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 vertsatile Uno Rev3 is capable of more than just a board for experimental and education projects ...

Arduino is one of those boards that has become synonymous with hacking and making. Since its introduction in 2005, over 700,000 official Arduino boards have been sold, along with untold millions of… ...