While this is a great little project, I was not very satisfied with the end result which didn’t sound very similar to a “real” theremin. The original theremin was an analogue instrument in which the audio frequency was produced by the interaction between two radio frequency oscillators at very nearly the same frequency which were controlled by the proximity of a musicians hand to a metal rod aerial. A third oscillator controlled by the proximity to a metal loop of the musicians other hand was used to control the volume. this gave a sound which was continuous when started by the proximity of a hand, whose frequency and volume could be varied smoothly. You can see an example in the picture above (from the Wikipedia article on Theremin)
The first thing I resolved to do was to use two ultrasonic sensors instead of one. I used the same circuit as the one in the raspberrypi.org article but built it twice on the same breadboard. The first circuit used pins 4 and 17 as in the original, and the second one, used pins 23 and 24 for the trigger and echo signals instead, otherwise being identical. I built the circuit on a RasPiO ProHat board which gives convenient access to all the GPIO pins, as shown in the photographs below.
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