Abstract: Access control system in which a magnetically coded key or card is inserted into a reader which reads the recorded code. The key is coded by small permanent magnets embedded in non-magnetic body of the key at positions on a 4 .times. 4 regular grid of possible positions. The reader includes 16 Hall Effect devices which are disposed adjacent keyway in the reader so as to register with the 16 possible magnet positions on the key when the key is inserted. Signals are generated in those Hall effect devices which register with magnets in the key and these signals are amplified to produce reader output.