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Ursa Minor keyboard

Fellow Redditor u/scissor_rock_paper finished his custom split PCB design dubbed Ursa Minor.
Published February 6, 2022

The Ursa Minor is the first PCB project by Mark Story aka scissor_rock_paper: a low profile split, abundant in keys (num row included), with a single on-board microcontroller and IO expander.

I built a handwired build with the same layout earlier this year. I wanted to go deeper into the custom keyboard pool. So the logical step was to try a PCB. I spent a few months learning KiCad and redoing my design several times – scissor_rock_paper.

The GitHub repo contains two versions of the PCB:

  • The base model has a basic board design. It has has no frills and a smaller parts list. It was intended to be used caseless with self-adhesive feet, or bare on the switch pins/diodes.
  • The bling version builds on the base one and adds per-key RGB lighting via RGB matrix drivers over I2C.

The keypad shape and thumb arc remind me of Ursa Minor – scissor_rock_paper.

Thus, the PCB's silk screen sports constellations on the back:

Ursa Minor GitHub repo


  • ATMega32u4 microcontroller
  • MCP23017 IO expander
  • Single MCU – Instead of dual microcontrollers, this design uses only one microcontroller paired with an IO expander. This lets more cost effective components be used and only requires a single flash operation when updating firmware.
  • IS31FL7333 RGB controllers (optional)
  • USB-C ports for upstream and interconnects
  • Thumb arc – The thumb cluster follows the thumb's natural arc, and all keys are reachable in 1u of space. Unlike many thumb arcs, this one requires no reaching under the palm.
  • No Frills, or Frills – The base model is a simple stripped down keyboard. It has no lights, no encoders, just 58 switches and wires. The bling version includes support for per-key RGB, and case mounting holes for more elaborate enclosures in the future.

The specific build in the picture features Choc Red switches (lubed with Krytox205g0), MBK Legend Glow caps and cables from Zap Cable Kits.

Published on Sun 6th Feb 2022. Featured in KBD #64 (source).

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