Chaos Theramin

(6″ x 8″, wood, glass, obsolete electronics)
Building on the feel of the crackly era of old radio and employing vintage technology, this interactive piece explores the universal progressive descent into chaos. By sensing electrical fields, the viewer’s proximity engenders transformations of sound from a pure tone into noise.

I made this art piece as part of a series that all fit inside small wooden boxes that mount on the wall. They have a slightly steampunk look and are built using outdated technology. This one uses two vacuum tubes to implement a theramin circuit which responds to the proximity of the hand of someone who comes near its antenna.

Unlike a traditional theramin, the antenna on this one does not control pitch and volume. Instead it controls some parameter in a chaotic noise oscillator which makes a non-linear system move from simple oscillation, through a set of period doublings, into a noise-rich wave. This produces an auditory tone which is not very pleasant but is quite interesting as it changes quite sensitively and subtly with your hand position.

There is also a green magic-eye indicator tube which gives a very 1920s look and visual feedback of what is happening as you move your hand around.

Chaos Theramin
There is a huge amount of electronics crammed into a small space in this device and it was a challenge to build on a variety of levels. However, I do like a good challenge!

For those people interested in more details, I present below the schematic (which is rather a working schematic, with random notes, so may not be completely “final”). I will now give a technical walkthrough.

Chaos Theramin SchematicThe first part I will describe is how the device senses the presence of someone’s hand. This is done by detecting small changes in the capacitance around the art piece. For this purpose it has a brass antenna. This was very kindly turned on a lathe for me by my good friend Kevin MacDonald.

The circuit uses an ECC82 dual triode to act as an oscillator and buffer. The oscillator is a sine wave oscillator with a nominal frequency of 455kHz. However, the presence of someone’s hand detunes the oscillator away from this frequency. The output of the oscillator is taken through a ceramic filter which is a 455kHz bandpass filter. Due to the filter characteristic, the final amplitude of the signal will vary depending on how far away from 455kHz the oscillator has been detuned. After passing through a cathode follower using the second half of the ECC82, the signal goes to a diode detector and this turns the output into a negative going control voltage which is useful for a variety of purposes.

The whole design of this oscillator and detector was taken from a vacuum tube theramin design at Art’s Theramin Page. The circuit is quite versatile and I hope to use it in other projects, e.g. to control motion. The only issue with it is that tuning is quite critical. There is a small 27pF trimmer that must be adjusted to get the right behavior, and I found that it needed to be adjusted in situ for each different location of hanging the device on a wall. The theramin was sensitive to the presence of wall studs, etc., and annoyingly would be silent and work perfectly when it was first turned on, but then, once the tubes warmed up, it would drift and start making a sound continously. Careful adjustment of the trimmer allowed one to fix this.

The anode supply for these tubes was set to +50V to avoid electric shock risk. Actually, an important omission on the schematic above, but present in the actual piece, is a 330pF capacitor in series with the antenna to provide DC isolation.

EM80 IndicatorI decided that it would be important to have a visual indication of the state of the control, and so I made use of an EM80 indicator tube which I always found very mysterious as a child. These look really quite cool and have around 16V of control voltage change for the complete range of visual sweep. An op-amp buffer was used to get the correct 0 through -16V grid voltage from the theramin circuit detector output.

The main issue with the EM80 that I found was that its phosphor screen burns in really easily, so you definitely don’t want to leave the unit turned on all day.

The noise maker in this art piece is a non-linear chaotic oscillator called the Chua circuit. It took quite a lot of experimenting to get this to work correctly and find the right component values. However I didn’t write them all on the circuit, and I sold the art piece, so tough luck replicating it!

The Chua circuit uses an inductor, two capacitors, and two op amps. It has one resistor value that affects the behavior and changes it from producing a slightly distorted sine wave to a more and more complex waveform at the output. The system goes through multiple period doublings until it ends up in a chaotic attractor and sounds progressively more distorted as you move your hand closer. Below you can see the phase space waveform during this change created by monitoring on X and Y the voltages on either side of the main resistor.

In order to get the control voltage from the theramin to affect this circuit, I used a light dependent resistor attached to an LED inside a black tube. Small changes in the LED current were enough to sweep the effect over the full range.

The output of the oscillator is fed to a TDA7052 power amplifier and drives a small speaker. It is quite loud. The art piece has a door that opens if you really want to let the sound out, but it is quite a noise!

I had to use two transformers in the power supply. One provides the low voltage supply for the tube filaments and the dual voltages (around 16V) for the op-amps. This transformer is a 6.3V-0-6.3V type and the center tap is grounded. The other transformer is a 60V-0-60V type and provides regulated +50V for the ECC82 anodes from the center tap, and uses a voltage doubler to get +250V for the indicator tube anode.