125KHz proximity card

Proximity 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 14443 (Proximity Card) standard. There is also a related ISO 15693 (Vicinity Card) standard. Proximity cards, also known as prox cards have a range of 0-3 inches in most instances, allowing the user to leave the card in one's wallet, or purse. The price of the cards is also low, 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 built-in chip of 125KHz cards are EM4100, TK4100, TK28 optional.