Echo Tone Oscillator

This simple circuit amplitude-modulates a single tone in such a manner to approximate the echo effect one might obtain, for instance, with sonar apparatus. Any reader who has been on a sonar-equipped vessel will realize that some imagination is required to appreciate the similarities among this simple scheme and the "real thing." However, this circuit might adjunct a small model (of a submarine, perhaps) to add the dimension of sound.

Long ago, I was indelibly impressed with myriad sci-fi adventure serials, such as "Voyage to the Bottom of the Sea." There, a variety of simple audio effects, either provided by tape or phonographic reproduction, or by the most rudimentary laboratory apparatus, created the incidental sounds that gave these episodes an added dimension of reality. I loved to devise ways of making these sounds on command, and thus designed and constructed a number of appropriate electronic circuits, well represented by this example. The original renditions were entirely made of discrete (usually germanium) transistor arrangements. Later, I converted many of these designs into integrated-circuit equivalents, which both decreased their size (allowing them to fit into pockets!), and their cost.

Lay-out and construction technique is left to the builder. The use of a substantial ground bus and direct, short wiring, is suggested in the area of ICs U1, U2, and U4 and their associated components, to prevent unwanted oscillations. The two major parameters, "pitch" and "rate" may be alternatively made variable by replacing the relevant resistors, RPITCH and RRATE with 250,000-ohm potentiometers in series with 10,000 ohm resistors. I selected the parameters of about 970 hertz for the pitch, and 16 hertz for the clock rate, merely on personal preference, for what seemed to be the best "Science Fiction Coefficient." Click here for an audio sample of this circuit's output.

April 14, 2000

Source documents dated March 16, 1989 and September 13, 1994

Text and image ©2000 by Arthur Harrison

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