A passive infrared (PIR) sensor is an electronic sensor that detects movement by measuring infrared (IR) light radiating from objects in its field of view. The term “passive” refers to the fact that the sensor does not emit any energy of its own; it only detects the IR radiation already present.

How PIR Sensors Work 🚶‍♀️

All objects with a temperature above absolute zero emit thermal energy in the form of infrared radiation. A PIR sensor is designed to detect changes in this radiation.

A typical PIR sensor has two pyroelectric sensors, which are materials that generate a charge when exposed to heat (IR radiation). These two sensors are wired to cancel each other out.

• Idle State: When the sensor is idle, both halves receive the same amount of ambient IR radiation from the surrounding room, so the output remains at zero.
• Motion Detected: When a warm body, like a person or animal, enters the sensor’s view, it first crosses one half of the sensor. This creates a positive differential change in the charge between the two halves. As the body moves, it then crosses the second half, creating a negative differential change.

The sensor’s electronic circuitry detects these changes or “pulses” and sends a signal to a connected device, which can then trigger an action like turning on a light or sounding an alarm.

Common Applications 💡

PIR sensors are small, inexpensive, and low-power, making them ideal for a variety of applications, including:

• Security Systems: They are a core component of burglar alarms and security cameras, detecting intruders by sensing their body heat.
• Automatic Lighting: They save energy by automatically turning on lights in a room or hallway when someone enters and turning them off when they leave.
• Smart Home Devices: PIR sensors can be integrated into smart home systems to automate tasks like activating appliances or adjusting thermostats based on occupancy.
• Automatic Doors: Many public buildings use PIR sensors to automatically open doors when a person approaches.