What Is a PIR Sensor? How They Work, Where They Fail, and How to Stop False Alarms
By the DC Fire & Security engineering team — installing and maintaining fire and security systems since 2010. Updated July 2026.
Quick answer
A PIR — passive infrared — sensor detects the body heat of a person moving across its field of view. 'Passive' because it emits nothing; it just watches for a moving warm shape against the background through a segmented lens. PIRs are the standard motion detector in burglar alarms, the trigger in security lights, and the 'smart motion' in many cameras. They're brilliant and cheap, with three known weaknesses: heat sources that mimic movement (sun through glass, radiators, hot pipes), small warm things that aren't intruders (pets, spiders on the lens), and bad placement — which is behind most 'faulty' PIRs we're called to.
How a PIR actually works
Everything warm emits infrared. A PIR's pyroelectric element sits behind a faceted lens that splits the room into zones — as a warm body crosses from one zone to the next, the element sees infrared rise and fall, and enough of those transitions in quick succession equals 'movement'. That's the whole trick, and it explains every PIR behaviour: why they detect movement *across* their view better than movement straight toward them, why a stationary intruder eventually disappears to them, and why anything that changes temperature quickly — a sunlit wall, a radiator kicking in, a draught over a hot pipe — can look like a person.
It also explains the fixes. Point PIRs away from glazing and heat sources, mount them at the height the lens was designed for (usually about 2.4m), and they're extremely reliable — the same physics has run the industry for decades.
The variants, and when each is worth paying for
| Type | What it adds | When it's the right call |
|---|---|---|
| Standard PIR | The baseline detector in every alarm system | Most rooms, most systems |
| Pet-tolerant PIR | Lens geometry and processing that ignores warm bodies under a stated weight (typically up to ~25kg) at floor level | Any armed area a cat or dog can reach — but it must be mounted exactly as specified, and a cat on a bookshelf defeats the geometry |
| Dual-tech (PIR + microwave) | Only alarms when BOTH heat movement and microwave-detected motion agree — false alarms drop dramatically | Conservatories, garages, workshops, anywhere with temperature swings or draughts that plague standard PIRs |
| PIR with anti-masking | Detects someone covering or spraying the lens while the system is unset | Higher-security graded systems; often an insurer requirement at Grade 3 |
| Curtain PIR | A blade-shaped zone instead of a fan — watches a door, window run or corridor without covering the room | Protecting entry points while people or pets use the space behind |
| Outdoor PIR | Weatherproofing plus processing tuned to ignore foliage, small animals and temperature swings | Driveways and perimeters — expect (and want) conservative detection outdoors |
PIRs in cameras and lights are the same part, different job
The 'motion detection' in security lights and many cameras is the same PIR technology. In a floodlight it switches the lamp; in a camera it wakes recording or fires a notification. Same physics, same failure modes — a camera that notifies you about headlights sweeping a wall, or a floodlight that triggers on the neighbour's cat, is a PIR with placement or sensitivity problems, not a mystery.
One practical difference: alarm PIRs are certified components of a graded system and their sensitivity and zones are set by the engineer at commissioning. Camera and light PIRs are usually user-adjustable — if yours over-triggers, sensitivity and detection-zone masking in the app are the first stops. For the alarm version of this problem, the false-alarm guide walks through causes in order of likelihood.
The placement mistakes behind most 'faulty' PIRs
- Facing a window — sun through glass is a moving heat source every clear morning; the classic 9am false alarm
- Above or opposite a radiator, boiler or heat pipe — heating cycles read as movement in cold snaps
- In a conservatory as a standard PIR — the temperature swings are exactly what PIRs hate; dual-tech or don't bother
- Wrong height or tilted — the lens zones land in the wrong place; a pet-tolerant PIR mounted low stops being pet-tolerant
- Left dirty or webbed — a spider on the lens is a warm moving object one centimetre from the element; the single most common false-alarm cause we attend. Clean lenses on a schedule
- Detecting through the letterbox draught — hallway PIRs positioned where cold air moves over them on windy nights
PIR quality is a system property, not a gadget spec
The same detector performs completely differently at 2.2m clean on a stable internal wall versus tilted above a radiator facing patio doors — which is why 'which PIR should I buy' is usually the wrong question. On a professionally designed system, detector choice, mounting and zoning are decided together against how the building is actually used, then proven at commissioning and kept honest by servicing.
If your current system's PIRs are the problem — random triggers, blind spots, a pet that can't be armed around — that's rarely a rip-out. Swapping the right two detectors for pet-tolerant or dual-tech units and re-siting one more is a routine visit. What a burglar alarm costs covers the numbers for bigger changes.
Frequently Asked Questions
What does PIR stand for?
Can a PIR sensor detect through glass?
Do pet-friendly PIRs actually work?
Why does my PIR sensor keep going off at the same time each day?
What's the difference between a PIR and a microwave sensor?
How long do PIR sensors last?
PIR problems, or an alarm that cries wolf?
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