Wiegand decoder

Many security access control systems incorporate a data protocol called Wiegand. Wiegand was named after its inventor John R. Wiegand and the term, while defining a particular data format, is more commonly used to describe the Wiegand Effect.

The Wiegand Effect is exhibited when the internal magnetic fields in specially prepared wires swiftly reverse polarity when exposed to external magnetic fields. This reversing magnetic field can be captured as a negative-going electrical pulse.

The Wiegand data protocol incorporates two separate logic lines, a "one" line and a "zero" line. As the Wiegand effect can only produce negative-going electrical pulses, both lines will individually go low to indicate the presence of a bit. Wiegand wire (wire specially developed to exhibit the Wiegand Effect) is very hard to duplicate. This is why the technology is heavily used in security access cards and "pin" cards.

This simple decoder circuit can be used to convert dual channel, nega--
tive-going electrical pulses from a Wiegand magnetic reader head to standard hexadecimal format.

The circuit consists of a 74HC00 quad NAND gate and two cascaded 74LS164 8-bit shift registers. Where necessary, the number of shift registers can be increased to suit the bit depth of the Wiegand data stream. For example, we used 26-bit Wiegand for our experiments (two parity bits + three bytes of data) and this required four 74HC164s.