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Modified:21:46, 22 October 2013
A Theremin is decribed in Wikipedia as...
"... originally known as the aetherphone / etherophone, Thereminophone or termenvox / thereminvox is an early electronic musical instrument controlled without contact from the player. It is named after its Russian inventor, Professor Léon Theremin, who patented the device in 1928."

This version I found on the internet many years ago but have only recently got round to building it. Normally it is both frequency and volume controlled moving your hands near the antennae of the Theremin but this version only seems to alter the frequency.

Anyway I decided to try it out, designed a PCB, and it worked straight away, very impressive. I have yet to house it properly with a decent ground plane and antennae but it is quite fun to play with, difficult to produce a tune worth listening to though.

As I said I did not design this and do not know much about Theremin design as I had no information with the circuit. But after examining it, I think it is basically 2 high frequency oscillators (around 300KHz when measured) of almost identical frequency, mixed together to generate 2 further frequencies f1-f2 and f1+f2. The first one, the difference f1-f2 is filtered through to the output via a low pass filter, thus keeping all the audible frequencies and filtering out the high frequencies. As you pass your hand near the antennae, the effective capacitance alters altering the frequency ofone of the oscillators thus producing a "beat" frequency of the difference in the audio range.


IC1 forms the first oscillator and IC2 the second. These are mixed using Q3 and then filtered by IC3 part of which is a Sallen-Key type filter. The transistor in the original circuit were 2N5484 types but I couldn't find these so used 2N5457.

When setting up the circuit, connect an antennae of about 8 inches to the Ant. input and adjust VR1 and C11 until the pitch drops so that it is silent. When moving your hand near the antennae, the pitch should rise. It takes a litle more adjustment to get the range and sensitivity correct.

The output is about 1v peak to peak and is fed to the input of a general purpose audio amplifier.

PCB Dimensions = 102mm x 51mm

The PCB holds VR1 and C11 which are used to "tune" the circuit. I will when I house it properly connect a potentiometer and an old radio tuning capacitor mounted on the case to allow easy tuning.

IC holders were used for the ICs and a PCB pin for the connection to the antennae. CN1 provides the audible output signal.

Admittedly on the prototype I just couldn't route the -V connection so this was conencted to the IC pins 7 using wire underneath the PCB.


PCB layout - (PCB Wizard)

Circuit Diagram - (jpg image)

Written by Phil Townshend 2007
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