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Table of contents
What is a PIR motion sensor?
How does a PIR motion sensor work?
Active Motion Sensors
Applications of Motion Sensors
Frequently Asked Questions
Why it’s called a passive sensor?
What is the range of the PIR sensors?
What are active motion sensors?
Last Updated: Mar 27, 2024

Motion Sensors

Author Ayushi Poddar
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A motion sensor, also known as the motion detector, is a device designed to detect and measure movement. Unlike other sensors, a motion sensor is typically an embedded system that consists of three major components that are a sensor unit, an embedded computer, and a hardware unit, or what we call the mechanical component. These three parts vary in size and configuration, and motion sensors can be customized to perform particular functions.

What is a PIR motion sensor?

A PIR Sensor or a Passive Infrared Sensor is a motion sensor that can measure the infrared light radiating from objects. These PIR sensors are used as motion detectors in security alarms and automatic lighting applications. A general PIR motion sensor consists of three pins which are -

  • Pin1 is the drain terminal of the device and is connected to a 5V DC supply.
  • Pin2 is the source terminal of the device and is connected to the ground terminal via a 100K or 47K resistor, and this pin is also the output pin of the sensor.
  • Pin3 is connected to the ground.
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How does a PIR motion sensor work?

A PIR motion sensor detects animal or human movements in a specified requirement range. The sensor is made up of a pyroelectric sensor, which can detect the different levels of infrared radiation. The detector itself does not emit energy, but it passively receives it.

PIR sensor is a passive sensor, and as mentioned before, it doesn't radiate energy to space, rather, it receives the infrared radiation from the human body to make an alarm. 

Any object with any temperature always radiates infrared rays to the outside world. The surface temperature of the human body is around 27 degrees Celsius to 36 degrees Celsius, and most of its radiant energy is concentrated in the wavelength range of 8um to 12um.

PIR sensors are classified into infrared detectors and control sections, and the most widely used infrared detector is the pyroelectric detector. The pyroelectric detector is used as a sensor for converting human infrared radiation into equivalent electricity. When the human infrared radiation is directly irradiated on the detector, it will cause a temperature change to output a signal. Usually, a plastic optical reflection system or plastic Fresnel lens is used as a focusing system for infrared radiation.

In the detection area, the detector's lens receives the infrared radiation energy of the human body through the clothing and focuses on the pyroelectric sensor. Suppose the human body moves in the surveillance mode. In that case, it enters a certain field of view in sequence and then walks out of the field of the view, the sensor sees the moving human body for a while and then stops to see it. Thus the infrared radiation of the human body constantly changes the temperature of the pyroelectric material. 

Active Motion Sensors

Active motion sensors have both a receiver and a transmitter. It detects motion by observing changes in the amount of sound or the radiations reflecting back into the receiver. When an object alters or interrupts the signal field, an electric pulse is sent to the computer which then interacts with the mechanical components. The common types of active motion detectors use ultrasonic sensor technology; these motion sensors produce sound waves to detect the presence of objects. There are also microwave sensors (which emit microwave radiation), and tomographic sensors (which transmit and receive radio waves). The passive motion sensors unlike the active sensors do not have a transmitter. 

Applications of Motion Sensors

PIR Motion sensors are small, inexpensive and consume low power, and are very easy to use hence they find their application in a lot of devices and fields. They are commonly used in appliances and gadgets used in homes or businesses. These motion sensors can activate floodlights, trigger audible alarms, activate switches, and even alert the police.

Frequently Asked Questions

Why it’s called a passive sensor?

It is called a passive sensor because the sensor doesn't radiate any kind of infrared rays of its own.

What is the range of the PIR sensors?

The indoor PIR sensor can detect distances from 25cm to 20m and the outdoor one can detect from 10m to 150m.

What are active motion sensors?

Active motion sensors have both a receiver and a transmitter. It detects motion by observing changes in the amount of sound or the radiations reflecting back into the receiver.


In this article, we have extensively discussed motion and the PIR motion sensors. We hope that this blog has helped you enhance your knowledge regarding the same, and if you would like to learn more, check out our articles here

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