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.
Bill of Materials
- 1 x LM13600/13700 or NJM13600/13700, dual OTA + buffer
- 1 x TLV2374 or similar quad rail-to-rail CMOS opamp, tolerating ±9V supply rails (18V diff.)
- 1 x MZ/AZ/TS34063 switching converter (buck/boost/buck-boost)
- 1 x CdS or CdSe light-dependent resistor (LDR) a.k.a. photoresistor
- 1 x green LED, 5mm
- 1 x Schottky rectifier diode, >1A continuous current: 1N5819/SR07 or similar)
- 2 x Signal pn-junction diodes: 1N4148 or similar
- 1x 22K potentiometer
- 1x any-value potentiometer (preferably 100K or above for power
- 1x SPDT switch
- 47µH inductor – or similar value. Self-wound absolutely OK.
- Assorted capacitors and 1/4 W resistors (see schematic)
- Film capacitors for the oscillator. Styroflex, Mylar both OK
- 0.2 ohm resistor for negative regulator current set
Gerber files, as well as the EAGLE schematic/board file, will soon be available. Until that point, you can either construct the circuit on