Commodore 64 Theremin

As L';(AKA lftkryo) explains, the theremin is one of the earliest electronic instruments. It’s played by moving one’s hands in front of two antennas: one to control the pitch, the second for volume.

Åkesson's build is quite unique in that it uses theremin technology to interact with a C64 and produce sound via its SID (Sound Interface Device) chip. The result is an instrument that is controlled like a theremin, but has a distinctly chiptune sound, exactly as you might expect from C64 hardware.

A nice demo of the device in action can be seen in the video below as well as an explanation of how this device, and a theremin in general, works based on capacitive interactions (not RF). The write-up dives a bit more into the specifics, and how he finally settled on using a 555 timer to help produce the proper signals.

Åkesson also goes over some of the design choices in the original theremin, and this C64 version in more detail, which should be especially interesting to those that have both a musical and electronics background. In order for the C64 to properly measure the 555 oscillator, he's employing the C64’s two 16-bit pulse counters as asynchronous edge detectors, freeing inputs from constraints of the C64 clock.

Code includes a calibration mode to account for the user’s size, along with filtering to avoid jittery outputs.