How HC-SR501 PIR Sensor Works & How To Interface It With Arduino

The HC-SR501 PIR (Passive Infrared) sensor detects motion by measuring changes in infrared levels emitted by surrounding objects. The sensor features a special multi-faceted plastic cover called a Fresnel lens, which focuses infrared energy onto a pyroelectric sensor element inside. When a warm body, such as a person, enters the sensor's field of view, it cuts across different zones of the lens, creating a differential change in infrared radiation between the sensor's dual halves. This generates a small electrical voltage, which is amplified by the onboard BISS0001 IC. If the signal exceeds a specific threshold, the HC-SR501 sets its digital output pin to HIGH. The Arduino Uno continuously monitors this digital signal using a simple input pin read. When the Arduino detects a HIGH state from the PIR sensor, it executes code to turn on the red LED connected to one of its digital output pins through a 220-ohm current-limiting resistor. The resistor ensures the LED does not draw excessive current, protecting both the LED and the Arduino. Once the moving object leaves the area and infrared levels stabilize, the sensor's output returns to LOW, and the Arduino turns the LED off. Power is supplied to the entire circuit via the Arduino's 5V and GND rails, powered either by USB or an external battery.