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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
2006 American Society for Engineering Education K-12 Workshop
Copyright  2006 Dennis Silage Temple University
Page 1 of 14
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 276142) 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
or a net negative charge).
Figure 2
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
2006 American Society for Engineering Education K-12 Workshop
Copyright  2006 Dennis Silage Temple University
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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.
Why is that so? How come things are backward?
Figure 4
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
2006 American Society for Engineering Education K-12 Workshop
Copyright  2006 Dennis Silage Temple University
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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 or
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, Ω), 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
2006 American Society for Engineering Education K-12 Workshop
Copyright  2006 Dennis Silage Temple University
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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 Ω
Any value of resistance can be solved by substituting a different value of current I.
Close standard values of resistors would be 22 Ω or 27 Ω. However, resistors offered
by Radio Shack have a ± 5% tolerance and values 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
2006 American Society for Engineering Education K-12 Workshop
Copyright  2006 Dennis Silage Temple University
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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 Ω are only available as either 10 Ω or 100 Ω. 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=IR
solve for R
R=V/I
solve for the inverse of R 1 / R = I / V
the sum of the currents
I = I1 + I 2
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
(Ω Ω)/ (Ω + Ω) = Ω
Extending this to four resistors in parallel,
two at a time for our circuit here means that
four 100 Ω resistors in parallel would be 25
Ω.
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
2006 American Society for Engineering Education K-12 Workshop
Copyright  2006 Dennis Silage Temple University
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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
diode.
Figure 9
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
amplification.
2006 American Society for Engineering Education K-12 Workshop
Copyright  2006 Dennis Silage Temple University
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Figure 10
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
collector (cathode) to emitter (anode) unless IR light is inputted from
an external source.
Figure 11
The maximum collector to emitter
voltage is 70 V which is much
larger than our battery 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 photodetector 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.
2006 American Society for Engineering Education K-12 Workshop
Copyright  2006 Dennis Silage Temple University
Page 8 of 14
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 Ω 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 Ω.
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:
2006 American Society for Engineering Education K-12 Workshop
Copyright  2006 Dennis Silage Temple University
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I = (3.2 – 0.2) V / 200
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
2006 American Society for Engineering Education K-12 Workshop
Copyright  2006 Dennis Silage Temple University
Page 10 of 14
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 modules 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
2006 American Society for Engineering Education K-12 Workshop
Copyright  2006 Dennis Silage Temple University
Page 11 of 14
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
RS 273-053
RS 278-149
RS 270-401
RS 271-1331
Infrared emitter and detector
Mini buzzer
Component PC Board (2)
AA battery holder (4)
100 Ω resistors
AA batteries (4)
2006 American Society for Engineering Education K-12 Workshop
Copyright  2006 Dennis Silage Temple University
Page 12 of 14
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.
2006 American Society for Engineering Education K-12 Workshop
Copyright  2006 Dennis Silage Temple University
Page 13 of 14
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.
Dennis Silage, PhD K3DS
Professor
Temple University
[email protected]
astro.temple.edu/~silage
2006 American Society for Engineering Education K-12 Workshop
Copyright  2006 Dennis Silage Temple University
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