Intruder Alarm Circuit Diagram

Circuit Diagram

Description

This is a circuit which sounds on the momentary breaking up of a guard circuit comprising a magnet and a reed switch a small permanent magnet is fitted at the edge of the door to be guarded, a normally open reed switch is fitted on the door frame or a wall facing the magnet.
In the normally closed condition of the door, the reed switch is very close to the magnet and hence the reed contact is kept closed. When the door is opened the reed switch contact opens. This triggers a scr, the gate of the scr is connected to its cathode through the reed switch. As long as the switch is closed, the scr dose not fire, but when the door is opened the switch also opens and the scr fires, supplying voltage to the alarm.

Engineers note:

This alarm looks so simple but it is more effective and fool proof the audio signals can further be amplified according to the area or the place it is fixed and the major plus point is that even if the door is closed after opening or in case if the door is opened an closed immediately the alarm keeps on blowing continuously. Constructional details are also added along with this article for better understanding.

Constructional Details:

 Source -http://www.electronguide.com/circuits_pages/IntruderAlarm.html