Theremin circuit is a device which generates musical effects in response to the capacitive proximity of a human hand.
This is the only musical instrument known to the writer that is played without being touched!
The Theremin is named after Professor Theremin who, in 1928, amazed New York audiences when he demonstrated his ability: "to obtain music from the ether".
How the Circuit Works
The instrument has two rods protruding from its housing, each one forming one 'plate’ of a capacitor.
The performer's hands become the second 'plates’ when held near the rods.
The capacitive changes engendered in one rod controls the pitch of the tone,the other rod responding by varying the amplitude of the output. simple experimental circuit is illustrated .
The plate should be around 30cm. square. It is placed next to a radio receiver tuned to a fairly strong station around 900 kHz.
The slug of the coil is then adjusted to obtain the most pleasing tone.
When the hand is moved near the plate,the pitch of the tone will change.
Hi, any idea what the specs should be for the inductor? The old Lafayette 32-H-4108 (man that company brings back memories) has disappeared completely off the grid and can’t get any info on it. Also, couldn’t the inductor/radio idea simply be replaced with a small transformer to directly drive a speaker, or if voltage too low, at least as an output into an amplifier? If so, what specs would you recommend for the transformer? thanks! Really neat simple little circuit for my daughter who has recently taken an interest into electronics to play with.
Hey Mike, yes the old Lafayette 32-H-4108 coil is gone now and no specs exist but you can replace it easily. With the 500 pF capacitor in the circuit, the oscillator needs an inductor in the 300 µH to 600 µH range. You can wind this yourself on a 10–12 mm plastic tube using about 30–40 turns of 24–26 AWG enameled wire, single layer. Remove or add turns to tune.
About using a transformer instead: that will not work sorry, because this circuit is an RF oscillator (hundreds of kHz), not an audio oscillator. A speaker needs audio, so an audio transformer cannot replace the RF coil. You must keep the LC tank.
If you want to drive a speaker, you can detect the RF and feed the resulting audio into a small LM386 amplifier, or build a simple heterodyne stage like a classic theremin.