Arduino Uno is the most popular and widely used development board. It is powered by an ATMega328P microcontroller. It is the most popular choice among the community because it’s, cheap, easy to learn ...

The voltmeters that [Tom] bought used a 8 bit STM8S003F3P6 microcontroller. He reverse engineered the device and re-created the schematic to find out where the I2C and programming pins would be.