Typically, a theremin uses two separate Colpitts LC oscillators, one of which can be slightly varied in frequency. The two frequencies are mixed together, and demodulated to reveal a beat frequency. If the two oscillators are at the same frequency, there is no beat or audio, but if they are off because of the proximity of your hand, a difference or beat frequency results, which is the audio output of the theremin. A 4011 quad gate, IC1, is the heart of this theremin. Taro gates are used for each of the two required oscillators running at 250 kHz. For the aerial, use a metal toilet-tank float. It provides much better sensitivity than a length of wire. The two RF signals are mixed, then amplified by IC2, an LM741 op amp. Audio is detected by D1, a 1N34 diode. Another LM741, IC3, is set up as an adjustable bandpass filter; still another LM741, IC4, further amplifies the audio for IC5, an LM386 audio amplifier.
Required Field 
