Security cameras
What is a PIR sensor?
A PIR (passive infrared) sensor detects motion by picking up the changes in heat a person or an animal causes when passing in front of it. It is passive because it emits nothing: it only receives. It uses very little power, which is why it is the usual sensor on battery cameras, automatic lights and alarms.
How it works
Every warm body gives off infrared radiation. A PIR sensor watches several zones and reacts when a heat source moves from one to another, in other words when something warm moves. That is why it detects someone walking across its view better than someone walking straight toward it.
Image based motion detection, by contrast, compares changes between video frames. It also reacts to shadows, rain, branches or changes in light. A PIR ignores all of that because none of it is moving heat, which is why it produces fewer false alerts.
Its limits
The infrared it senses does not pass through glass, so a PIR cannot detect through a window. Its range is limited, normally a few meters, and it loses sensitivity when the surroundings are close to body temperature.
It can also be triggered by pets, a vehicle with a warm engine or a current of hot air. Many devices let you adjust sensitivity to cut down on those alerts.
What to check before you buy
- On a battery camera, the PIR is what lets it last for weeks: confirm it has one.
- Look for adjustable sensitivity if you have pets or the camera faces the street.
- Mount it so people walk across its view, not straight at it.
Guides to help you choose
Frequently asked questions
Yes. It detects heat, not light, so it works the same in the dark. It is independent of the camera's night vision.