Proximity card

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. Modern proximity cards are covered by the ISO/IEC 14443 (Proximity Card) standard. There is also a related ISO 15693 (Vicinity Card) standard. Proximity cards are powered by resonant energy transfer and have a range of 0-3 inches in most instances. The user will usually be able to leave the card inside a wallet or purse.

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. The card readers communicate in Wiegand protocol that consists of a data 0 and a data 1 circuit. The earliest cards were 26 bit. As demand has increased bit size has increased to continue to provide unique numbers. Often, the first several bits can be made identical; these are called facility or site code. The idea is that company Alice has a facility code of xn and a card set of 0001 through 1000 and company Bob has a facility code of yn and a card set also of 0001 through 1000.

Lots of companies use proximity cards to control physical access. An employee holds their card within a few inches of the reader; the reader receives a unique id from the card and transmits it to some central computer that tells it whether or not to open the door. This is rather magical, considering that the tag is credit card-thin and contains no battery. The trick is the same as for RFID tags. The reader constantly transmits a rather strong carrier; the tag derives its power and clock from this carrier, kind of like a crystal radio. The tag changes how much carrier it reflects back at the reader—loosely, it makes the circuit across its antenna more like a short or more like an open—to transmit its code.

The reader and the tag both have antenna coils tuned to the carrier frequency; they work like a loosely-coupled resonant transformer. Or at least that's the theory. I couldn't find any credible documentation on the protocol used by the particular proximity cards that were available to me (Motorola's Flexpass). I did find a datasheet that claimed that they worked with a 125 kHz carrier. I wound a couple dozen turns of magnet wire on a 4" form, taped it to a reader, and 'scoped the coil. There was indeed a 125 kHz sine wave, large, a few volts peak to peak. The cards did, at least, work at 125 kHz.