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 key at positions on a 4 .times. 4 regular grid of possible positions. The reader includes row of 4 Hall Effect sensors adjacent keyway entrance so that the 4 rows of magnet positions pass successively across them. Signals generated in sensors are transmitted to signal storage units and extracted from the storage units on withdrawal of the key. Activation of sensors and transmission of signals is controlled by successive interruption by leading edge of key of light beams along the keyway.