So I made a $5 Theremin with an OTA and a few other things...

[DrGonzoDK]

EDIT: The schematic is available here for the interested!

Actually, I was going to build a fancy ring modulator (and probably still am). But I was playing around with the prototype (three "panes" of strip board), and decided I could a more interesting form of pitch control than simple pot turning. So having ruminated over this over dinner, I ran in and speed-soldered an "optical theremin controller" - a low-angle power THT LED and an LDR in an adjustable divider (to adjust for background light and general sensitivity, etc. - and got this


http://ee.david.promo/theremin-open.jpg

...and when closed

http://ee.david.promo/theremin-closed.jpg

Of course - there's a video of my /fantastic/ skills at making 1950s ghost movie noises -
http://ee.david.promo/Theremin.mp4

Apologise for the whining in the background - it's some sort of modulation with the heater radiator in the background (it's not audible "in person").
The tinkling sound in the background is my cat trying to figure out what is making that noise (cat tag on collar)

And, for the interested, here's the schematic - http://ee.david.promo/optotheremin/opto-schematic.pdf. If you buy cheap IC's - the NJM13600 instead of the LM13700, and the TL062 instead of the TLV2372, you can definitely make this for $5! The reason the oscillator uses an op amp in its feedback loop, is the improved stability against DC offsets, rather than using the integrated Darlington buffer of the LM13700.

And nope, the LED/LDR combo isn't an ideal control. This probably will get turned into that ring modulator prototype. But it's a fun project, doesn't take too long, and is genuinely fun to play with! The nice thing, also, is that it's basically a VCO. So many types of control can be rigged in.

The actual output waveform (the triangle - the square is just a comparator-derived waveform from the triangle) is very nice and symmetrical, with very limited distortion,


An improvement on this would be to use the second OTA - and a second LDR/LED pair - to create the amplitude modulation function of the Theremin. That way, it wouldn't be at a constant volume, but instead be controllable with the other hand. That's why i love OTAs. They are so damn flexible :-P