This is a percussion and drone synth module based on ideas presented in the 2015 Hackaday article “Logic noise: More CMOS cowbell!” by Elliot Williams. (See also Cmos noise bell .) The PCB and front panel are Kosmo format but the same circuit could be implemented in Eurorack format, or other ±12V formats, with no changes other than PCB, panel, and jack dimensions.
The module consists of two separate sections. One is a 2-oscillator circuit based on the “cowbell” section of the Hackaday article. The other is a 4-oscillator circuit, lying sort of halfway between the “cowbell” and “hi hat” designs and designed to use most of the IC functionality left over from the “cowbell” side. Whether it leans more toward bell or cymbal sounds I leave for you to decide. The two sections differ only in the number of oscillators and XORs used.
Aside from a few component value changes, notably a larger capacitor allowing longer decay times, there are a couple of significant modifications to Williams’s design:
- The 2-diode “VCA” has been replaced by a 4066 analog switch “VCA” as discussed in the article’s comments and at http://files.electro-music.com/forum/viewtopic.php?highlight=diode+vca&t=23960.
- A switch has been added to bypass the EG/VCA section, sending the output of the final XOR straight to the output buffer. This allows using the module for noisy drones.
Pots are used for variable oscillator frequencies and amplitude envelope decay rates.
Schematics, Gerber files, BOM, build notes, and other information at GitHub - holmesrichards/NoiseBells: Percussion module based on Hackaday CMOS Noise article
Extra boards and panels should be up on Tindie early next week. Sold out!