This circuit uses invisible infrared light to detect the movement of people through the door. A short beep will be generated when the infrared beam breaks. So it is ideal to monitor the passages in shops, banks etc where many people are moving.
Two Infrared LEDs always emit continuous infrared beam to the Photodiode. The IR LEDs and Photodiode are placed on the opposite frames of the door and properly aligned. Resistor R1 is the current limiter giving around 70 mA current through the LEDs which is necessary to increase the output of IRLEDs. IC1 is designed as Current to Voltage converter with the reverse biased photodiode connected to its inverting input. The non inverting input is directly grounded.
Resistor R2 and VR1 forms the feedback loop to adjust the sensitivity of the IC. Normally the Photodiode generates a small current by accepting the energy from the infrared Beam. This tiny current will be amplified by IC1 and gives a high output. This forward biases T1 and it conducts. The emitter current from T1 keeps T2 off since it is a PNP transistor. Since T2 is off, Buzzer remains silent. In short, in the standby mode, LED glows indicating the active state of circuit and buzzer remains off.
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