This door opening alarm alerts you of intruders. You can use it for up to three doors. You simply need to fit a small unit including reed switch on each door frame and fix a magnet on the moving door such that the magnet aligns with the reed switch when the door is closed.
A separate unit incorporating the power supply, three LEDs and a buzzer is to be kept by your side inside your room. A three core ribbon cable from this unit goes to each door unit. One core goes to the positive terminal, second to the negative terminal and third to the output of the unit. When the door is closed, the reed switch terminals are shorted and the alarm does not sound.
Door opening alarm circuit
When door 1 is opened by someone, transistors in the corresponding door unit conduct and the buzzer sounds. LED1 glows to indicate opening of door 1. Due to diode latching action, the alarm will sound continuously even after the door is closed. It can be stopped only by pressing the reset switch of the door unit.
Regulated 9V to 12V DC for operating the circuit is derived from AC mains and fed to the three units mounted on the doors. Battery-backup is also provided. When all the three doors are simultaneously opened, all the three LEDs will glow.
This arrangement can be extended for more doors by increasing the number of door units connected to the audio-visual indication unit.
The article was first published in June 2005 and has recently been updated.