How does a security passcard function?

A passcard resembles a credit card, with a magnetic stripe that can be read by a card reader. SrdjanPav / Getty Images

Security passcards are frequently used to provide access to restricted areas and buildings. The passcard might offer general access, meaning it doesn't store any personal data, or it could be individually encoded, containing specific details about the cardholder.

Generally, the data stored on an encoded security passcard consists of:

An individually-encoded passcard closely resembles a credit card. The back of the security passcard has a magnetic stripe, also known as a magstripe. This stripe is composed of minuscule iron-based magnetic particles embedded in a plastic-like material. Each particle acts as a tiny bar magnet, measuring around 20 millionths of an inch long. The magstripe resembles the material found in cassette tapes.

The magstripe is capable of being "written" on because the minuscule bar magnets can be magnetized with either a north or south polarity. When the bars all align in the same direction, the card remains blank. To encode data, a process called flux reversal is used. Essentially, the polarity of a bar is flipped, so that the north pole faces the north pole of the adjacent bar (N-N

) or the south pole faces the south pole (S-S). This reversal alters the magnetic field, which can be detected by the card reader. Since there are two possible flux reversals, N-N or S-S, two distinct states of information are created. This works similarly to the binary system of 1s and 0s used in computing.

The data writing process requires an encoder. This encoder contains an electromagnet, which serves as the encoding head. The solenoid is shaped like a ring, with a small gap in the middle. The north and south poles of the solenoid are positioned across from each other, creating a magnetic field. The strength of this field changes depending on the amount of current flowing through the solenoid. If the tiny magnetic bars are in the gap of the solenoid, the strength change can flip their polarity. By changing the polarity in a specific order, the encoder writes data to the card.

A card reader is designed to interpret the information stored on the magstripe. The reader detects the changes in the magnetic field caused by flux reversals on the card's magstripe. Most readers rely on one of three methods to read the card:

While most general-access cards use a magstripe, some employ alternative methods for access. One such method is embedding a tiny radio transmitter in the passcard. These cards can be "active" (containing a small battery) or "passive" (powered by the radio receiver). When the card is near the radio receiver, typically just a few feet or inches away, the security system detects the transmitted signal and grants access to the building or area.

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