Natural Ergonomic Keyboard, Type A
This is a column-driven split keyboard using three custom PCBs connected with ribbon cables. The left and right PCBs are passive: they only have the diodes and switches needed to make the matrix. The center PCB has an Adafruit Feather 32u4 and MCP23017 expander.
The left matrix has its rows and columns directly connected to the Feather. The right matrix has its rows connect to the Feather (using the same pins as the left matrix) and its columns connected to the expander. The expander uses the LUFA hardware TWI driver.
Bluetooth is enabled.
Make and install this keyboard (after setting up your build environment):