Introduction to PIR Sensor: What Is It and How Does It Work?

A PIR Sensor is an electronic device designed to detect motion by measuring changes in infrared radiation (heat) in the environment. PIR stands for “Passive Infrared”. This means PIR sensors don’t emit any radiation or signals of their own. They only “listen” for changes in infrared radiation from objects in their surroundings

These sensors are commonly used for security and automation purposes, as they can detect the movement of people, animals, or objects within their detection range.

Key Components

The HC-SR501 PIR Sensor is a motion detection sensor widely used in various electronic projects and applications. It is designed to detect infrared radiation emitted by objects, such as the human body, which allows it to sense motion.

Now, let’s discuss the main components of the HC-SR501 PIR Sensor.

Pyroelectric Sensor

The pyroelectric sensor is the core component of the PIR sensor. It detects changes in infrared radiation.

Fresnel Lens

Fresnel Lens is used to focus the infrared radiation onto the pyroelectric sensor.

BISS0001 IC

It is a high input impedance operational amplifier.

Time Delay Adjust

Set the delay time of the sensor.

• Counterclockwise or Left: reduces the delay time, with the delay being approximately 5 seconds when fully left.
• Clockwise or Right: increases delay time, with the delay being approximately 5 minutes when fully right.

Sensitivity Adjust

Adjust the sensitivity or distance of the sensor.

• Clockwise or Right: the sensitivity of the sensor increased, and the sensing distance increased by about 7 meters.
• Counterclockwise or Left: the sensitivity of the sensor decreased, and the sensing distance decreased by about 3 meters.

Triggered Mode Control

• Repeatable(H) mode: the output pin will go high when a person is detected within range and return to low after a specified duration.
• Non-Repeatable(L) mode:  the output pin will go high when a person is detected within range and will stay high as long as he/she stays within the limit of the sensors range. Once the person has left the area, the pin will go low after a particular time.

Pin Configuration

• GND: Connect to the Arduino’s GND pin.
• OUT: Connect to the Arduino digital pin.
• VCC: Connect to the Arduino’s 5V pin for power.

How It Works

The working principle behind a PIR sensor is relatively simple. It detects infrared radiation emitted by warm objects (such as human bodies) in its field of view. When a person or animal moves across the sensor’s detection area, the sensor notices a change in the amount of infrared radiation and triggers an output signal, typically activating a connected system, like an alarm or lighting system.

Pin Wiring

The Difference between IR Sensors and PIR Sensors

Visit Easy Understanding IR Sensor: What Is It and How Does It Work? for more details about IR sensors.

1. Detection Mechanism:

• IR Sensor:
  An IR sensor detects infrared radiation emitted or reflected by objects. It typically measures the infrared light (heat or reflected infrared rays) in a specific wavelength range. IR sensors can be active or passive.
  
  • Active IR sensors emit infrared light and measure the reflection of that light from objects.
    
  • Passive IR sensors only detect the infrared radiation naturally emitted by objects.
    
• PIR Sensor:
  A PIR sensor, on the other hand, is a passive sensor that specifically detects changes in the infrared radiation in its environment caused by motion (usually from warm bodies like humans or animals). PIR sensors don’t emit any infrared light but detect heat changes, usually in the 8–14 micrometer range of infrared radiation.

2. Use Cases:

• IR Sensor:
  Used in a variety of applications, including distance sensing, object detection, and temperature measurement. Common uses include proximity sensors, temperature sensing (like thermometers), and in robotics for navigation.
  
• PIR Sensor:
  Typically used in motion detection systems, such as in security alarms, motion-activated lighting, and automatic door openers. PIR sensors are mainly focused on detecting movement rather than measuring distances or temperature.
  

3. Sensitivity:

• IR Sensor:
  Can detect infrared radiation from a variety of sources, not necessarily from warm objects. It may not be sensitive to movement alone, but more to the presence of objects emitting or reflecting infrared light.
  
• PIR Sensor:
  Highly sensitive to changes in heat within its detection field. PIR sensors are specifically tuned to detect the movement of warm bodies (like humans) and are designed to ignore static objects.