Abstract: A door lock includes circuitry for signalling the condition and operation of the lock. The lock has a bolt and two separate locking mechanisms, each operable to lock the bolt in its projected position. A bolt position signal switch generates a signal indicating the position of the bolt, either projected or retracted. Two lock signal switches are operated by the respective locking mechanisms to provide respective lock position signals. An intruder alarm using the lock includes a perimeter alarm subsystem, an interior alarm subsystem and an alarm conditioning control. The control is responsive to an unlocked condition of the door as signalled by the bolt position signal to disable both the perimeter and the interior alarm subsystems. When the door is locked from outside the premises protected, the control activates the perimeter and interior alarms, while locking of the door lock from inside locks the perimeter alarm only.