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. 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. Here is the schematic diagram of the circuit:

One Chip Theremin Circuit circuit schematic

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]