A PIR (Passive Infrared) sensor is an electronic device that detects motion by measuring infrared (IR) radiation emitted by objects in its field of view.
It is like a pair of “invisible eyes” that detect movement by sensing heat from people, animals, or warm objects. It doesn’t send out signals—it just “watches” for changes in heat.
It is commonly used in security alarms, automatic lighting systems, and smart home applications.
How PIR Sensors Work
- Detecting Heat Changes: Every object emits infrared radiation based on its temperature. Humans and animals are warm, so they stand out from the background. The PIR sensor detects changes in infrared levels when a warm object (such as a human or animal) moves in front of it.
- Two Tiny Sensors Inside: The PIR sensor has two heat-detecting sensors inside. If nothing moves, both sensors see the same heat level. If something warm moves in front of it, one sensor detects a change first, then the second. This difference tells the PIR sensor: “Hey! Something is moving!”
- Fresnel Lens (The Plastic Cover) : Most PIR sensors use a Fresnel lens to focus and extend the detection range. It helps focus heat signals from a wide area. This PIR lets the sensor “see” movement from farther away.
What Happens When Motion is Detected?
When motion is detected, the sensor sends a high signal. This can turn on lights, activate alarms, or send a signal to a microcontroller (like an ESP32 or Arduino).
Features of PIR Sensors
- Passive operation: It does not emit any radiation, unlike active infrared sensors.
- Low power consumption: Ideal for battery-operated devices.
- Wide detection range: Typically between 3 to 12 meters.
- Adjustable sensitivity and delay: Some models allow tweaking the range and duration of the output signal.
Common PIR Sensors
HC-SR501
The HC-SR501 is a widely used, adjustable PIR motion sensor module that operates with a voltage range of 4.5V to 20V and offers a detection range of up to 7 meters, depending on sensitivity settings.
It features two adjustable knobs: one for sensitivity, which controls how far the sensor can detect motion, and another for the time delay, which sets how long the sensor stays active after detecting motion (ranging from 5 seconds to 5 minutes).
It works best For Arduino UNO R3, Mega 2560 with 5V voltage.
SR602
The SR602 Mini PIR Sensor is a compact, low-power motion detector designed for small electronic projects. Operating at 3.3V to 5V, it is ideal for use with microcontrollers like the ESP32 , ESP8266 and Arduino.
The sensor has a detection range of 3 to 5 meters and a fixed delay time of approximately 2 seconds, meaning it stays active for 2 seconds after detecting motion. Its tiny size makes it perfect for space-constrained applications, such as wearable devices, battery-powered IoT systems, and smart home automation.
Unlike SR501 PIR sensors, the SR602 consumes very little power, making it a great choice for energy-efficient projects. and it works with ESP32 or ESP8266 with 3.3V voltage.
