The Arduino Uno-compatible board has an MCS-51 (often called 8051 instead) instead of the usual ATmega328P/ATmega168. Specifically, [ElectroBoy] uses the AT89S52.

The miniscule size of this self-balancing robot makes it a cool project. It actually uses the motor and wheels from a small toy car. But when you look into how the balancing act is performed it get… ...