See a writeup here: https://imateapot.dev/homemade-power-meter/
This is the complete code for a crank-based power meter, transmitting over BLE to a bike computer. Schematics and a hardware parts list for the hardware, as well as notes on calibrating, are on the wiki. In all, it's a full instruction set for building your own cycling power meter, compatible with any bike computer that supports Bluetooth (i.e. any Wahoo or Garmin).
This is a fun project and I trained with it just great for a year. It can handle the bumps and (minor) hits from the road, but I did struggle to get it any kind of water resistant. I'd say that was the biggest pain point. It's also not a super attractive form factor, but function > form, right?!
As far as accuracy, it's within a reasonable margin of error when compared with commercial units, and usually on the low side. As far as training goes, it's for certain valuable, because consistency is most important. As far as ego goes, I guess it's not the friendliest. I tested these times and durations with about these results:
- Wahoo Kickr, the programmed 20 min FTP test. This meter read 9% lower.
- Computrainer, 40 min at ~190 watts. This meter read 6% higher.
- Favero Assioma Duo, 40 min at ~190 watts. This meter read 6% lower.
This project is licensed under the terms of the MIT license.