Automatic Room Light Controller circuit Using PIR Motion Sensor And Transistor.


The main idea of this project is when any human, animals or any object come in the range of PIR motion sensor, the light in the room must glow automatically. This project can be used in smart room where automatic control of light is required. This circuit can also be implemented any where like staircase or front gate etc., and when the PIR sensor detects person movement it will triggers transistor and makes light glow.

This project can plays a important role in saving the electrical energy. A PIR Motion Sensor will automatically detect when someone enters the room to glow the light. After a few minutes(adjusted by your desires) if there is no movement detected, the lights will then automatically turned off.

PCB Manufacturer

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Bill Of Materials

1BC547NPN transistor1
2SCHR501PIR motion sensor1
3Power Adapter12 volt1
4Led Strip12 volt1

PIR Motion Sensor

A passive infrared sensor is an electronic sensor that measures infrared light radiating from objects in its field of view. This type of motion sensor is only the receptor of infrared waves and does not release any infrared beam so they are called passive infrared sensor.

PIR Motion Sensor

Working Principle of PIR motion sensor

All the living beings emits infrared radiation to surrounding. Warmer the object more will be the infrared emission.

The Passive Infrared sensor is a thermal infrared sensor that detects the motion of objects by these infrared radiations variations in environment.

The sensor has two or more equal slots made up of Pyroelectric material that is very sensitive to Infrared. When there are no object in the sensor range, both its slots sense the same amount of Infrared radiations.

When a warm body comes in the range of the PIR motion sensor, it passes through both the slots one after the other. The moment it intercepts the first half of the sensor, a positive differential change is caused between the two halves. Similarly, when it leaves the detection area a negative differential change occurs between both the slots. These changes in the pulses are the indication to the sensor that there is some motion in its detecting zone.

PIR Sensor Working

Modes and Adjustment of PIR Sensor Light


PIR sensor waveform

Normal Modes

When the Passive infrared sensor detects motion, it sets the output to High until a certain predefined time by Potentiometer. This mode is independent of the movements during the output HIGH time. In the figure above, t1 is first motion detected. The output go high when the first motion is detected. t2 and t3 are second and third motion detected but in this case the output is not set high as it falls in delay interval. After the output is set for some predefined time there is fix delay. This mode is also independent of the movements during this fixed delay time. The output is set high for the fourth motion (t4) but the output is not high for t5 as it fall in fix delay time.

Re-Triggering Mode

In this mode when the PIR sensor detects motion, it sets the output to High until a set adjusted time but if it detects motion during its output High state 1, it again triggers the sensor to increase the output high period of the sensor.

In re-triggering mode, t6 is the first motion detected. t7 does not trigger the sensor because it is within the fixed delay interval. t9, t10, t11, which occurs after t8 triggers the sensor, extend the time delay since they occurred within the adjusted delay interval.


Sensitivity Adjust

With the help of sensitivity Potentiometer, We can adjust the working range of this PIR sensor.

At the beginning, you must play with this potentiometer so that you can set the appropriate setting for your desired purposes.

If you increase sensitivity it is beneficial for long range.

If you decrease sensitivity it is beneficial for short range.

Delay Time Adjust

The Delay Time indicates how long the PIR sensor will keep the Output High (ON) after detecting motion.

We can set the delay time from few second to five minutes.

Circuit Diagram

Circuit diagram of PIR motion sensor and led strip

The above figure is the circuit diagram of Automatic Room Light Controller. It has very simple circuit which contian PIR Motion Sensor, Transistor, Led strip and power supply.


Emitter – GND (-ve terminal)

Base – Data pin of PIR motion sensor

Collector – ( – ve pin of LED strip)

PIR sensor

Vcc – Positive terminal of battery ( +12V)

Gnd – Negative terminal of battery.

Data – Base of transistor

Led strip

+VE terminal – Positive terminal of battery (+12V)

-VE terminal – Collector of transistor.

Working Principle

We are using 12 Volt power supply and led strip of same 12 Volt rating. We know that, If we directly connect our led strip to 12 volt power supply, the led will glow. But the objectives of this project is to glow the led only when the sensor detects the motion. If we measure the voltage across the data pin of PIR sensor, we get 0 volt when no motion is detected and 3.3 volt when motion is detected.

We have use BC547 transistor as a switch. If the voltage at base cross the threshold limit (ie 0.4 volt) it triggers the base emitter junction of the transistor. We have connected data pin of the PIR sensor to base of the transistor.

The positive terminal of LED strip is directly connected to +12 volt and negative terminal of the led strip gets the ground only when base of the transistor is triggered. When the motion is detected, there is 3.3 volt across the data pin of the sensor which is enough to trigger the base emitter junction of the transistor. The led strip gets the ground and it glows.

When the motion is not detected, there us 0 volt across the data pin of the sensor which cannot triggers the transistor. The led strip cannot get the ground and it does not glow.

Youtube Video

Image During Trial

motion detector using pir motion sensor
Motion detector using pir motion sensor

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