Infrared Motion Sensing Relay Switch

T.K. Hareendran is an electronics hobbyist, freelance technical writer and circuit designer


This IR motion sensor relay switch circuit is designed for use with all kinds of medium-power automobile/domestic 12V DC loads. It is a simple solid-state relay (SSR) switch, controlled by a standard passive infrared (PIR) motion sensor module. A PIR sensor is an electronic device that can measure IR light radiating from objects in its field-of-view.

Apparent motion is detected when an IR source with one temperature (such as a human being) passes in front of an IR source with another temperature (such as a wall). The PIR sensor module, centred on a PIR sensor, has elements made of crystalline material that generates an electric charge when exposed to IR radiation.

IR motion sensor relay switch circuit

Fig. 1: PIR motion sensor module and its dome shaped cover (inset)
Fig. 1: PIR motion sensor module and its dome shaped cover (inset)
Fig. 2: IR motion Sensor Switch
Fig. 2: IR motion Sensor Switch

Changes in the amount of IR striking the element change the voltages generated, which are measured by an onboard circuitry. The module contains a special filter called Fresnel lens, which focuses IR signals onto the sensor element. As ambient IR signals change rapidly, onboard circuitry triggers the output to indicate motion. Fig. 1 shows the PIR motion sensor module. It is usually hidden behind a translucent dome shaped cover as shown in the inset in Fig. 1.

Circuit operation

Circuit diagram of the IR motion sensor relay switch is shown in Fig. 2. It is built around a PIR motion sensor module (connected across CON2), MOSFETs BS170 (T1) and IRF9540 (T2), rectifier diode 1N4007 (D1) and a few other components.

The PIR sensor module has a 3-pin connection for Vcc, output and ground, and provides a single output that goes high when motion is detected. It also has a 2-pin jumper selection for single or continuous trigger output mode. The two positions are labelled H and L (Fig. 1).

When the jumper is at H position, output goes high whenever the sensor is triggered and retriggered. In position L, output goes high and low like a monostable timer every time the sensor is triggered. Retriggering pulse does not affect the output duration.

The circuit presented here is in repeatable trigger mode and turns T2 on when sensor output goes high. Output of the PIR sensor module is connected to the base of T1 (BS170) via resistor R1.



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