Theremin Interrupter

[profdc9]

Hello Tesla Fans,

Yesterday I got a wild hair and thought: wouldn't it be cool to control your Tesla coil with a Theremin?  Well, I designed an interrupter that I think should do just that.   It has two plates which are capacitively coupled to your hands.  One plate controls the pitch (or bps) of the DRSSTC, the other the volume or the length of the interrupter pulses. 



I have included the schematic and the board I laid out to try it.  Here's how it works:

Each of the 74HC04 have a pair of oscillators on it.  One of the HC04's is for the pitch, the other for the volume.  One of the oscillators has its capacitor hooked up to a plate so that by moving your hand next to the plate, it changes the capacitance and therefore the oscillation frequency.  The other is adjusted so that when you hand is not near the plate, the frequency matches the other oscillator.  Using a XOR gate and a low-pass filter, the beat frequency between these two oscillators is derived.

For the volume, (HC04 U3), the beat frequency is converted to a voltage by converting the beat frequency to a series of pulses of a uniform length, and then low-pass filtering these pulses.

For the pitch, the beat frequency is used to derive a ramp signal that resets each time the cycle of the pitch beat frequency is low.  By using a comparator with the voltage derived from the volume, a pulse-width-modulated signal is derived to modulate the length of this pulse.

An analog output is also provided as well.   To do this, transistor Q2 is a voltage follower with its collector tied to the output of U2D.  When U2D is low, current flows through D10 and the capacitor C13 is discharged.  When U2D is high, the voltage follower limits the charging voltage of C13 to one diode drop below the voltage at C10 which is the volume-proportional voltage.  This is low pass filtered to get a sinusoidal-like signal.

Anyways, I'm still working on getting a DRSSTC up and going, hope to have it Theremin controlled!

Dan

[Mads Barnkob]

A little while ago I built the OpenTheremin v3 and it can be modified to output MIDI, which could then be fed to a DRSSTC without any problems.

I do believe that you should either sit in a faraday cage with your theremin or stay at a great distance to the coil, to avoid it ends up in some loop of the sparks interfering with the note generation.

[Hydron]

It might be fine if the theremin tank circuit and DRSSTC tank circuit operate sufficiently far away from each other in frequency, and the broadband noise produced by the coil isn't too bad. I do like the idea!

[futurist]

Did you build your circuit? I tried to build several theremins and they all turned out to be just good noise synthesizers. Biggest problem was to tune them, which Gaudi solved well in openTheremin v3, using varicap diodes
Check his openTheremin page http://www.gaudi.ch/OpenTheremin/ It's excellent and you can really play something on it

Last year I tried what Mads suggested, modifying openTheremin for MIDI output and then feeding that to onetesla MIDI interrupter
At the time openTheremin MIDI firmware had some problems so it kinda worked, the whole setup was glitchy/lagging even without DRSSTC being powered on. Lagging was probably caused by the computer and next time I would try connecting openTheremin directly, which will need some mods on the openTheremin MIDI firmware. So at the end DRSSTC sounded pretty bad and I couldn't play anything recognizable

edit: I hope that I'll have some free time this month to finally play with the DRSSTC, I bought wire mesh for the faraday cage so I might revisit theremin interrupter

[profdc9]

I know it's tricky to make a theremin stable, but others have done it, and I'm going to try it.  I put both of the oscillators, the variable and fixed oscillator, on the same chip 74HC04, so that hopefully the temperature drift of the two oscillators will cancel somewhat.  As for component values, that's a bit harder, because probably you have to use a capacitor with low temperature drift.  I am going to use plates rather than antennas initially because it's easier to get more capacitance variation that way, and they'll be placed close to the circuit mounted on the side of the enclosure.

Due to feature creep, I added a little bit so it could be a theremin by itself by putting on a simple amplifier chip, and corrected a few problems.

Dan