He’s packed the entire device (called the Drum8 Vintage) into a single ATtiny84 14-pin DIP package ... [Jan] is offering the chip itself for $20, and has made a limited edition version that ...

One of Atmel’s smallest microcontrollers, the ATtiny ... of the chip design. The RFID circuit only uses two of the ATtiny’s pins and neither of which is the ground or power pin.