Abstract: A detector lock assembly comprising a cylinder body having a spaced apart front and rear end with a bore therein extending along an axis and a plug rotatable in the bore on the axis with means defining a keyway in the plug and adapted to receive therein a key of a defined length and configuration to permit rotation of the plug relative to said cylinder body. Detection means is operatively associated with the cylinder body at the rear end thereof so as to establish an electrical circuit when a key or other object is inserted within the keyway that has a length that is less or greater than the defined length for the key.