We design open-hardware electronics. We also sell some of these in kit form, including cases we 3D print ourselves.
A simple MIDI and CV/Gate sequencer. It has 16 steps, a small screen and a single rotary button. A sequencer cannot be much simpler.
An extensible retro computer based on the 6502 processor. It has the processor, RAM and ROM on the mainboard, and can then be extended by whatever you decide. Check out the Planck 6502 website
This is a VIC20 emulator that runs on the Raspberry pi pico. It’s board is tiny, about 5x5 cm, but can almost completely emulate the VIC20.