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Super Simple FM TransmitterBy Sean Michael Ragan Category: Electronics Difficulty: Difficult View Comments

FM transmitters can be complicated to build, but not this one — it’s about the easiestyou can possibly make. And though the science of radio is well understood, there’sa magical, emotional quality about it that we don’t often stop to appreciate. You willnot forget the first time you pick up a transmission broadcast from a device yousoldered together, yourself, from a few bits of copper, carbon, plastic, and wire.

I am indebted to Jim and Kat of Sonodrome for first introducing me to thatexperience, through this very circuit, which I first built on a pre-etched PCB from akit they offered for sale as recently as 2011.

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Weekend Projects ­ Super Simple FM Transmitter

This design was originally popularized by Japanese multimedia artist TetsuoKogawa. The circuit itself is a slight variation on Kogawa’s simplest FM transmitterdesign, and the method of building it is sometimes referred to as “Manhattan style.”It uses a piece of copper-clad circuit board but, rather than etching the circuit tracesthrough the copper layer, a large piece of continuously-plated board is used tomake all the circuit’s ground connections, and small sections of plated board areglued to the surface to form nodes or “pads” that are insulated from ground. Besides being a convenient way to assemble circuits using minimal tools, thisbuilding method encourages you to think about circuits in an interesting way — asgroups of connections that are either grounded or “floating above” ground.

This transmitter uses ten on-board components and will transmit a monaural audiosignal about 30 feet. It is possible to extend that range by adding an antenna, andMr. Kogawa’s website has more information about how to do that.

NOTE: Depending on where you live, operating an FM transmitter — even a veryshort-range one like this — may be illegal without a license. Unless you attach anantenna, it’s very unlikely that anyone will notice or complain about anytransmissions you may make with this device. On the other hand, it’s very difficult topredict, before construction is complete, just where on the FM band this transmitterwill broadcast. Use due caution during testing, and make sure you understand thelaw in your area before attaching the battery.

PARTS

FM Radio Receiver RadioShack #12-586Transistor, NPN, 2N3904 RadioShack #276-2016Battery clip, 9V RadioShack #270-324Phone plug, mono, 1/8" RadioShack #274-286Stranded copper wire, 22 gauge, red and black RadioShack #278-1218Capacitor, ceramic disk, 0.01μF (2) RadioShack #272-131Capacitor, ceramic disk, 10pF (2) from RadioShack #272-801Battery, 9V, rechargeable Ni-MH, or alkaline RadioShack #23-748 or #23-853

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Solid copper wire, 18 gauge, 4" length RadioShack #278-1223Hook-and-loop strips RadioShack #64-2363Resistors, 1/4W (3) 1 each: 470Ω, 10K, and 27K, from RadioShack #271-0312Copper-clad board, about 5cm × 5cm RadioShack #276-1499Capacitor, electrolytic, 1μF–33μF such as RadioShack #272-1026. Mine camefrom RadioShack assortment #272-802.

TOOLS

1/4-20 bolt for forming the coilWire stripper/cutter RadioShack #64-224Pliers, mini long-nose RadioShack #64-062Soldering iron and solder RadioShack #64-2071Small fileScissorsUtility knifeStraightedgeFM radioAudio source with 1/8" phone jackGlue, cyanoacrylate, medium viscosity aka super glue or crazy glue

STEPSPROJECT STEPS1. Form the coil.

2. Cut the board.

3. Mount the coil.

4. Add capacitor C3 and resistor R2.

5. Add the electrolytic cap, resistor R1, and capacitor C2.

6. Add the transistor.

7. Add the 10pF caps, resistor R3, and the battery clip.

8. Attach the phone plug.

9. Tune it up!

10. Mount the battery.

Step #1: Form the coil.Prev

Strip about 4" of 18AWG solid copper wire and wind 4 turns around the threads of a 1/4-20bolt.Turn the coiled wire off the bolt as if you're unthreading a nut, and clip each lead to about1cm.Bend little "feet" on the ends of the leads and adjust them so the coil will stand upright.

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Holding a pair of pliers in each hand, grab the coil's leads and stretch it evenly along itslength until the feet are 12mm apart on center. You may need to even out the coil spacingwith a screwdriver or other tool.

Step #2: Cut the board.

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Use a straightedge, a utility knife, and the edge of a table to score and snap a 5cm × 4cmrectangle from the copper-clad board. This will be your ground plane.Score and snap a 5mm × 5cm strip of copper-clad board, then score it crosswise at 5mmincrements. Snap along these lines, with pliers, to create several 5mm × 5mm "pads." Youonly need 5, but you may want to make a couple extra.Smooth the corners and edges of the ground plane and the pads with a small file. Beespecially careful to remove any sharp copper burrs that might cause cuts on handling.

Step #3: Mount the coil.

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Apply a small drop of cyanoacrylate glue to the underside of one of the pads. It doesn'ttake much. Use tweezers or small pliers to carefully position it in the center of the groundplane. Wait a few seconds for the glue to set.Glue a second pad to the board, above and to the left of the first, along a line running atabout 135° with respect to the long centerline of the board, as shown. Position the secondpad along this line so that there's about 12mm between the centers of the first and secondpads. Wait a few seconds for the glue to set.Solder the coil across the 2 pads as shown. This will be easiest if you pre-tin the surface ofeach pad, and both coil feet, before applying heat to reflow the solder and join the tinnedareas.

Step #4: Add capacitor C3 and resistor R2.

Solder a 0.01μF ceramic disk capacitor (C3) between pad 2 and the ground plane, and trimaway any excess leads. It doesn't especially matter where you connect to the groundplane, for this or any other connection in the project.Glue pad 3 to the board somewhere below and to the left of pad 2, as shown. You wantenough space between pads 2 and 3 to fit the body of a 1/4W resistor.

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Solder a 1/4W 27K resistor (R2) between pads 2 and 3, as shown. Trim any excess leads.

Step #5: Add the electrolytic cap, resistor R1, andcapacitor C2.

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Glue pad 4 to the ground plane just to the left of pad 3. Space the pads to match the leadspacing on your electrolytic capacitor. Solder the electrolytic cap (C1) between pads 3 and4, making sure the negative (–) lead is connected to pad 4.Solder a 10K resistor (R1) between pad 3 and ground.Solder a 0.01μF ceramic disk capacitor (C2) in parallel to the 10K resistor between pad 3and ground.

Step #6: Add the transistor.

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Bend the transistor's 3 leads, as shown.Glue pad 5 to the board directly to the right of pad 1. Make sure it's close enough to pad 1that one of your transistor's leads can reach between them.Solder your transistor across pads 1, 3, and 5, as shown. The collector connects to pad 1, thebase to pad 3, and the emitter to pad 5. Trim any excess leads.

Step #7: Add the 10pF caps, resistor R3, and thebattery clip.

Solder one 10pF ceramic disk cap (C5) across the transistor's collector and emitter, (i.e.between pads 1 and 5), and a second 10pF ceramic disk cap (C4) between pad 1 and theground plane. Trim any excess leads.NOTE: For more convenient adjustment of the transmitting frequency, replace the 10pFcapacitor (C4) between pad 1 and ground with a 20pF variable or "trim" cap. If you use a

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variable cap, the frequency can be adjusted simply by turning the trimmer shaft with asmall screwdriver.Solder a 470Ω resistor between pad 5 and the ground plane. Trim any excess leads.Connect a 9V battery clip to the board, as shown, by soldering the red lead to pad 2 andthe black lead to the ground plane.

Step #8: Attach the phone plug.

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Unscrew the threaded housing from the tip-shield (TS) mono phone plug and set it aside.Solder a 4" length of red stranded wire to the center "tip" contact, and a 4" length of blackstranded wire to the outer "shield" contact.The shield contact has built-in prongs that can be crimped over onto the wires to providestrain relief for the solder connections. Use small pliers to fold these prongs over and crimpthe wires beneath them, being careful not to crimp so hard you damage the wire insulation,bend the tip contact onto the shield contact, or otherwise short the 2 connections.Slip the threaded housing over the wires and tighten it onto the plug threads again. Solderthe free end of the red wire to pad 4, and the free end of the black wire to the groundplane.

Step #9: Tune it up!

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Attach your 9V battery to the battery clip and insert the phone plug into an audio sourcelike an MP3 player or smartphone. Start a song or other easily-recognizable audio trackplaying, then turn on your radio and scan through the FM band to locate the transmission.TIPS:

Start with your receiver right next to the transmitter.A digital tuner with precision down to 0.01MHz may be helpful.Be patient and careful. Scanning is a bit tedious, but if you get impatient you may missthe signal altogether and mistakenly believe the transmitter isn't working.If you scan the entire band and can't locate your signal, try changing the orientation ofyour receiver's antenna with respect to the board and scanning again.It's best to run your audio source on battery power when you are first isolating thetransmitting frequency. If you have to run it from mains power, make sure electricallynoisy devices like fluorescent lights, TVs, and computer monitors are not active on thesame circuit at the time.

You can tune the transmitting frequency by changing the spacing between turns in thecoil. Closing the spacing will lower the transmitting frequency, while opening it up willraise the transmitting frequency. You can also use a variable capacitor for tuning (see Step7).

Step #10: Mount the battery.Next

NOTE: This transmitter design needs very clean, smooth power, which is one of thereasons we choose to run it from a battery. Power from a "wall wart" or other AC adapter issmooth enough for most DC applications, but not for this radio transmitter. Using an ACadapter to power this transmitter is likely to cause the signal to be too noisy to use.Use scissors to cut a strip of hook-and-loop fastener (velcro) tape to fit the length of your

9V battery.Separate the hook and loop sides of the tape, remove the backing from each, and apply thehook (scratchy) side to the bottom of the transmitter board. Apply the loop (fuzzy) side toone of the battery's 2 largest faces.Attach the transmitter to the battery using the velcro during use. When depleted, thebattery can be removed and separated from the transmitter for recharging.

SEAN MICHAEL RAGANI am descended from 5,000 generations of tool-using primates. Also, I went tocollege and stuff. I write for MAKE, serve as Technical Editor for MAKE magazine,and develop original DIY content for Make: Projects.

smragan.com // Etsy // shapeways // Thingiverse

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Toad of Toad Hall • 37 minutes ago

Very interesting. Quite illegal, of course.But, oh dear, didn't they teach you to look after your tools?

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