CMOS-555 Theremin
My friend Wolfgang Dorninger asked me for a recommendation for a Theremin workshop. He wanted a cheap and beginner friendly DIY Theremin kit. He sent me 4 links and I should choose my favorite. After watching a Youtube video and listening to sound examples , I came to the conclusion that all of them were either not playable, ugly sounding, not DIY friendly or too expensive and decided to design my own Theremin with these specs :
- about 10€ for PCB + components
- standard components still in production, no programmable parts
- simple standard inductors for coils
- no variable capacitors. inductors or other adjustable parts beside pitch control
- simple operation with only a pitch antenna
- playable distance range of at least 50cm
Well, I underestimated the effort to realize this small project. It took me 6 prototypes until I was satisfied with the result. Here it is :
circuit description :
Master and Local Oscillator are Schmitt-Trigger LC-oscillators. My design winner uses only a TLC555CP CMOS timer, a series LC tank, and a resistor. I once got an industry donation of hundreds of 330uH coils, so I had to use them ( I guess that any 330uH with low DC resistance will work here). Due to the DC coupled feedback path via L1, this kind of oscillator always starts up reliably. R1 sets the antenna amplitude and protects U1 from this high voltage (>=150V). Local oscillator U4 uses 2 (empiricaly found ) 3.9pF capacitors instead of the antenna. D1 and D2 are mixing both oscillator frequencies and provide 1.2V bias for a transistor amplifier with a gain of R5/R6. Lopw pass filter R4,C3 keeps RF away from audio out. Voltage regulators U2, U3 have a double function. They isolate both oscillators with separate power supplies which prevents locking of the 2 RF frequencies, which would result in zero audio output. This adresses a problem from which all "single chip" oscillator pairs suffer. The second function is voltage tuning of both oscillators. With a supply range from 4..6V, the 1.5MHz oscillators can be detuned by about +- 10kHz. tuning range can easily be doubled by increasing one oscillators supply voltage while decreasing the other oscillators supply voltage. 1k potentiometer RV1 has a grounded wiper and acts as 2 variable resistors to set the 2 supply voltages.
Meanwhile (May 2023) almost 30 CMOS-555-Theremins were successfully built in DIY workshops in Austria, Spain and Germany.
Some funny applications :
"Beeremin" an interactive sound installation with 3 CMOS-555 Theremins and beer cans as antennas performed by visitors of the Mappa de Ruido Noise Festival 2023, Mallorca :
prepared CMOS-555 Theremin by Ralf Schreiber : electromechanical treatment of antenna and coils with aluminium foils :