Sandy Freckle

pulse/burst generator

In Australia, there is an entrepreneur called Dick Smith who ran a chain of electronic hobby stores in the 70s and 80s (now he makes breakfast cereal). He used to publish circuit books every year (and sell kits for them), I no longer have the issue but remember the finer points of a circuit for a burp/fart generator.

It was simply 3 transistor based multivibrators running at different frequencies. The trick was that they were loosely connected by low valued resistors causing them to influence each other. This would result in unpredictable sounds that rarely repeated. Don’t expect it to actually sound like gas escaping from an orifice, think of it is a random pulse/burst generator

This version has pots to adjust the frequencies of each section and an input for turning the sound on and off rather than a switch. It can be fed gates or envelopes, which give a more gradual onset and tail as the sound starts and drops off.

Feel free to build it with whatever capacitor values you like. The ones suggested on the PCB (and heard in the demo vid) get into audible frequencies, but in some ways it is more interesting to use larger valued capacitors to get just pulses that will then ping filters and phase-shifters.

It is also interesting to feed it audio rate signals and it becomes a waveshaper/distortion module.

Sandy Freckle was a wonderful villain played by actor Rock Hampton in the Kath & Kim series. He was sleazy and annoying but irresistible and funny at the same time, much like this module.

https://www.nonlinearcircuits.com/modules/p/sandyfreckle