PIR sensors or Passive Infrared Sensors are certain type of sensors that detect infrared radiation. All warm-blooded beings such as humans or animals emit certain amount of IR radiation or heat which can be detected by IR sensors. Active IR sensors emit IR radiation and measure the reflected radiation. If anything with heat moves between the emitter and the sensor, the radiation is disturbed and a signal is triggered but in Passive IR sensors, they do not emit any IR beams but detect the infrared radiations from outer source. When an object is in the field of view of a sensor it provides a reading.

The type of IR sensors used in automation are usually Pyroelectric motion detector type i.e. the motion of an IR body is detected. PIR sensors have convex lens that are designed to collect the infrared radiation from the different spatial zones. Usually there are a pair of sensors that detect IR radiation in the surrounding under normal condition the signal produced by both of them are cancelled out but when a person enters the feild of view one of the sensor will detect more radiation than the other and a difference signal is generated. We can make use of this signal to drive a switching transistor, MOSFET or a relay using an amplifier or use as an input to a micro-controller depending on the application.


1) PIR sensor

2) 12v Relay rated for 250V AC, 7A

3) BC 547 or 2N7000 or equivalent

4) Resistor 220 ohm quarter watt

5) A 12v DC supply circuit

6) Diode 1N4007


The circuit is pretty simple as shown. We use the signal from PIR sensor as the base threshold to turn on the transistor completing the path for the relay coil. Thus the relay is energized by the 12v supply, as a result the incoming phase line will be connected to the lamp as the relay switches to its normally open position.


The Sensor works for a voltage range of 5-20V DC. A different type that works on 230VAC is also available. According to the data sheet the sensor can detect up-to a distance of 7 meters and a conical field of view of about 110 degrees.

The sensitivity and the delay can be adjusted using the two trimmers provided. The delay means the period of time for which the sensor will keep output the signal high after it has been triggered once. The sensor can work in two modes: Single or Non-Repeat mode and Repeat mode.

In single mode the sensor triggers upon detecting a human motion and will stay ON for the time set by the delay trimmer (say 20 seconds) then after 20 seconds the output will go low even if a human is present. It will re-trigger back to high/ON after a few seconds (Block time) if the human is still present. Block time is time when the sensor goes low after it was once triggered and during which it is disabled or will not detect any motion (By default its 3 seconds).

In repeat trigger mode the sensor will trigger on presence of motion of a person and will keep re-setting the delay timer till the person leaves. So only after 20 seconds (or how you set the delay trimmer) since the person has left, the sensor will output low.

The jumper set can be used to configure between the two modes. In this setup we use the Repeating trigger mode.


The PCB was designed in proteus design suit but since it is a small circuit it can be done using a copper dot board instead of a PCB


For the relay as well as the PIR sensor we need a DC power source. Battery is not recommended as it will drain rather quickly. I used a 12V SMPS power supply unit since it is light weight and compact.

Connections are simple and straight forward as in the diagram and it all need to fit inside a gang box and should be mountable on the wall. Keep the lens/dome of the PIR oriented in such a way that it can detect presence.

Step 5:

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