PROJECTS Amateur Scientist
Listening
to Light
Written and photographed by Forrest M. Mims III
Build a photophone to send
and receive sound on a beam
of modulated sunlight
United States public domain via Wikimedia
84 makezine.com
Over the years I’ve designed many electronic
circuits that transform steady (DC) or modulated
(AC) beams of light into sound. These circuits
have many applications, some of which are
described here.
BASIC LIGHT-TO-TONE RECEIVER
During my senior year at Texas A&M in 1965,
I modified a simple 2-transistor oscillator by
connecting it to a speaker and replacing its single
resistor with a light-sensitive cadmium sulfide
(CdS) photoresistor. The circuit was silent when
placed at the end of my dormitory’s darkened
200-foot hallway. When a fellow student struck
a match at the far end of the hall, the circuit
emitted a loud buzz that amazed me as much as
the students who were watching.
You can repeat this demonstration with the
simple audio oscillator circuit shown in Figure
A
.
This circuit is easily assembled on a solderless
breadboard. It features a 555 timer chip thats
connected to a CdS photoresistor across pins 4
M78_084-88_AmSci_F1.indd 84M78_084-88_AmSci_F1.indd 84 7/12/21 1:07 PM7/12/21 1:07 PM
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