Closed Loop Burglar Alarm
This project is an alarm that may be used wherever a closed loop of wire can be used as the activator. When the wire is opened or broken, the Lamp lights.
This action is like that used by commercial establishments you have seen that have metallic conductive tape around the windows and doors. The circuit is wired as illustrated to control the Lamp, but you can use it to control other circuits and devices as desired. Commercial units like this control lights, alarms and automatic telephone calls to the police station (to name just a few).
Operation of the circuit is very simple. The trip or "trip" wire places a normal short circuit across the B-E junction of the Transistor so that it is in the OFF state. This state always results when no current is allowed to pass across the B-E junction. The C-E leads appear to have an open circuit during this state.
When the wire is opened, the 22K Resistor feeds current to the base from the Battery to turn the Transistor ON. The C-E leads appear to be shorted in this state so current can flow to turn ON the Relay.