This schematic diagram show a single chip Theremin circuit. Theremin is an electronic music instrument which sense hand movement to control the tones/frequency. This Theremin circuit uses two separate Colpitts LC oscillators to produce a beat frequency. The frequencies of two Colpitts LC oscillators are mixed and then rectified.
Here’s the figure of the schematic diagram.
This rectification demodulate the mixed signal
to get the beat frequency which is in audible range. This beat frequency or
difference is the real Theremin’s output. The oscillator is operated at high
frequency (inaudible) to get wide audible frequency range of beat frequency
when two oscillator output is mixed. This circuit uses a 4011 quad gate
to construct the high frequency oscillator operating at 250kHz.
The metal
probe that is used to sense your hand produces only small frequency shift
in term of percentage of original frequency, that’s why we need to derive the
beat frequency to get wide audible frequency range as the result of high
frequency shifting. The IC2, an LM741 is used to amplify the mixed signal
before rectification. The D1 will rectify the mixed signal to detect the
audio (the beat frequency).
This audio
signal is then filtered by an adjustable bandpass filter IC3. The further audio
amplification before power amplifier IC5 is done by IC4. The metal
toilet-tank float is used for the hand probe since is has better
sensitivity than a simple wire antenna, but any conductive material will work.
[Circuit’s schematic diagram source: seekic.com]
