A proximity card reader is a type of reader that eventually replaced card readers that used the Wiegand effect. This effect was used in magnetic card readers that were very popular when the technology was invented. Each of the keycards used in these readers had strands of Wiegand wiring embedded in them. When the wires were read by the reader device, they changed the magnetic state of a series of coils. This then opened the electrically locked door. Wiegand cards were harder to fake and were more durable than both magnetic stripe cards and barcodes, but today, they have been eclipsed by the proximity card reader.
A proximity card reader, which is still sometimes called a Wiegand output reader even though it no longer relies on the Wiegand effect, creates a field anywhere from one inch to twenty inches around it. This field reacts with the correct cards, which each contain a simple LC circuit. When the card enters this field, the field causes a coil inside this circuit to become excited. This then charges a capacitor that turns on the integrated circuit. The circuit sends the card number to the coil, which then transmits the number to the card reader. Many proximity card readers will still actually read the Wiegand cards as well, allowing companies to upgrade to a proximity card reader without completely redoing every security card.
Many proximity card readers use a 26-bit Wiegand format. This format makes use of a site or facility code. This code is unique to the location, but it is imprinted in all of the cards in the set. This means that one organization can have a set of cards that have their own unique card numbers but have the same facility code. Another organization may have cards that have the same numbers, but since the facility code is different, the cards won’t work for readers installed at the first organization. This means organizations can all have cards that start their numbering at one instead of every card needing its own unique number.
However, there is one issue with proximity card readers and numbering—there is no overall organization that keeps track of facility numbers. This means that two different proximity card manufacturers could use the exact same facility number for their readers, which in turn would compromise security for the organizations who used these readers. While this compromise would not be intentional, it could lead to some serious problems. Because of this security issue, many manufacturers have started using formats that go beyond the standard 26-bit Wiegand format.