The Opto-Theremin came into existance because of work I did on a high-quality oscillator circuit for a ring modulator (which is still in the works). Wanting to use only half of an LM13700 operational transconductance amplifier (OTA) for the oscillator, and the other half for the modulation, I worked on a way to improve the basic single-OTA designs, which were quite lackluster.
In fact, they had awful wave shapes and weren't stable - there'd be periods of different frequency sloping in there, which ruined the point. Eventually, however, I struck on a design that was both stable and had near-perfect triangular wave-shape. Win!
So, over dinner that night, I thought - why not make this into a theremin? An LDR, a sensitivity control circuit as well as an LED would surely do the business! And, as a matter of fact, it did. At this point, the circuit is completely drawn out, and a PCB design is available, as well, for those wanting their very own ultra-cheap theremin fun.
Gerber files, as well as the EAGLE schematic/board file will soon be available. Until that point, you can either construct the circuit on protoboard (quite easy) and enjoy the sight of the PCB ;-)
David Christensen is an electronics fanatic, especially in the audio and low-frequency areas. The author can be contacted at m e (at) davidh.info.