Theremin Circuit

Posted on Sep 11, 2012

Two identical integrated circuits, U1 and U2, known as `hex inverters` are used for the theremin's primary functions. They are CMOS (Complimentary Symmetry Metal Oxide Semiconductor) devices, typically used in digital circuits to perform a logic function called `inversion.` Each IC contains six identical sections; thus the term `hex inverter.` U1 sections A and B, in conjunction with R1, R2 and C1, form the theremin's variable oscillator that operates in a frequency range around 73kHz. The antenna forms one-half of a variable capacitor that is part of this oscillator's frequency-determining network, and the player's hand forms the other half.

Theremin Circuit
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As the distance between the hand and the antenna varies, so does the capacitance, and therefore the oscillator's frequency. U1 section C buffers the variable oscillator's output to provide isolation from the rest of the circuit. U2 sections A and B, in conjunction with R13, R14, RV1, and C9, comprise the theremin's local oscillator, adjusted with the PITCH ZERO CONTROL potentiometer and ZERO CAL potentiometer RV1. RV1 is a miniature "trimmer" type, mounted on the circuit board, used to calibrate the local oscillator's frequency range. With RV1 properly adjusted, the local oscillator's frequency will equal the variable oscillator's frequency with the hand furthest away from the antenna. Under these conditions, the phase relationship of the two oscillators will be constant due to their small, but finite capacitive coupling, so no audible tone will be produced. U2 section C buffers the local oscillator's output to provide isolation from the rest of the circuit. The three remaining U2 sections are not used, so their inputs are grounded. Because the theremin's oscillators have some sensitivity to power supply variations, VR1, a low-dropout voltage regulator IC, with C5 and C6, is used to furnish a steady 5 volts. Rectifier CR1 protects the circuit from accidental battery reversal, and R8 prevents excessive current from causing CR1 and the battery to heat under such a condition. The voltage drop across R8 is less than 200...

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