The Opto-Theremin

Google+ Pinterest LinkedIn Tumblr


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

Integrated Circuits

  • 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)

Active Devices

  • 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 cosumption reasons)
  • 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


Oscillator Section Schematic
Optothermin Control Section Schematic
Optothermin Supplies Section Schematic
Optothermin Output Section Schematic


Gerber files, as well as the EAGLE schematic/board file, will soon be available. Until that point, you can either construct the circuit on aprotoboard (quite easy) and enjoy the sight of the PCB 😉

Optothermin PCB Schematic

David is a true electronics fanatic, with a special love for analog circuit design, especially as it pertains to audio, power supplies and measurement instruments. Not afraid to use a microcontroller when the situation calls for it, though. And, of course, a big fan of cats!

Write A Comment