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Turning Capacitance into a Signal

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David LeviDavid Levi 04/11/2017 at 03:431 Comment

How does a theremin convert a wave of the hand into music? The first step is to convert capacitance into a usable frequency signal.

Capacitance is how easy it is to store energy between objects in an electric field. By moving things (like your hands) closer to the theremin, capacitance increases. A hand far away from an antenna will only have a few electric fields. A hand that is closer will have more electric fields and thus more capacitance.

This capacitance connects to a dual timer. In the schematic below, each side is a complete 555 timer. The antennas act at the capacitor. Only one resistor is needed, because these 555 timers are configured in 50% mode.

To simplify part count, Control Voltage (Pins 3 and 11) are left floating. For more precise measurements, these pins should be bypassed to ground with their own capacitors, but in this application they made no noticeable difference.

Typically, in this configuration, the output of this circuit would be taken off Dischage (Pins 1 and 13). I had no issue taking the output of the, well, Output (Pins 5 and 9). This decision removed the need for pull-up resistors.m

The output frequency of the oscillator depends on how long it takes to charge and discharge a capacitor "C" through a resistor "R". A 555 timer in 50% duty cycle configuration has an output frequency "f" of:

In practice, the pitch antenna oscillates around 800 kHz and the volume antenna oscillates around 500 kHz. This number varies depending on the environment, but the microprocessor will adapt. The chosen values of R keeps oscillation below 1 MHz. The two oscillators must be two dissimilar frequencies to prevent interference. Pitch should be the higher of the two frequencies, since the sensitivity of the pitch antenna is more important than the volume antenna.

When a hand moves toward an antenna, capacitance increases and frequency decreases. Up next, I'll be explaining some of the things that goes on inside the microprocessor to turn this frequency signal into something audible.

Discussions

Todd wrote 04/21/2017 at 06:59 point

This was informative!

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