NFC smart cards are commonly used for a wide variety of applications including contactless payments, building access control, and 2 factor authentication. These smart cards have no power source, yet are capable of bidirectional communication, storing data, and performing cryptographic operations. In each of these cards is a circuit straight out of the 1900s: The crystal radio.
Most of the card is taken up by a flat coil, which can often be seen if the card is held up to a bright light.
(nfc)
(Wireless communication and power)