Abstract: A latch tongue of a lock has a pair of contact points which are respectively connected to the gate and the cathode of a silicon controlled rectifier. When the lock is closed, the latch tongue is in a latch tongue housing which connects the points together thereby providing a connection between the gate and cathode. Additionally, the closing of the lock with a key causes a series connection of the anode and cathode of the rectifier with a bell and a voltage source. When the latch tongue is removed from the housing, the gate to the cathode connection is broken thereby causing the application of the voltage of the source to the bell.