Working of the Circuit
The circuit continuously transmits infrared beam to the IR receiver and if that beam is broken by the entry of any person the alarm will turn ON for a preset time period, this time period can also be adjusted by the 1M variable resistor.
The circuit is divided in two parts which are transmitter and receiver. The transmitter part of the circuit is very simple it only consist of an IR LED and a 100 ohms resistor. The operating voltage of the transmitter circuit is 3V to 5V. The receiver part of the circuit consist of three ICs, one TIP125 transistor and other passive components. The receiver part of the circuit can be operated from 5V to 12V DC.

Placement and Alignment
A proper placement and alignment is very important for proper working of the circuit. Both the IR transmitter and IR receiver LEDs should be placed facing each other at a distance of 1 meter to 10 meters. The distance also depends on the quality of IR Tx and Rx LED pair.

Circuit Adjustments
After the circuit is completely built it will require some adjustments for the first time. First set the 1M variable resistor to its lowest possible value so the 555 timer will only activate the alarm for few seconds. Now adjust the 100K variable resistor for the max distance between the IR Tx and Rx LEDs. After these adjustments set the 1M variable resistor for the required alarm switch ON time.

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Here is an effective burglar / intruder detection alarm system to detect burglary or intrusion at home or anywhere needed. The circuit is using an invisible infrared beam link that should be placed at door or window or any required place to detect the entry of intruder.
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Burglar / Intruder Detection Alarm using Infrared Technology