One night a couple weeks ago, I rigged up a basic synthesizer which generates an audio signal for 3.5mm jacks with just a couple transistors, resistors and capacitors.
To generate the signal, I used a simple flip flop oscillator. There are two methods to change the frequency (pitch) of the signal: a) changing the capacitance of one (or both) sides of the oscillator. b) changing the flow of electricity (in this case, via a potentiometer).
In this circuit, I decided to use both methods just for fun. When pressing one of the pads (which were just hacked together with some tape and aluminium foil), one is able to change the amount of capacitors in series, which results in a lower frequency due to the longer charge time. The potentiometer acts as a wobble nob by changing the voltage passing across the capacitors.
So here it is!!