Motion Activated PIR Relay Circuit

The submit reveals an easy motion activated PIR relay circuit widely available for activating lights only in the occurrence of a human, as a result conserving important electric power.

Following is an ordinary circuit that activates a relay when a living being (a human) is detected by the PIR sensor. Here PIR symbolizes Passive Infrared sensor. It doesn’t deliver any infrared radiations to sense the occurrence of a living being but meanwhile it detects the infrared radiations released by them.


This circuit consumes a HC – SR501 PIR motion sensor that may be the heart of the circuit. Originally when the motion is detected by the sensor, it delivers a small signal voltage(usually 3.3 volts) that may be provides to the base of the transistor BC547 by way of a current control resistor and thus, its output goes high and it switches the relay on. This relay could possibly be designed to function with a electrical bulb or a tubelight, night lamp or anything else that works well on 220VAC.

This circuit is generally found in gardens, so that at night, when we go for a stroll in the garden, the circuit switches on a light automatically and it continues to be lit until we are in the sensor’s vicinity and it gets turned off when we walk away from that place and for this reason minimizing the electricity costs. Here’s a back view of the sensor HC-SR501…


The sensor comprises of two preset resistors widely available to regulate the delay time and sensing range. The delay potentiometer could be changed to determine the time for which light continues to be on. The sensor when obtained, it consists of the default mode ‘H’ which signifies the circuit switches on the light when motion is detected and it continues on for preset time and after the preset time lapses, if the sensor could still sense motion, it can not switch the light off and if it might not detect the motion, it switches off the light. Here I will discuss the technical knowledge of the sensor HC-SR501…

Working voltage range: 4.5VDC to 12VDC.

Current Drain: <60uA

Voltage output: 3.3V TTL

Detection distance: 3 to 7 metres(can be adjusted)

Delay time: 5 to 200 seconds(can be adjusted)

Considered one of the weaknesses of PIR motion sensors is that its output goes high although a rat or a dog or some other animal moves in front of it and it switches on light unnecessarily. In cold countries, the sensor’s sensing range enhances. On account of low temperature, infrared radiations released by humans travel more distances and for this reason producing unwanted switching of lights. If fitted in backyards, you will find possibilities of activating of light when a car passes by because the radiations provided by hot engine of car fools the sensor.


D1, D2 – 1N4007,

C1- 1000uf, 25V,

Q1 – BC547,

R1 – 10K,

R2 – 1K,

L1 – LED(green)

RY1 – Relay 12V

T1 – Transformer 0-12V.


PIR Motion sensor built unit

PIR Motion sensor built unit

After conducting the manufacturing of the circuit, enclose it in an effective casing and use a separate casing for the sensor and join the sensor to circuit applying long wires so that you could place sensor at the place you require to similar to a garden and circuit is going to be inside in order that the circuit is secured from weather.

And you should definitely apply a separate PCB for relay.

Also, don’t neglect to exercise a perfect relay with correct current and voltage rating. You may use a terminal block which hooks up to the relay’s switching contacts, and manage it as proven in image so that you could adjust the electrical device hooked up to relay contacts effortlessly.


The ways to access this sensors save electricity to great extents. It may possibly decrease your electricity bills too!


If the above design is designed to function with an alarm and a lamp such that both the loads function in the course of night but the alarm only in the course of day, then the diagram could be improved in the following means.


PIR alarm with light circuit with day night automatic switching action