I've been working off and on for the past several months on a from-scratch split column staggered keyboard design for the past several months, and it's finally in a state where I'm ready to show it off here!
I'm currently using this as my daily driver (and have been been since mid-February).
The 3D model is built in OpenJSCAD and the design is fully
open source; a 3D viewer link can be found on the Github project README.
I'm using a Arduino Pro Micro knock off (yay $3 boards on Aliexpress!) and the QMK firmware. An HDMI cable attaches the two halves -- cables are easy to find and it allows enough conductors to not need to use an IO expander chip on the second half.
No assembly guide as of yet, but you can hand wire it just as you can the Atreus (which has a helpful guide already).
