, 050C3.50 covers 50p ea.

WIRE WOUND RESISTORSW21 or sim 2.5W 10 of one value C1

R10 OR15 OR22 2R0 4R7 5R0 5R6 8R2 10R 12R 15R 18R 20R22R 27R 33R 47R 56R 62R 91R 1208 180R 390R 430R 470R680R 820R 910R 1K15 1K2 1K5 1K8 2K4 2K7 3K3 3K0 5K0 R05(50 milli -ohm) 1% 3w 4 FOR C1W22 or sim 6W 7 OF ONE VALUE £1R47 R62 1R0 1R5 1R8 3R3 6R8 9R1 12R 20R 24R 27R 33R 51R56R 62R 68R 100R 120R 180R 220R 390R 560R 620R 910F11K01K2 1K5 1K8 2K2 2K7 3K3 3K9 4K7 8K2 10k 15K 16K 20KW23 or sim 9W 6 of one value C1

R22 R47 1R0 1R1 56R 62R 100R 120R 180R 220R 300R 390R680R 1K0 1K5 5K1 10KW24 or sim 12W 4 OF ONE VALUE C1R50 2R0 9R1 18R 22R 27R 56R 68R 75R 82R 100R 150R 180R2008 220R 270R 400R 620R 1K0 6K8 8K2 10K 15K

PHOTO DEVICESSLOTTED OPTO-SWITCH OPCOA OPB815 £1.302N5777 50pTIL81 PHOTO TRANSISTOR £1TIL38 INFRA RED LED 5/C14N25, OP12252 OPTO ISOLATOR 50pPHOTO DIODE 50p 6/C2MEL12 (PHOTO DARLINGTON BASE n/c) 50p

LEDs RED 3 or 5mm 12/£1 100/C6LEDs GREEN OR YELLOW 10/C1 100/C6.50LEDS ASSORTED RD/GN/YW + INFRA/RED 200/£5

100/£40FLASHING RED OR GREEN LED 5mm SOp

STC NTC BEAD THERMISTORSG22 220R, G13 1K, G23 2K, G24 20K, G54 50K, G25 200K,G16 1M, RES o 20°C DIRECTLY HEATED TYPE C1 eaFS22BW NTC BEAD INSIDE END OF 1" GLASS PROBERES (e 20°C 200R £1 eaA13 DIRECTLY HEATED BEAD THERMISTOR 1kres. Ideal for audio Wien £2 ea

CERMET MULTI TURN PRESETS 3/4"OR 20R 100R 200R 250R 500R 2K 2K2 2K5 5K 10K 47K

50K 100K 200K 500K 2M2 50p each

IC SOCKETS6 pin 15/C1 8 pin 12/C1 14/16 pin 10/£1 18/20 pin 7/£1,22/24/28 pin 4/C1 40 pin 30p

SOLID STATE RELAYS40A 250V AC SOLID STATE RELAYS £18

POLYESTER/POLYCARB CAPS100n 63v 5mm 20/£1 100/C31000/£251n/3n3/5n6/8n2/10n 1% 63v 10mm 100/e61On/15n/22n/33n/47n/68n 10mm rad 100/C3.50100n 250v radial 10mm 100/C3100n 600v sprague axial 10/£1 100/£6 (£1)2u2 160v rad 22mm 100/C1010n/33n/47n 250v ac x rated 15mm 10/C1470n 250v ac x rated rad 4/C11U 600V MIXED DIELECTRIC SOp ea.

RF BITSMINIATURE CO -AX 5011 URM95 100m/£12

Ex -equipment, with BNC tailsTRIMMER CAPS ALL 4/50pSMALL 5pf 2 pin mounting 5mm centresSMALL MULLARD 2 to 22pF 4/50pSMALL MULLARD 5 to 50pF 4/50plarger type grey 2 to 25pF black 15 to 90p1TRANSISTORS 2N4427 60pFEED THRU CERAMIC CAPS 1000pF 10/C1

MINIATURE RELAYS Suitable for RF5 volt coil 1 pole changeover C15 volt coil 2 pole changeover C112 volt coil 1 pole changeover £1

MONOLITHIC CERAMIC CAPICITORS10n 50v 2.5mm 100/C4.50100n 50v 2.5mm or 5mm 100/£6100n ax short leads 100/C3100n ax long leads 100/C6

100n 50v dil package 0.3" rad C10/100

STEPPER MOTORS4 PHASE 2 9V WINDINGS £3.50

MAIL ORDER ONLYMIN CASH ORDER £3.00 OFFICIAL ORDERS WELCOME

UNIVERSITIES COLLEGES SCHOOLS GOVT. DEPARTMENTSMIN. ACCOUNT ORDER £10.00

P&P AS SHOWN IN BRACKETS (HEAVY) ITEMS65p OTHERWISE (LIGHT) ITEMS

ADD 15% VAT TO TOTALELECTRONIC COMPONENTS

BOUGHT FOR CASH


GLOBAL ELECTRONICS1 WINKLEY STREET

LONDON E2 6PT

The name for your components,

ONLY QUALITY BRANDED COMPONENTS

LOW PRICES - NO MINIMUM ORDER

LARGE QUANTITY PRICING

HAVING PROBLEMS FINDING ACOMPONENT? - CONTACT US!

PHONE FOR OUR FREE. CATALOGUE

10% OFF CATALOGUE PRICES WITHTHIS ADVERT!!

OFFER VALID UNTIL FEBRUARY 1990

NEW PRODUCTS BEING ADDEDCONSTANTLY

PHONE: 01-739 8199

INDEX TO ADVERTISERSA & G Electronics 53

A.D.M. Electronics Supplies 54

Astronomy Now 47

B.K. Electronics IBC

Blackmore Electronics 54

Boscad 37

Bull J. 51

Cambridge Computer

Science Ltd 54

Chartwell Braft 31

Classified Ads 53-56

Coles Harding 54

Component Solutions 48

Cooke International 54

Cricklewood Electronics 30

C.R. Supply Co. 54

Deansgate 53

E lect royal ue 44

Eskan. 55

Fraser Electronics 53

Global Electronic Services 62

Greenbank Electronics 62

Hanney, L.F. 53

J.P.G. Electronics 54

Keytronics. 61

K-Tek 54

London Electronics College 53

Maplin Electronics 41, OBC

Mauriton Electronics 55

Number One Systems 26

Omni 53

PC Publishing 55

Philpot J. 54

Phonosonics 44

PM Components 41

Program Now 34

Proto Design 55

Radio and Telecommunications

Correspondence School 53

R.J. E. Components 55

Specialist Semiconductors 20

Spiers Electronics 37

Stewart of Reading 62

Suma Designs 44

Tandy IFC

Technical Info Services 55

Tech nom at ic. 10,11

T.K. Electronics 48

PLEASE MENTION PRACTICAL ELECTRONICSWHEN REPLYING TO ADVERTS

iT i .A

USED TELESCOPES IN THE COUNTRYTEKTRONIX OSCILLOSCOPE 2235 Duel True 100MHZ Dr ey Sweep.3750

PHLIPS OSCILLOSCOPE PM3217 Duel Trace 50IAHZ Deley9roop....1500

GOULD 1421 DO Some Dud Tree amorz....... _ ...................1750

THMOAR PL1200M0030V 2A Twice. Ord Made 019411 Motors...1200

FEEDBACX FSG 606 Steno Frettion Goner" C1011i2 DMZ (PIP 67).....1203

SOLARTRON 7045 likernstor 44601 LED 33 lupe Adoiltlenuel Fr 67)..1125

LOGIC PROBE Nye IMAM/Once (RIP 33).._...._._._................._..118

FARAELL Oeolletor LFM3 10H110MHZ SineSquem...............................1200

RACAL 0915 Fn q Counter 10HZ-520MHZ (Clyde EMn).......................1150

RACAL 1003 Senn Uridersal Cotner Toners..fiern.. 1150

MAROSNI ATTENUATORSTF2162DC-11/{2 COO elm 011103 (PIP 173.135

HATFIELD ATENUATORS 50 ohm DC250MHZ 01000640 ohm DC1MHZ

(Nee Pats 01351 (P&P 64) °SOILS°

H. P. 174A Duel Trace 100MHZ Cel ey Sweep. Tny. .................. .1500TELEOUIPMENT 013 Our True MHZ Deley Setep.............. ./300GOAD 053000A Duel Trace AOMHZ DM Sweep TV TN. /300TELECIU IRA ENT 0755 Duel Treat 50MHZ DO/ %eh /775COSSOR 0.3U150 Our Trees 35M HZ Dolly Sem ..../180GOULD OS1100 Dud Trace 30MHZ TV Tng .6210S.E. LABS SM111 Duel Tram MHZ.. ....1130

1175rpPH C1S255 Cmol True 15MHZ fri6. .6180

PH LIPS PM 3233 DA Beam 10IAHZ TV Trig_ 1100GOULD 0S250B Dal Trace 19A HZ TV Thy .6160MIS IS JIM A SPIKE 1WIT *TIMM NALAILE

TEKTROMX nib. DUAL TRACE 601/112. SWEEP DRAY MTH

WWI, PROBES, FRONT COVER, POUCH. ONLY BOO.AVO MULTIMETERS (P&P 10 all

Avo 8s; tis end kindly Vermont WM Brines A Leeds................from 650

TEST LEADS tor Awe Red I Back with 2 croo Ceps 6 2 mode

(PAP E3)_.15

Blade EVER READY Cue for An, Un-used_APIP 64)............._..115-

MARCOS TF2015 MAFIA 10-5201011 Sip Gen kith 1F2171 ..... 1450MARCONI 1F2015 NONA %inchroriser TF2171................................1300MARCONI 1F2016 MGM 100.13-1201AHZ Sg GeneOTF2173 ..... ........../400

MARCONI TF2016 mined Synchronism' TF2303DYMAR 1525 AMR( 0.1 . 184 MHZ Sig Gin

H. P. 6208 Signal Generr or 7.11 GHL.. /400H.P. 6168H. P. 616B Spied Generator 1.114.2GHZ 1400

FE RROG RAPH RT52 Records COW Sol.. 1270

WOELKE Wow I Flutter Meter M E1011.. /100t PitnP R MINA Two CHANNEL M &Winder

Sp :ifFEED3ACXFG61Ge 0.01413hZ indOtt...................1150MARCONI Automatic Dstorfion Mar TF2337A 400HZor Measures dorm lo 0.01%. /100LEVELL OSCILLATORS TG/52/1C200 kenos from. .1135

MARCONI TF2700 UNNE ASIR LCR BROGE Story from .............1150

AVOTRANSSTOR ANALYSER MD. CT442 mimeo shit tVeh Brants&

opening Inelmchono ONLY 325 (PAP 67)

MARCONIAF Poser IA oter 7803A 20HZ.35KHZ 20mW 10W. 210

M snot ..ONLY 325 (PIP 67)

MARCONI AF Power Meter 71152471. CO3001AHZ 0S. 25W.

NEW EQUIPMENTHAMEG OSCILLOSCOPE H311035 Tools Trice 100**Q, DeleyTmehase.....6726

HANES OSOLLOSCOPE HM ASCU( TRACE tOMHZ Delay Sweep.........1575

HAMEG MODULAR SYSTEM *MEG OSCILLOSCOFERA 203.6 Mame WIZ Coroporraostor..1314HAMEG OSCILLOSCOPE HM205.2 Duel Tram 20M HZ DIU Storepo..../527

Al other models mailable A all osciloccopes sipped wi It 2 probesHA8001 M in Frame with HM 8032 Sine Wen Wenger 20HZ 20M HZ

HM8030.2 Fimition Generator 0.1 1MHZ Sine/Sciananyile and

HIA8011.2 Id lAultimeler 4.5 Cog( ..£375 BLACK STAR EMINENT (RIP al Mee 15)

APOLLO 10 1001411Z Cotner Timer Riem/PenodiMine Memel mc..._...ine

APOLLO 100 1=1.2 (AS Wm wAh more functions). _1215

METEOR 100 FREOUENCY COUNTER 100MHZ.

METEOR 600 FREQUENCY COUNTER 600MHL _1126

METEOR 1000 FREOUENCY COtXTER 1GHZ..... ..... .... ..... .... /170JUPITER 500 FUNCTION GENERATOR 0.1H1300101Z,SineSe7rri_.....1110

ORION CCLOUR EAR GENERATOR PeIGNAIdeo. .1200

Al other Bleck Sr Equipm ea meter..

SACELTA MC321 COLCUR BAR GENERATOR RF Bends 1731476 Video,Sound Carrier, UNA. d (PIPIT) /125

COLOURBAAGEN RolmShe, Pooholsoblo BO Camille with Miry Choir Adapt Untast(PIP 1

Libre Dolour Br Womb; KG1 8 to* Prisms (PIP 64) oat/..... £40ABGEAR CROSSHATCH GENERATOR Type CM6038.OB CrosehrScholr* Rester. Mira or BATTERY _Unused 11 t Used 612 (PAP )

TRIO RF SGNAL GENERATOR Type 5G412 100KHz.3C4AHLUnused(PAP 67). . .ONLY £50

ADVANCE SG628 AM 1500012120MHz .145 (PIP £7)MICAOCAMEX CUB MONFTOR1( CCLOUP .................................100

HUNG CAANG OHM 7030 31t2 481, Had hold 28 noon induangtompAczc 0.1% Aec. (ptp E4) 13950Ad above OHM 6010.0.25% Act. .633.50

C'*9 C"nhY 6'" .1 3TELEPHONES YES! REAL DIAL TYPE TELEPHONES

that don't slide arcund lie desk. Type No. 746 supplied with

standard BT plug (Used)....... Ceily ES each . QualtydiSCOUnt cuCALOSCOPE PROBES 9ottehoblo ol; ol 0 (P&P En .111

Used equipment - with 30days guaranMe. Manuals suppled, if possible. This is a VERY SMALL SAMPLE OF STOCK. SAE or

Telephone bt Lists. Please check mailability before ordering. CARRIAGE al units £16. VAT to be added b total of Goods &Carriage.

STEWART OF READING110 WYKEHAM ROAD, READING RG61PL, BERKS RG61PL

Tel: 0734 68041 Fax:0734 351696Callers welcome 9 am - 5.30 pm (until 8 pm Thursday)

Interak 1SINGLE BOARD COMPUTER

"SBC-1"°

-4°o

A computer doesn't have o ook like you'd expect a computer to look.It doesn't have to have a keyboard and a screen and floppy disks andso on,

The SBC-1 has the bare minimum of chips a Z80 computer can haveand still be a computer: A 4 MHz Z80 -CPU chip, an EPROM chip (upto 32K), a static RAM chip (up to 32K) and a pair of 8255A I/O (inputoutput) chips giving 48 individual lines to waggle up and down. Thereare one or two additional "glue" chips included, but these are simple"74LS" or "HC" parts.

A star feature is that no special or custom chips Cie PALs, ULAs, ASICsetc) are used - and thus there are no secrets. The Z80A is the fastestand best established of all the 8 -bit microprocessors - possibly thecheapest too!

Although no serial interface is included, it is easy for a 280A to waggleone bit up or down at the appropriate rate - the cost is a few penceworth of code in the program: why buy hardware when software will do?

Applications already identified include: Magnetic Card reader, miniprinter interface, printer buffer, push button keypad, LCD alphanumericpanel interface, 40 -zone security interface for auto sending of securityalarms, code converter (eg IBM PC keyboard codes to regular ASCII),real time clock (with plug in module), automatic horticultural irrigationcontroller.

By disabling the on -board Z80A-CPU this card will plug into our Interak1 CP/M Plus disk -based development system, so if you don't fancyhand -assembling Z80 machine code you don't have to!

The idea is (if you are a manufacturer) you buy just one developmentsystem and then turn out the cheap SBC-1 systems by the hundred. Ifyou are really lazy we can write the program for you and assemble theSBC-T cards so you can get on with manufacturing your product, leavinall your control problems to us.

GreenbankFor more details write or phonp us:

Greenbank Electronics, Dept (E7P), 460 New Chester Road, RockFerry, Birkenhead, Merseyside. L42 2AE. Tel: 051-645 3391.


POWER AMPLIFIER MODULES -TURNTABLES -DIMMERS -LOUDSPEAKERS -19 INCH STEREO RACK AMPLIFIERS

PRICES INCLUDE V.A.T. * PROMPT DELIVERIES * FRIENDLY SERVICELARGE S.A.E., 30p STAMPED FOR CURRENT LIST.

teThlage17114:WMI:11l1algalM19111114.1 Supplied ready built and tested.

OMP POWER AMPLIFIER MODULES Now enjoy a world-wide reputation for quality, reliability andperformance at a realistic price. Four models available to suit the needs of the professional and hobby market. i.e Industry.

Leisure, Instrumental and Hi -Fl etc. When comparing prices. NOTE all models include Toroidal power supply, Integral heat sink.

Glass hbre P C B.. and Drive circuits to power compatible Vu meter. Open and short circuit proof.

THOUSANDS OF MODULES PURCHASED BY PROFESSIONAL USERS

OMP100 Mk 11 Bi-Polar Output power 110 wattsR.M.S. into 4 ohms, Frequency Response 15Hz -30KHz -3dB, T.H.D. 0.01%, S.N.R. -118dB, Sens. forMax. output 500mV at 10K, Size 355 x 115 x 65mm.PRICE £33.99 + £3.00 P&P.

NEW SERIES II MOS-FET MODULES

OMP/MF 100 Mos-Fet Output power 110 watts R.M.S.into 4 ohms, Frequency Response 1Hz - 100KHz-3dB, Damping Factor, >300, Slew Rate 45V/uS,T.H.D. Typical 0.002%, Input Sensitivity 500mV, S.N.R.-125dB. Size 300 x 123 x 60mm.PRICE £39.99 + £3.00 P&P.

OMP/MF200 Mos-Fet Output power 200 watts R.M.S.into 4 ohms, Frequency Response 1Hz - 100KHz-3dB, Damping Factor >300, Slew Rate 50V/uS,T.H.D. Typical 0.001%, Input Sensitivity 500mV, S.N. R.-130dB. Size 300 x 155 x 100mm.PRICE £62.99 + £3.50 P&P.

OMP/MF300 Mos-Fet Output power 300 watts R.M.S.into 4 ohms, Frequency Response 1Hz - 100KHz-3dB, Damping Factor >300, Slew Rate 60V uS,T.H.D. Typical 0.0008%, Input Sensitivity 500mV,S.N.R. -130dB. Size 330 x 175 x 100mm.PRICE £79.99 + £4.50 P&P.

NOTE:- MOS-FET MODULES ARE AVAILABLE IN TWO VERSIONS, STANDARD - INPUT SENS, 500mV BAND WIDTH 100KHzPFC (PROFESSIONAL EQUIPMENT COMPATABLEI - INPUT SENS. 775mV, BAND WIDTH 50KHz, ORDER STANDARD OR PEC

Vu METER Compatible with our four amplifiers detailed above. A very accurate visualdisplay employing 11 L.E.D. diodes (7 green, 4 red) plus an additional on off indicator.Sophisticated logic control circuits for very fast rise and decay times. Tough moulded plasticcase, with tinted acrylic front. Size 84 x 27 x 45mm.PRICE £8.50 + 50p P&P.

LOUDSPEAKERS

OMP VARISPEED TURNTABLE CHASSIS.

OPTIONAL MAGNETIC CARTRIDGES

* MANUAL ARM * STEEL CHASSIS * ELECTRONIC SPEED CON-TROL 33 8 45 * VARI PITCH CONTROL * HIGH TORQUE SERVODRIVEN DC MOTOR * TRANSIT SCREWS * 12' DIE CAST PLATTER *NEON STROBE * CALIBRATED BAL WEIGHT * REMOVABLE HEADSHELL * CARTRIDGE FIXINGS * CUE LEVER * POWER 220 240V50 60Hz * 390,305mm * SUPPLIED WITH MOUNTING CUT-OUTTEMPLATE

PRICE £59.99 + £3.50 P&P.

STANTON AL500 GOLDRING G850PRICE £16.99 + 50D P&P PRICE £6.99 + 50o P&P

LARGE SELECTION OF SPECIALIST LOUDSPEAKERSAVAILABLE, INCLUDING CABINET FITTINGS, SPEAKERGRILLES, CROSS-OVERS AND HIGH POWER, HIGH FRE-QUENCY BULLETS AND HORNS, LARGE S.A.E. (30pSTAMPED) FOR COMPLETE LIST.

McKENZIE:- INSTRUMENTS, P.A., DISCO, ETC.8 OHMS IMPEDENCE

8" 100 WATT C8100GPM GEN PURPOSE, LEAD GUITAR, EXCELLENT MID., DISCO.RES. FREQ. 80Hz FREE), RESP, TO 14KHz. SENS, 99dB PRICE £28.59 +£2.00 P&P.10" 100 WATT C10100GP GUITAR, VOICE, ORGAN, KEYBOARD, DISCO, EXCELLENT MIDRES, FREQ, 70Hz. FREQ. RESP, TO 6KHz. SENS, 100dB PRICE £34.70 + £2.50 P&P.10" 200 WATT C10200GP GUITAR, KEYBOARD, DISCO. EXCELLENT HIGH POWER MID.RES, FREQ. 45Hz FREQ. RESP, TO 7KHz. SENS, 103dB PRICE £47.48 + £2.50 P&P.121 100 WAIT C121000P HIGH POWER GEN, PURPOSE. LEAD GUITAR, DISCO.RES, FREO, 45Hz. FREQ, RESP, TO 7KHz. SENS. 98dB. PRICE £36.66 + £3.50 P&P.121 100 WATT C12100TC TWIN CONE) HIGH POWER WIDE RESPONSE. P.A., VOICE, DISCO.RES, FREQ, 45Hz. FREQ. RESP, TO 14KHz. SENS, 100dB. PRICE £37.63 + £3.50 P&P.IT 200 WATT C12200B HIGH POWER BASS, KEYBOARDS. DISCO, P.A.RES, FREQ. 40Hz. FREQ. RESP, TO 7KHz. SENS, 100dB PRICE £64.17 + £3.50 P&P.17' 300 WATT C12300GP HIGH POWER BASS LEAD GUITAR, KEYBOARDS, DISCO, ETC.RES, FROG, 45Hz FREQ, RESP, TO 5KHz. SENS, 100dB PRICE £85.79 + £3.50 P&P.15" 100 WATT C15100BS BASS GUITAR, LOW FREQUENCY, PA., DISCO.

PRICE £53.70 + £4.00 P&P.SEND, 90012HES,HHEQ, z. z.15" 200 WATT C15200BS VERY HIGH POWER BASS.RES, FREE), 40Hz FREQ, RESP, TO 4KHz. SENS, 99dB PRICE £73.26 + £4.00 P&P.15" 250 WATT C15250BS VERY HIGH POWER BASS.RES FRET) 40Hz FREQ. RESP, TO 4KHz. SENS, 99dB PRICE £80.53 + £4.50 P&P.15" 400 WATT C15400BS VERY HIGH POWER, LOW FREQUENCY BASS.RES, FROG, 40Hz. FRED, RESP, TO 4KHz. SENS. 102dB PRICE £94.12 + £4.50 P&P.18" 400 WATT C18404BS EXTREMELY HIGH POWER, LOW FREQUENCY BASS.RES. FREQ, 27Hz FREQ, RESP. TO 3KHz. SENS. 99413 PRICE £167.85 + £5.00 P&P.

EARBENDERS:- HI-FI, STUDIO, IN -CAR, ETC.ALL EARBENDER UNITS 8 OHMS EXCEPT EB8-50 AND EB10-50 DUAL 4 AND 8 OHM.BASS, SINGLE CONE, HIGH COMPLIANCE, ROLLED FOAM SURROUND8" 50 WATT EB8-50 DUAL IMPEDENCE, TAPPED 48 OHM BASS. HI-FI, IN -CAR.RES, FREQ, 40Hz. FREQ, RESP, TO 7KHz. SENS, 97dB PRICE £8.90 + £2.00 P&P.10" 50 WATT EB10-50 DUAL IMPEDENCE, TAPPED 4:8 OHM BASS, HI-FI, IN -CAR.RES, FREQ. 40HZ. FREQ, RESP, TO 5KHz. SENS, 99dB PRICE £12.00 + £2.50 P&P.10" 100 WATT EB10.100 BASS,HI-FI,RES, FREQ, 35Hz. FREQ, RESP, TO 3KHz. SENS, 96dB. PRICE 027.50 + £3.50 P&P.12" 60 WATT EB12-60 BASS, HI-FI, STUDIO.R ES, FR EQ, 28Hz. FROG, RESP, TO 3KHz. SENS, 92dB PRICE £21.00 + £3.00 P&P.12" 100 WATT EB12-100 BASS, STUDIO, HI-FI, EXCELLENT DISCO.RES, FREQ, 26Hz. FREQ. RESP. TO 3KHz. SENS, 93dB PRICE £32.00 + £3.50 P&P.FULL RANGE TWIN CONE, HIGH COMPLIANCE, ROLLED SURROUND5V." 60 WATT EB5-60TC (TWIN CONE) HI -Fl, MULTI -ARRAY DISCO ETC.RES, FREQ, 63Hz. FREQ, RESP, TO 20KHz. SENS, 92dB PRICE £9.99 + £1.50 P&P.6'/"60 WATT E66-60TC (TWIN CONE) HI-FI, MULTI -ARRAY DISCO ETCRES. FROG, 38Hz FREQ, RESP, TO 20KHz. SENS, 94dB. PRICE £10.99 + f1.50 P&P.8" 60 WATT EB8-6OTC (TWIN CONE) HI-FI, MULTI -ARRAY DISCO ETC.RES. FREE), 40Hz. FREQ, RESP, TO 18KHz. SENS, 89dB PRICE £12.99 + £1.50 P&P.10" 60 WATT EB10-60TC (TWIN CONE) HI -Fl. MULTI -ARRAY DISCO ETC.RES, FREQ, 35Hz. FREQ, RESP, TO 12KH, SENS, 86dB PRICE £16.49 + £2.00 P&P.

TRANSMITTER HOBBY KITSPROVEN TRANSMITTER DESIGNS INCLUDING GLASS FIBREPRINTED CIRCUIT BOARD AND HIGH QUALITY COMPONENTS

COMPLETE WITH CIRCUIT AND INSTRUCTIONS

3W FM TRANSMITTER 89108MHz. VARICAP CONTROLLED PROFESSIONAL PER-

FORMANCE, RANGE UP TO 3 MILES, SIZE 38 it 123mm, SUPPLY 12V Ii 0.5AMP.PRICE 04.49+ £1.00 P&P

FM MICRO TRANSMITTER (BUG) 100.108MHz VARICAP TUNED COMPLETE WITH

VERY SENS FET MIC RANGE 109390m, SIZE 56 x 46mm. SUPPLY 9V BATT. PRICE

£8.62 + £1.00 P&P

3 watt FMTransmitter

ValPOSTAL CHARGES PER ORDER fl 00 MINIMUM. OFFICIAL ORDERS WELCOME FROMSCHOOLS. COLLEGES. GOVT. BODIES, ETC PRICES INCLUSIVE OF V.ki. SALES COUNTER.

VISA ACCESS ACCEPTED BY POST. PHONE OR FAX

OMP MOS-FET POWER AMPLIFIERS,HIGH POWER TWO CHANNEL 19 INCH RACK

THOUSANDS PURCHASEDBY PROFESSIONAL USERS

NEW MXF SERIES OF POWER AMPLIFIERSTHREE MODELS:- MXF200 (100w + 100w)

MXF400 (200w + 200w) MXF600 (300w + 300w)All power ratings R.M.S. into 4 ohms.

FEATURES: * Independent power supplies with two Toroidal Transformers * Twin LED. Vu meter: * Rotaryindended level controls * Illuminated on oft switch * XLR connectors * Standard 775mV inputs * Open and shortcircuit proof * Latest Mos-Fets for stress free power delivery into virtually any load * High slew rate * Very lowdistortion * Aluminium cases * MXF600 Fan Cooled with D.C. Loudspeaker and Thermal Protection.

USED THE WORLD OVER IN CLUBS, PUBS, CINEMAS, DISCOS ETC.

SIZES:- MXF 200 W19" x H31/2" (2U)x D11"MXF 400 W19" x H51/4" (3U)x D12"MXF 600 W19" x H51/4" (3U) x D13"

MXF200 £171.35PRICES: MXF400 £228.85

MXF600 £322.00SECURICOR DELIVERY £12.00 EACH

OMP LINNET LOUDSPEAKERSTHE VERY BEST IN QUALITY AND VALUE

MADE ESPECIALLY TO SUITTODAY'S NEED FOR COM-PACTNESS WITH HIGH OUTPUTSOUND LEVELS, FINISHED INHARDWEARING BLACK VYNIDEWITH PROTECTIVE CORNERS,GRILLE AND CARRYING HANDLE.INCORPORATES 12" DRIVER PLUSHIGH FREQ. HORN FOR FULLFROG. RANGE 45Hz-20KHz BOTHMODELS 8 OHM, SIZE H18" x W15"x 012"

CHOICE OF TWO MODELS

POWER RATINGS QUOTED IN WATTS RMS FOR EACH CABINET

OMP 12-100 (100W 100dB) PRICE £159.99 PER PAIROMP 12-200 (200W 102dB) PRICE £209.99 PER PAIR

SECURICOR DEL..- £12.00 PER PAIR

PIEZO ELECTRIC TWEETERS -MOTOROLA

OMP SLIDE DIMMER1K WATT & 2.5K WATT

CONTROLS LOADS UP TO 1KW 8 25KW,SUITABLE FOR RESISTIVE AND INDUC-TIVE LOADS BLACK ANODISED CASE.READILY FLUSH MOUNTED THROUGHPANEL CABINET CUT-OUTS. ADVANCEDFEATURES INCLUDE.-

. FULL 65mm SLIDETRAVEL

. NEONMONITOR INDICATOR FLASH OVERRIDEBUTTON* HIGH 8 LOW LEVFIPRESETS* FULLY SUPPRESSEDToes 800

IKW H128 W40 055mm2 5KW H1213,W76,,D79mm

PRICES:- 1K WATT £15.992.5K WATT £24.99 60p P&P

PIEZO ELECTRIC TWEETERS - MOTOROLAJoin the Piezo revolution. The low dynamic mass (no voice coil) of a Piezo tweeter produces an improved transientresponse with a lower distortion level than ordinary dynamic tweeters. As a crossover is not required these units canbe added to existing speaker systems of up to 100 watts (more if 2 put in series). FREE EXPLANATORY LEAFLETSSUPPLIED WITH EACH TWEETER. TYPE 'A' (KSN2036A) 3" round with protective wire

mesh, ideal for bookshelf and medium sized Hi-fispeakers. Price £4.90 each + 50p P&P.TYPE 'El' (KSN1005a) 31/2" super horn. For generalpurpose speakers, disco and P.A. systems etc. Price

TYPE £5.00 each + 50p P&P.TYPE 'C' (KSN6016A) 2" x 5" wide dispersion horn. Forquality Hi-fi systems and quality discos etc. Price £6.99each + 50p P&P.TYPE 'D' (KSN1025A) 2"x6" wide dispersion horn.Upper frequency response retained extending down tomid range (2KHz). Suitable for high quality Hi-fi systemsand quality discos. Price £9.99 each + 50p P&P.TYPE 'E' (KSN1038A) 33/4" horn tweeter with attractivesilver finish trim. Suitable for Hi-fi monitor systems etc.Price £5.99 each + 50p P&P.LEVEL CONTROL Combines on a recessed mountingplate, level control and cabinet input jack socket.85x 85mm. Price £3.99 + 50p P&P.

TYPE

TYPE C TYPE E

TYPE D

STEREO DISCO MIXERSTEREO DISCO MIXER with 2 x 5 band L&graphic equalisers and twin 10 segment L.E.D.Vu Meters. Many outstanding features 5 Inputswith individual faders providing a useful com-bination of the following: -3 Turntables (Mag). 3 Mics. 4 Line including CDplus Mic with talk over switch Headphone Moni-tor. Pan Pot L. & R. Master Output controls.Output 775mV. Size 360 x 280 x 90mm. Supply =>-22O -240v.

Price £134.99 -- £4.00 P&P

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B. K. ELECTRONICS Dept PEUNIT 5, COMET WAY, SOUTHEND-ON-SEA, ESSEX SS2 6TR

TEL: 0702-527572 FAX: 0702-420243


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