Abstract: This device comprises a housing having a circular keyway, and a key having a cylindrical light-transmissive shank insertable into the keyway to energize a lamp, which directs light longitudinally along the shank and radially outwardly through a plurality of openings in the annular wall of the shank and onto a plurality of light pipes which are secured in openings formed in the key housing to open on the keyway at axially and angularly spaced points therealong. The opposite ends of the light pipes confront upon light-sensitive elements which form part of an electrical control circuit that produces an output which unlocks the device, when all of the light sensitive elements are energized, but which energizes an alarm if less or more than all of the light sensitive elements are energized.