1976 Phototheremin

1976 Phototheremin is a circuit I spotted in a Forrest Mims III book called Optoelectronic Projects Vol. 1, printed by Radio Shack in 1976.  Its the most minimal sound generator I’ve seen, consisting of only seven parts including the battery, speaker, and power switch.

 1976 phototheremin schematic black

Q1, Q2 = general purpose NPN, PNP transistors: 2N3904 / 2N3906, BC549/BC559, etc.
C = 0.01uF - 0.1uF
B = 1.5V or 3V coin cell
SPKR = 8 ohm

 Assembly Guide

Forrest Mims provides some insight on the early origins of this circuit:

That circuit was adapted from a very early transistorized code practice oscillator that I bought at an electronics store. I used the same circuit to pulse an IR LED in the various travels aids for the blind I designed and built. The circuit was also used in the light flasher for model rockets that became the first product we developed at MITS–the company that introduced the Altair 8800 microcomputer in 1975.

It operates by charging a capacitor with current from the battery.  The rate of current flow into the capacitor is controlled by a light-dependent resistor, aka photocell.  When the voltage on the capacitor reaches a threshold, the transistors act as a switch, releasing a short burst of energy through the speaker.  This produces a single ‘click’ of audio.  The capacitor is discharged and immediately begins recharging, thus the cycle continues.  The rate at which the clicks come out is controlled by how much light is falling on the photocell.  Under normal room lighting, this rate is fast enough to produce an audible pitch.

With careful gestures, it is possible to play a musical scale on the 1976 Phototheremin.  Identify a light source that will be your controller and stand facing it.  Hold the phototheremin in your hand, and vary the angle of your elbow as you raise and lower the unit.  Using your critical ear, you can relate the angle of your elbow to notes on a musical scale.  Of course, this is not a true theremin because it isnt controlled by radio waves.  However, the gestures involved in playing it are similar.

1976 Phototheremin kit

PCBv2