the marconi challenge: infrared data transmission as thesilage/marconichallengek-12ws.pdf ·...

2006 American Society for Engineering Education K-12 Workshop Copyright 2006 Dennis Silage Temple University

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The Marconi Challenge: Infrared Data Transmission

as the 21st Century Crystal Radio

Dennis Silage, PhD

Electrical and Computer Engineering Temple University

[email protected]

Introduction The Marconi Challenge is a new electrical and computer engineering K-12 outreach program that addresses the design objectives of wireless data communication and is suitable for students from junior high school to college. The Marconi Challenge was originally conceived to celebrate the 100th anniversary of Guglielmo Marconi’s transatlantic wireless transmission in 2001. In 1901 Marconi succeeded in transmitting a radio signal across the Atlantic Ocean. The wavelength of the transmitted signal was approximately 1500 meters, and input power was measured in kilowatts. In contrast, the Marconi Challenge requires that junior and senior high school experiment with the transmission of an infrared (IR) light signal at a wavelength of 940 nanometers and power measured in milliwatts to a receiver at the greatest distance from the transmitter. The Marconi Challenge offers unique opportunities for experimentation and learning for junior and senior high school students. IR digital data communication is used to provide an educational experience in electronics, the use of semaphores to represent information, and the optical transmission and reception of IR light. Unlike low power, unlicensed radio frequency (RF) transmissions, whose electronic circuitry is more complicated, IR semiconductor components are inexpensive and the circuitry is easy to comprehend, construct and utilize. Figure 1


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In a junior and senior high school educational module, the students are first instructed on the simple principles of the IR light emitting diode (LED) transmitter and IR photodetector receiver, as described here. Standard components for the Marconi Challenge include a single high-output IR LED as a transmitter, and an IR phototransistor as a detector Infrared Emitter Diode In the Marconi Challenge the transmission of a signal by infrared (IR) light using a semiconductor diode emitter and phototransistor detector is introduced. The emitter and detector are inexpensive and commonly available (RadioShack 276-142) as shown in Figure 1. The specification for the IR diode emitter (the blue tinted device) is shown in Figure 2. From these specifications you see that the maximum reverse voltage is 5 V (volts). This means that when the diode is not conducting or reverse biased no more than 5 V can be used. The IR emitter diode is a junction of semiconductor type P (a deficit of electrons or a net positive charge region) and type N (a surplus of electrons Figure 2 or a net negative charge). When the battery voltage source is connected as shown in Figure 3, the positive terminal of the battery attracts negative electrons away from the PN junction of the IR diode emitter. The negative terminal attracts holes away from the PN junction. The insulating depletion region widens and no electron current flows. Figure 3 When the battery voltage source is connected as shown in Figure 4, the negative terminal of the battery supplies electrons to the PN junction of the IR diode


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emitter and current flows. The maximum forward current of the diode with conduction is 150 mA (milliamperes) or 0.15 A (amperes) which must not exceeded. The arrow in Figure 3 and Figure 4 shows the direction of electron flow but that not common the convention! The common convention for current flowing would show the arrow reversed. Figure 4 Why is that so? How come things are backward?

The choice of which type of electricity is called "positive" and which "negative" was made around 1750 by Ben Franklin, early American scientist and man of many talents. Franklin studied static electricity, produced by rubbing glass, amber, sulfur etc. with fur or dry cloth. Among his many discoveries was proof that lightning was a discharge of electricity, by the foolhardy experiment (he claimed) of flying a kite in a thunderstorm. The kite string produced large sparks but luckily no lightning, which could have killed Franklin.

Franklin knew of two types of electric charge, depending on the material one rubbed. He thought that one kind signified a little excess of the "electric fluid" over the usual amount, and he called that "positive" electricity (marked by +), while the other kind was "negative" (marked -), signifying a slight deficiency. It is not known whether he tossed a coin before deciding to call the kind produced by rubbing glass "positive" and the other "resinous" type "negative" (rather than the other way around), but he might just as well have.

Later, when electric batteries were discovered, scientists naturally assigned the direction of the flow of current to be from (+) to (-). A century after that electrons were discovered and it was suddenly realized that in metal wires the electrons were the ones that carried the current, moving in exactly the opposite direction. Also, it was an excess of electrons which produced a negative electric charge. However, it was much


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too late to change Franklin's naming convention.

Ohm’s Law

Basic electronics requires an understanding of voltage, current and resistance. For this we are indebted to Georg Simon Ohm, who was born in 1787 in Erlangen, Germany.

In 1805, Ohm entered the University of Erlangen and received a doctorate. He wrote elementary geometry book while teaching mathematics at several schools. Ohm began experimental work in a school physics laboratory after he had learned of the discovery of electromagnetism in 1820.

In two important papers in 1826, Ohm gave a mathematical description of conduction in circuits modeled on Fourier's study of heat conduction. These papers continue Ohm's deduction of results from experimental evidence in an electrochemical cell and, particularly in the second, he was able to propose laws which went a long way to explaining results of others working on galvanic electricity.

The basic components of an electrochemical cell are:

1. Electrodes (X and Y) that are made of electrically conductive materials: metals, carbon, composites ... 2. Reference electrodes (A, B, C) that are in electrolytic contact with an electrolyte 3. The cell itself or container that is made of an inert material such as glass o r Plexiglass 4. An electrolyte that is the solution containing ions. Using the results of his experiments, Georg Simon Ohm was able to define the fundamental relationship between voltage, current, and resistance. What is now known as Ohm's law appeared in his most famous work, a book published in 1827 that gave his complete theory of electricity.

The equation I = V / R is known as "Ohm’s Law". It states that the amount of steady current (I) through a material is directly proportional to the voltage (V) across the material divided by the electrical resistance (R) of the material. The ohm (whose symbol is the Greek letter omega, O), a unit of electrical resistance, is equal to that of a conductor in which a current of one ampere (named after Andre-Marie Ampere) is produced by a potential of one volt (named after Alessandro Volta) across its


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terminals. These fundamental relationships represent the true beginning of electrical circuit analysis. Ohm's law is usually expressed by rearranging the formula as:

V = I R.

Infrared Emitter Diode Transmitter Circuit The simple infrared (IR) emitter diode circuit for the Marconi Challenge transmitter is shown in Figure 5. The value of the resistor can be calculated using Ohm’s Law. The voltage drop across the IR emitter diode can range from a typical amount of 1.3 V to a maximum of 1.7 V, as shown in Figure 2 . A single alkaline battery AA cell (when new) is about 1.6 V. Figure 5 To get current consistently to flow in this circuit the battery should be greater than the worst case IR emitter diode voltage drop of 1.7 V. So two battery cells should be used for 1.6 + 1.6 V = 3.2 V. The least worst case IR emitter diode is 1.3 V and we’ll use that value because we want to calculate the worst case current flow. For this simple circuit, the battery voltage V will cause current I to flow. The resistor will have a voltage drop VR = I R. For our circuit we form a balance sheet of the battery voltage as a source and the voltage across the resistor and IR emitter diode voltage drop as a load. The source and load have to be equal (this is known as Kirchoff’s voltage law) or: 3.2 V = I R + 1.3 V The current I is not an unknown because it must be less than the maximum forward current of the diode with conduction of 0.15 A (150 mA) as shown in Figure 2. We’ll use a conservative 50% value of 0.075 A (15 mA): 3.2 V = (0.075 A) R + 1.3 V Solving for the only unknown, the resistance R in ohms: R = (3.2 – 1.3) V / 0.075 A R = 1.9 V / 0.075 A R ˜ 25 O Any value of resistance can be solved by substituting a different value of current I. Close standard values of resistors would be 22 O or 27 O. However, resistors offered by Radio Shack have a ± 5% tolerance and va lues that are only spaced as 1, 2.2, 3.3, and 4.7 (industry standard ± 5% tolerance spacing is 1, 1.2, 1.5, 2.2, 2.7, 3.3, 3.9, 4.3


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4.7, 5.6, 6.8, and 8.2). Unfortunately, resistors from Radio Shack in the range from 10 to 100 O are only available as either 10 O or 100 O. This is an example of an engineering tradeoff and a chance to learn something new. Figure 6 shows two resistors in series (top) and two resistors in parallel (bottom). Notice that in the series connection the total current I flows through both resistors. By Ohm’s Law V = I R we have that V = I (R1 + R2) or the equivalent resistor R is the sum of the two resistances. For the parallel connection the total current I is divided and flows through the resistors. That’s is, I = I1 + I2. Since by Ohm’s Law the voltage across each of the resistors must Figure 6 be equal (the balance sheet concept) we have that V = I1 R1 = I2 R2. Solving for the equivalent resistor R is an interesting exercise in electrical circuit theory and algebra: Ohm’s Law V = I R solve for R R = V / I solve for the inverse of R 1 / R = I / V the sum of the currents I = I1 + I2

substitution 1 / R = (I1 + I2) / V = I1 / V + I2 / V therefore 1 / R = 1 / R1 + 1 / R2

or by rearranging R = R = (R1 R2)/ (R1 + R2) So for the parallel connections of two resistors the equivalent resistor R is the ”product over the sum” (R1 R2)/ (R1 + R2). Dimensionally this is correct since we have (O O)/ (O + O) = O Extending this to four resistors in parallel, two at a time for our circuit here means that four 100 O resistors in parallel would be 25 O. This circuit can be easily constructed by twisting the component wires together on an insulating circuit board even without a permanent connection by soldering, as shown in Figure 7 . Figure 7


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A Radio Shack plastic holder (270-401) can contain one AA alkaline battery cell which can then be connected in series as shown in Figure 8. The red wire is the positive battery terminal and the black wire is the negative terminal To wire the battery holders in series connect the red wire of one to the black wire of the other. The remaining red and black wires are the positive and negative terminals of the battery. Figure 8 Remember to remove the battery from the holder when not in use so that the battery does not drain!

The cathode (the bar in the diode symbol or C) of the IR emitter diode is on the flat side of the package, as shown in Figure 2. The IR emitter diode by itself, of course, does nothing. We next have to build the IR receiver. Infrared Detector Phototransistor The IR detector phototransistor is a special transistor device in which photons of IR light induce current to flow. The transistor was invented at Bell Laboratories in December 1947 (not in 1948 as is often stated) by John Bardeen and Walter Brattain. The ungainly device is shown in Figure 9. 'Discovered' would be a better word, for although they were seeking a solid-state equivalent to the vacuum tube, it was found accidentally during the investigation of the surface states around a point-contact Figure 9 diode. Bell Labs kept their discovery quiet until June 1948 (hence the confusion about the date of discovery). They then announced it in a fanfare of publicity, but few people realized its significance, and it did not even make the front page of the newspapers! The common transistor has three wires, called the collector (C), emitter (E) and base (B) and is available in two types: NPN and PNP which differ in the direction of current flow, as shown in Figure 10. The arrow shows the direction of (Ben Franklin’s) current flow from a positive voltage source to a negative return. Current entering the base modifies the current that would normally flow from collector to emitter by a factor of 10 to 1000 times (or more). This process is called Figure 10 amplification.


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The IR detector phototransistor does not have a base wire or conventional electron base current. Rather photons of IR light perform the amplification of the collector to emitter current directly as shown in Figure 11. The specification for the IR detector phototransistor is shown in Figure 12. Since the phototransistor only has two leads (like a diode) you can consider that the collector represents the cathode and the emitter represents the anode for circuit analysis. The unique feature here, unlike a common diode, is that current does not flow from Figure 11 collector (cathode) to emitter (anode) unless IR light is inputted from an external source. The maximum collector to emitter voltage is 70 V which is much larger than our baterry voltage! However, the emitter to collector voltage is 5 V and represents the same concept and cautions that is described for the reverse voltage of the diode emitter in Figure 2. The maximum collector current of 50 mA represents the same concept and caution that is described Figure 12 for the continuous forward current of the diode emitter in Figure 2. The radiant power is 13 to 15 mW at a wavelength of 950 nm (nanometers or 10-9 m). Note that the peak sensitivity wavelength of the IR detector phototransistor is 850 nm and for the IR emitter it was 950 nm. This seeming mismatch is due to the semiconductor material difference for the IR emitter diode and the IR detector phototransistor. However, the spectral bandwidth (range) of the IR detector phototransistor is 620 to 980 nm and it works acceptably at 850 nm.

Infrared Phototransistor Receiver Circuit

The simple infrared photodector receiver circuit for the Marconi Challenge transmitter is shown in Figure 13. It can again be easily constructed by twisting the component wires together on an insulating circuit board even without a permanent connection by soldering, as shown in Figure 14. Two Radio Shack plastic holders (270-401) can again each contain one AA alkaline battery cell which can then be connected in series. The IR photodetector emitter (the arrow in the symbol or E) is the non-flat side of the package, as shown in Figure 12.


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To add to the realism that IR light is actually being transmitted we have used a piezoelectric sounder to convert electrical current to an audio tone (Radio Shack 273-073). The sounder not only indicates that IR light is being received but limits the current I that flow from the positive terminal of the battery. Figure 13

The sounder operates from 1.5 to 3 V and produces 75 decibels (dB) of sound at a nominal frequency of 300 to 500 Hertz (Hz). The sounder is can be considered as resistor (R) of about 200 O from the specification sheet which indicates that it draws a current of 0.015 A (15 mA) at the maximum voltage of 3 V.

R = V / I = 3 /.015 = 200 O.

The IR phototransistor has a typical voltage drop of about 0.2 V from the collector to emitter terminals when fully excited by photons. When in the dark the IR phototransistor draws very little current and the sounder is essentially off. Figure 14

To get current consistently to flow in this circuit also the battery should be greater than the worst case IR phototransistor voltage drop of 0.2 V and the minimal requirement of the sounder of 1.5 V. So two battery cells are again used for 1.6 + 1.6 V = 3.2 V. For our circuit we again form a balance sheet of the battery voltage as a source and the voltage across the sounder and fully excited IR phototransistor voltage drop as a load. The source and load have to be equal (this is known as Kirchoff’s voltage law) or: 3.2 V = I R + 0.2 V 3.2 V = I (200) + 0.2 V Solving for the only unknown, the current I in amperes: I = (3.2 – 0.2) V / 200


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I = 3 V / 200 I ˜ 0.015 A = 15 mA Such a small current also means that the sounder is not very loud! The chart below relates decibels to intensity in Watts per square meter (W/m2) and some common sources of sound. Note that the sounder produces only 70 dB, which is about the same intensity as a traffic in a busy street.

Source Intensity Intensity Level

Referenced to TOH

Threshold of Hearing (TOH)

1*10-12 W/m2

0 dB 100

Rustling Leaves

1*10-11 W/m2

10 dB 101

Whisper 1*10-10 W/m2

20 dB 102

Normal Conversation

1*10-6 W/m2

60 dB 106

Busy Street Traffic

1*10-5 W/m2

70 dB 107

Vacuum Cleaner

1*10-4 W/m2

80 dB 108

Large Orchestra

6.3*10-3 W/m2

98 dB 109.8

Walkman at Maximum Level

1*10-2 W/m2

100 dB 1010

Front Rows of Rock Concert

1*10-1 W/m2

110 dB 1011

Threshold of Pain

1*101 W/m2

130 dB 1013

Military Jet Takeoff

1*102 W/m2

140 dB 1014

Instant Perforation of Eardrum

1*104 W/m2

160 dB 1016


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Infrared Transmitter-Receiver The infrared transmitter-receiver configuration is shown in Figure 15. Because of the low power of the IR emitter diode the distance between the transmitter and receiver is not great as shown. This is another part of the Marconi Challenge.

Some questions that can be asked:

1. Is the IR photodetector circuit sensitive to other sources? Yes, incandescent lamps and bright sunlight can cause the sounder to operate.

2. Can you apply optical principles to increase the distance? Yes, lens and mirrors can be used quite effectively and can be used to animate lesson module on optics and ray tracing.

3. Can we encode information on the sounder? Yes, we can introduce the concept of semaphores in electronic communications starting with flags, proceeding to the Morse Code and modern coding of characters as the ASCII code.

4. Can more advanced experiments be supported for senior high school students? Yes, the basic concepts of IR data transmission can be used to introduce digital signaling. More details are available at astro.temple.edu/~silage


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Marconi Challenge Parts List The components of the Marconi Challenge are available at Radio Shack stores or on-line at www.radioshack.com. The total cost should be approximately $12. RS 275-142 Infrared emitter and detector RS 273-053 Mini buzzer RS 278-149 Component PC Board (2) RS 270-401 AA Battery holder (4) RS 271-1331 100 O resistors AA batteries (4)


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Optional equipment: Lens, mirrors, brackets, ruler (distance measurement), telegraph key (to make and break the IR emitter transmitter circuit for a semaphore), and an inexpensive digital volt meter.


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The Marconi Challenge is a Challenge for You! You can recognize that the simplicity and excitement of the Marconi Challenge could be the 21st century equivalent of the crystal radio which provided the impetuous for many high school students to turn-on to engineering and science. No, it’s not an iPod but at least the students can understand how it works!

For more information about electronics and communications , see the website of the American Radio Relay League www.arrl.org or a student-friendly website at hello-radio.org.

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