A number of great DIY synth projects have come out over the years, including the littleBits synth kit, the Moog Werkstatt 01, and Synthrotek's Atari Punk Console, just to name a few. But none are as small as David Levi's MicroSynth – a small business card that uses touch sensors to play music. Although it can be used as an introduction for job prospects, Levi made the board as a learning tool to experiment with music, electronics and signal theory.
The MicroSynth features a minimal design with no microcontrollers, no logic chips, buttons or even a 555 timer. Instead, it employs resistive touch sensors and traceroutes, so pressing a key allows a tiny amount of current to flow through the touchpad when depressed. There's just enough current to turn on a resistor, which creates a musical note.
"Musical notes increase in frequency exponentially. Every octave is a doubling of frequency. But the keyboard increases resistance linearly for each key pressed, and the VCO oscillator outputs a frequency that's linearly proportional to an input voltage," Levi writes on his project page. "For example, It's easy to design a system that will output 10, then 20, then 30, then 40 Hz. It's a lot trickier to design a system that outputs 10, 20, 40, 80 Hz."
The keyboard interface circuit uses pair of op-amps to convert those linear changes in the resistance for those exponential voltages. The build is truly one of a kind, and Levi has uploaded a detailed walkthrough of the MicroSynth for anyone who would like to recreate the build.