A burglar alarm operates by opening or closing of an electrical contact. For example, in a house, the doors and windows are pretected by switches near the hinges. The switches remain closed so long as the windows and doors are closed.
An opening of any one of these switches will trigger the alarm. Similarly, entry points may be protected by switches installed below doormats. These switches normally remain open and closing of any one will trigger the alarm. The burglar alarm circuit shown in figure below can accomodate any number of both types of switch.
In figure schematics, gates 2 and gates 3 form an RS flip-flop. When the “Arm” switch is put on, capacitor C3 keeps one input of gate 3 LOW for about 20 seconds. This resets the flip-flop and makes the output of gate 3 HIGH. The LED remains off. When capacitor C3 has charged, the alarm is ready for operation. The time taken by C3 to charge gives the owner an “exit-delay”, when he can arm the alarm, and gets sufficient time to move out and lock the premises without triggering the alarm.
If switches SW1, SW2, SW3 are open and S4 are closed all, the resistor divider network of R1, R2 keeps the input to gate 1 LOW. Its HIGH output keeps C2 charged via D1. A change in status of any one of the switches will make output of gate 1 go LOW and capacitor C2 starts discharging slowly. If the switch returns to its normal position within 10 second, no alarm is sounded. If it does not return to its original position within this period, the discharged capacitor C2 makes one ot the inputs to gate 2 go LOW and sets the flip-flop, thereby triggering the alarm.
The LOW going output of gate 3 lights the LED to give a visual indication. The time taken by C2 to discharge provides the owner with an “entry-delay” which permits him to open the door and disarm the circuit before triggering the alarm.