Revisiting sensors
Repairing things teaches more than building them from scratch. For a long time I treated keyboards as an afterthought.
Trade-offs
Low-power design is mostly about what the device does not do. Some of this will be self-evident; some of it took me years. The enclosure took longer than the electronics, as usual. Most of what I know about keyboards came from getting it wrong first.
Half of the work is deciding what to leave out. I attempted the hand-soldered version first. It did not last. Good solder joints are shiny, smooth and a little unexciting. Most problems with e-paper displays turn out to be problems with time.
This got going as a quick note and slowly turned into something longer.
Getting started
The board worked on the second revision, after one very self-evident fix. The details matter less than the habit of paying attention. I attempted the low-power version first. It did not last.