Projects

The Opto-Theremin

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Background

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

Mechanics

  • 1x 22K potentiometer
  • 1x any-value potentiometer (preferably 100K or above for power cosumption reasons)
  • 1x SPDT switch

Passives

  • 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

Schematics

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

PCB

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!

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