Proximity card (or Prox Card) is a generic name for contactless integrated circuit devices used for security access or payment systems. It can refer to the older 125 kHz devices or the newer 13.56 MHz contactless RFID cards, most commonly known as contactless smartcards. The price of the cards is also low, usually US$2-$5, allowing them to be used in applications such as identification cards, keycards, payment cards and public transit fare cards.
Proximity cards use an LC circuit. An IC, capacitor, and coil are connected in parallel. The card reader presents a field that excites the coil and charges the capacitor, which in turn energizes and powers the IC. The IC then transmits the card number via the coil to the card reader.