It’s really just an ATTiny85 – a favorite of the crowd that ... Four of the six IO pins on the ‘Tiny85 are broken out on a pin header; two are used to communicate with the BTLE module.

The RFID circuit only uses two of the ATtiny’s pins and neither of which is the ground or power pin. The ATtiny is equipped with protective diodes on its input pins, and if you apply an AC ...