Here are a couple of fun DIY synth projects by Michael Rucci: the Atari Paddle Synth, above, and the NES Advantage Synth.

The Atari Paddle Synth is a square-wave generator with a pulsing effect, hand built into an Atari paddle controller. When you press the trigger it plays the pitch controlled by the large original knob. When you flip the switch a LFO will start to pulse on and off. The speed of the pulse is controlled by the additional smaller knob and at high speeds you can get some interesting throaty sounds.

The NES Advantage Synth features two square-wave oscillators built into a NES advantage controller with turbo.

Rucci sells his creations at his Etsy store.