Squarewave

This is a direct copy of one of the circuits we used on the cardboard synth, it consists of three square wave generators, all are coupled together so each one modulates the pitch of the next. In the first two the modulation is controlled by a normal transistor, the last one is modulated by a crystal. It requires two cat's whiskers and the way it is set up with the crystal connected to the input of the inverter and to ground via the timing capacitor seems to be the best in terms of the range of sounds produced. It is very harsh, particularly when using chalcopyrite so ideally requires filtering.

This is still one of the most satisfying circuits for making sounds with natural semiconductors - in truth I have very little idea what happens in this circuit when you use two cat's whiskers like this, two back to back diodes or perhaps transistor like behavior under some circumstances. It doesn't matter very much, as this is a good circuit for being able to hear the difference between different minerals, as the overall noise, resistance, capacitance and semiconducting characteristics all seem to have a big effect on the sound.

Its won't work without a crystal connected, and it's quite easy to just pass through the previous oscillators output either directly or via the last inverter without oscillation happening, but the difference between this (which sounds quite 'clean') and the more noisy effects when the crystal starts interfering and doing it's thing is usually easy to determine.

The circuit uses two vactrols (paired leds and ldrs) so that the pitch of the first two oscillators can be modulated. The transistor driving the LEDs is probably not needed, but it was designed when I was experimenting with different control voltage sources, and I wanted to buffer incoming signals (and this makes it very sensitive). It's arranged so that when it's soldered together you can bend the LDR over the top of the LED - as you can see in the picture, I haven't enclosed these to light proof them.

R1 can be reduced to increase the modulator depth between oscillator 1 and 2. R8 and 9 as a voltage divider used to reduce the output signal to about a third to match other modules - and provide more headroom for filters later on.

Licenced under the CERN Open Hardware Licence Version 2 – Strongly Reciprocal

Inputs

• Three connectors for the crystal and two cat's whiskers via the on board connector.
• Two CVs to control the pitch of the first two oscillators.
• Audio input disconnected.

Controls

• OSC 1 pitch
• OSC 2 pitch

Outputs

• Audio signal

Schematic

PCB layout