AUDIO FREQUENCY LIGHT
TRANSMITTER AND RECEIVER
Figure
B
shows a light transmitter that applies
a stream of pulses to an ordinary red or near IR
LED at a frequency determined by R1. The values
shown provide a light beam that’s modulated at
about 600Hz.
Figure
C
shows a simple light receiver that
will detect the pulsating beam from the LED and
convert it into an audio tone. You can use this
TIME REQUIRED:
2–5 Hours
DIFFICULTY:
Easy
COST:
$15–$50
MATERIALS
All parts are widely available online.
» Solderless breadboard
» Hookup wire
» 9V battery and battery clip
» CdS photoresistor aka photocell. See text
for details; RadioShack offers a set of 5
(item 2761657) for experimenting.
» Silicon solar cell or photodiode See text.
» 555 timer IC chip
» Speaker, 8
Ω
impedance
» Capacitors, ceramic: .01µF (2) and 0.1µF (2)
» Capacitors, electrolytic: 4.7µF (1) and
100µF (1)
» Resistors: 470
Ω
(1), 1k
Ω
(2), 220k
Ω
(1), and
1M
Ω
(1)
» Variable resistor, 10k
Ω
aka potentiometer
» LED, red or near-infrared
» Op-amp IC chip, TLC271 aka operational
amplifier
» Audio amplifier IC chip, LM386
» Thin mirror or reflective material See text.
» Bluetooth speaker with clip and phone plug
(optional)
» Battery-powered radio and masking tape
» Sunglasses and hat
TOOLS
» Soldering iron and solder (optional)
FORREST M. MIMS III, an
amateur scientist and Rolex
Award winner, was named by
Discover magazine as one of the
“50 Best Brains in Science.” He
has been talking over light beams
since 1965. forrestmims.org
United States public domain via Wikimedia
C
This circuit is a light receiver that converts a pulsating
light into an audible tone.
85
make.co
and 7. You can slow the circuit’s audible frequency
range by increasing the value of C1.
This circuit works best with a CdS photoresistor
that has a high resistance when dark and a low
resistance when illuminated. CdS photoresistors
are widely available online. RadioShack offers
a set of 5 (item number 2761657) with a wide
range of sensitivity differences that allow you to
experiment to find the best results.
A
B
This circuit emits a tone whose frequency is controlled
by the intensity of light striking a CdS photoresistor.
This circuit causes an LED to emit a rapid string of
pulses that are converted into an audible tone when
the LED is pointed at a light receiver circuit.
M78_084-88_AmSci_F1.indd 85M78_084-88_AmSci_F1.indd 85 7/13/21 12:25 PM7/13/21 12:25 PM
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