I recently made an automatic toothbrush timer, using an accelerometer and a microcontroller. It is intended to be a fun and practical example for the emlearn-micropython machine learning library.

Below is a quick demo of the gadget in operation:

The project is available at: https://github.com/jonnor/toothbrush/
You will find there the software, firmware, data, ML training pipeline and 3d-printed parts.