It seemed like that whole way a Theremin behaves - instantly, sensitively, mysteriously over space, etc, would be a good thing for 'lots-of-stuff'. That much turned out to be true. The tricky bit was finding a reliable way to make it work at all! There's lots of circuits for Theremins all over the web and interestingly, lots of them don't work. Or are stupidly fussy, which amounts to the same thing most of the time! If you are of a mind to make something Theremin-like, let me share with you my all time favourite Theremin oscillator, found after (very) much research.
First a quick note on how they work. You get two oscillators, running at say 100kHz ish. From one you attach a long wire to the kind of fussy point on the circuit you would normally try and leave alone. The capacitance from your body (roughly speaking - I think there's a lot of interesting things to look into here) interacts with the oscillator circuit, causing it to loose energy and slow down a bit. By say a percent or two. That's not much of a big deal by itself. Its a small change and too high to hear anyway. The trick comes from mixing together the signals from the affected and unaffected oscillators through a really rubbish, distorting amplifier. What comes out is the difference between the two oscillator frequencies, plus a lot of mess. We ignore the mess because that's generally still too high to hear, but with the two oscillators just a little bit out of sync, the 'difference' signal is that useful, wailing tone.
Great, only a lot of the oscillator circuits you may find on the web either don't work or are needlessly complicated, with hard-to-get parts. But not this one. Credit is due, as far as I can tell, to some dedicated and inventive guys in Russia who's names I do not know. The design uses some very cheap, very outdated computer logic chips in a creative way. I call this iteration of it my 'Cyberman' oscillator, because my first working version, thanks to copious tinfoil, looked very much in the early Cyberman stylee. A CD4096 chip makes one oscillator, so you need a pair of them. A single transistor makes the dirty amplifier. A few minor bits and pieces and you're done! A quick note - the circuit shows a 20mH inductor connected to the aerial. That's optional (and easy to hand wind). If you put it in, then the Theremin will have a more 'linear' feel to the way it responds, probably. These things are not especially tame and predictable by their nature!
In another post, I'll show a circuit I like to add on, to smooth and improve the output. From there, should you choose, its quite easy to get a little ESP8266 (very small single chip computer) to read the output, turn it into data and do whatever you want with it... :)
(If you need more details, a bigger scrawly image of the circuit, etc, message me :) )