A special feature of this infrared burglar alarm is latching operation. Also, the circuit is highly sensitive.
The circuit comprises transmitter and receiver sections. Whenever IR beam between the transmitter and the receiver is interrupted, the alarm circuit is triggered and the buzzer sounds continuously. It can be reset only by pressing the reset button.
The transmitter section is built around IC 555 (IC1), which is wired as an astable multivibrator and produces about 38kHz frequency. IR LED1 emits 38kHz modulated signals up to a distance of 4.6 metres (15 feet).
The modulated IR beam from the transmitter continuously falls on the receiver section built around IR sensor TSOP1738, IC CD4011 and some discrete components. The receiver sensor TSOP1738 responds to 38kHz frequency. The modulated IR beam falling on receiver sensor TSOP1738 makes its output low and when somebody crosses the path of IR rays it senses this and its output goes high momentarily, which triggers the flip-flop.
The flip-flop circuit uses IC CD4011. Two NAND gates of IC2 are used here. Input pins 2 and 13 of IC2 are normally held high. When the IR beam falls on TSOP1738, the output of IC2 remains low. The alarm circuit de-activates due to cut-off condition of transistor T2 and the buzzer remains silent.
The IR transmitter and receiver units are mounted opposite to each other on the sides of entry door or gate. When the invisible beam between the transmitter and receiver is interrupted by anybody passing through, the output of the receiver goes low for a moment and triggers the flip-flop. Output pin 3 of IC2 goes high and latches at that position. The alarm circuit activates due to conduction of transistor T2 and the buzzer sounds until the receiver circuit is reset through switch S1.
Assemble the transmitter and receiver on separate, matchbox-size PCBs. For alarm purpose, use a piezobuzzer having internal oscillator.
The circuit works off 6 to 9V DC. Even unregulated supply can be used. After connecting the power supply, the alarm may trigger. Press reset switch to make the circuit ready for detecting burglars.