Oskitone has created several synthesizers, including the OKAY Synth followed by the upgraded version OKAY 2 Synth. Last month, Oskitone released the latest POLY555 Synth, a polyphonic synth based on 555 timers. This "spiritual successor" features 20 key controls along with a built-in amplifier and speaker.
If you're asking why 20 key controls? That's because the synthesizer has 20 555 timer circuits under the plexiglass window, which can be seen in the reference image. Each circuit comes with an LED that glows only when it's supplied power. Also, volume is controlled via a small 3D-printed wheel on the side.
The 555 timer circuit is called an astable multivibrator or commonly known as an oscillator, and is modified to have a 50% duty cycle with C1 acting as a bypass capacitor for stability. RV1 resistance is a 1k trimmer potentiometer, while the oscillator's frequency is determined by the C2 capacitor and the resistors R3 and RV1.
While the OKAY Synth may not be open source project, this time Oskitone has shared all the OpenSCAD code used to generate the 3D models, as well as the Kicad PCB files on GitHub.
If you're interested in purchasing the synthesizer, the model is available in two options — POLY555 DIY Synthesizer Electronics Kit and an assembled POLY555 Synth. The latter will be priced at $220 once released. An assembly guide is available if you plan to buy a DIY Kit along with the 3D-printed parts.