Open-source boards designed in KiCad by relic-se on GitHub, from 4 repositories. A board’s address here is its repository’s there.
relic-se/PicoSynth_Sandbox-hardware
Raspberry Pi Pico digital synthesizer board with I2S stereo audio i/o, MEMS microphones, MIDI i/o, a capacitive keybed, 1602 display, 2 rotary encoders, and LiPo battery. Designed for use with CircuitPython and synthio.
relic-se/C64UserPortMidi
Simple MIDI input and output interface using Commodore 64 user port. Utilizes an ATtiny2313 microcontroller and 6n138 opto-isolator.
relic-se/c64-eeprom
Simple AT28C64/256 EEPROM adapter for 2364/32 mask roms typically used for the Commodore 64 Kernal/BASIC/Character rom.