Infrared Alarm System for Detection in Doorways, Corridors, and Small Gates
This innovative infrared alarm system is ideal for detecting individuals moving through doorways, corridors, and small gates. Its transmitter emits an invisible infrared light beam, imperceptible to the human eye. When this light beam is disrupted by a person passing through, the receiver triggers a buzzer alert. The transmitter and receiver circuits featured in this design offer a substantial range of several meters, ensuring reliable detection irrespective of ambient light conditions. Only in rare instances where the receiver sensor is exposed to intense, direct sunlight, additional screening measures are necessary.
Modulated Infrared Transmission for Enhanced Sensor Response
In this system, the transmitter employs a modulated approach rather than emitting a continuous infrared signal. The 36-kHz carrier used to pulse the Infrared Emitting Diode (IRED) is itself switched on and off at a rate of about 300 Hz. This modulation technique is crucial because typical infrared sensors, including those recommended in the diagram, do not respond well to a continuous influx of infrared light. By intermittently switching off the IR source, even for brief intervals. IR detectors have the opportunity to recover, optimizing their ability to minimize responses to ambient light.
The transmitter circuit comprises two oscillators centered around the widely used 555 IC. The current-saving CMOS version TLC555 (or 7555) is employed here, or alternatively, the two 555 ICs can be replaced by a single TLC556 (or 7556). IC1 functions as the 300-Hz generator, while IC2 serves as the 36-kHz source. The IRED type LD274 is pulsed at a relatively high peak current through the driver transistor T1. For applications with limited IR beam coverage, resistor R5 can be increased to conserve current consumption.
Simplified Receiver Design for Effective Infrared Detection
The receiver design is equally straightforward and also relies on a CMOS 555 IC. When the sensor detects infrared light from the modulated transmitter, the reset input of the 555 IC is kept low, ensuring the buzzer remains silent. Components D1 and C2 function as a low-frequency rectifier, neutralizing the impact of the 300-Hz modulation on the transmitter signal. Upon interruption of the infrared light beam, the oscillator constructed around the 555 is activated, generating a warning tone. The test values indicated in the circuit diagram represent average DC levels measured with a Digital Voltmeter (DVM) under light and no-light conditions, although most test points carry rectangular or sawtooth waveforms.