Pulsed-light sensor
PARTS AND MATERIALS
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Two 6-volt batteries
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One NPN transistor -- models 2N2222 or
2N3403 recommended (Radio Shack catalog # 276-1617 is a
package of fifteen NPN transistors ideal for this and
other experiments)
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One light-emitting diode (Radio Shack
catalog # 276-026 or equivalent)
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Audio detector with headphones
If you don't have an audio detector already
constructed, you can use a nice set of audio headphones
(closed-cup style, that completely covers your ears) and a
120V/6V step-down transformer to build a sensitive audio
detector without volume control or overvoltage protection,
just for this experiment.
Connect these portions of the headphone
stereo plug to the transformer's secondary (6 volt) winding:
Try both the series and the parallel
connection schemes for the loudest sound.
If you haven't made an audio detector as
outlined in both the DC and AC experiments chapters, you
really should -- it is a valuable piece of test equipment
for your collection.
CROSS-REFERENCES
Lessons In Electric Circuits, Volume
3, chapter 4: "Bipolar Junction Transistors"
LEARNING OBJECTIVES
SCHEMATIC DIAGRAM
ILLUSTRATION
INSTRUCTIONS
This circuit detects pulses of light
striking the LED and converts them into relatively strong
audio signals to be heard through the headphones. LEDs have
the little-known ability to produce voltage when
exposed to light, in a manner not unlike a semiconductor
solar cell. By itself, the LED does not produce enough
electrical power to drive the audio detector circuit, so a
transistor is used to amplify the LED's signals. If the LED
is exposed to a pulsing source of light, a tone will be
heard in the headphones.
Sources of light suitable for this
experiment include fluorescent and neon lamps, which blink
rapidly with the 60 Hz AC power energizing them. You may
also try using bright sunlight for a steady light source,
then waving your fingers in front of the LED. The rapidly
passing shadows will cause the LED to generate pulses of
voltage, creating a brief "buzzing" sound in the headphones.
With a little imagination, it is not
difficult to grasp the concept of transmitting audio
information -- such as music or speech -- over a beam of
pulsing light. Given a suitable "transmitter" circuit to
pulse an LED on and off with the positive and negative
crests of an audio waveform from a microphone, the
"receiver" circuit shown here would convert those light
pulses back into audio signals.
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