[Nicholas] built a simple NFC tag using an ATtiny84 microcontroller, four resistors, three capacitors, a diode, and an antenna. It implements ISO 14443-3, a standard for identification cards, and can communicate with the NFC chip sets found in most new smartphones.
This standard uses on-off keying for communication, which makes the hardware slightly more complex than the AVR RFID tag that we saw a few years back. The antenna and a variable capacitor form an LC circuit tuned at 13.56 MHz, which is the carrier frequency for the protocol. The diode acts as an envelope detector, letting the microcontroller … Read the rest