Here is the short answer: a PIR sensor does not detect movement. It detects a change in infrared heat crossing the boundary between two zones of its lens — which is why it sees you walking across it beautifully and walking straight at it barely at all.
That one fact explains almost every complaint we hear at the counter. The light that ignores you coming up the path but fires when the neighbour walks past the end of the drive. The sensor that works in winter and sulks in a July heatwave. The detector that triggers every morning at seven when the sun hits the hall window.
PIR stands for passive infrared. Passive is the important word: it emits nothing, it only watches. We have been selling these from Villa Road since long before they were called sensors, and here is what actually matters.
1. How a PIR sensor actually works
Behind that white plastic lens is a pyroelectric sensor, and in front of it is a moulded Fresnel lens divided into segments. Each segment focuses a slice of the room onto the detector. As a warm body crosses from one slice into the next, the detector sees a sharp rise then fall in infrared energy. That change is the trigger — not the presence of heat, the change in it.
Three consequences follow immediately, and they are the whole article:
- Crossing beats approaching. Walk across the segments and you cut through several boundaries per second. Walk straight down the axis and you may stay in one segment the entire way.
- Contrast beats size. On a cold night a person is dramatically warmer than the background and detection range is at its best. On a 30°C afternoon, skin and background are close together and range drops.
- Anything that changes temperature fast is a candidate. Sunlight tracking across a wall, a heater cycling, an extract fan blowing warm air across the field of view.
Which brings us to the split that causes most of the wrong purchases.
2. Lighting PIR or alarm PIR? They are not the same product
Both are called a PIR sensor. They are not interchangeable, and buying the wrong one is the most expensive mistake in this category.
Lighting PIRs
These are mains devices. They sit in the 230V circuit and switch a load — which is why their headline rating is in watts. They have a lux adjustment (how dark before it works) and a time adjustment (how long it stays on). They do not talk to anything. They are a switch that decides for itself.
Alarm PIRs
These are detectors. They are low-voltage or battery-powered, they report to a panel, and their job is to signal a zone — never to switch power. They have features a lighting PIR has no concept of: pet immunity, anti-masking, tamper contacts, grade ratings. Their headline spec is range and protocol, not wattage.
Trade tip: if a spec sheet quotes a maximum load in watts, it is a lighting sensor, full stop. If it quotes a frequency in MHz or a grade under EN 50131, it is an alarm detector. That single check has saved more wasted trips to Villa Road than any other thing we tell people.
3. Choosing a lighting PIR: angle first, range second
Detection angle is what you match to the space, not range. A 12-metre sensor at 180° on a wall covers a drive properly. The same 12 metres at 360° on a ceiling covers a room properly. Put either in the other's place and it underperforms badly.
For an external wall, the Knightsbridge OS004B is the standard fit.
The OS004B is an IP44-rated 180° PIR sensor in black, with adjustable lux and timer and a 12-metre detection range. Where the wall is more exposed, the OS001B steps up to IP55 and widens to 200° over the same 12 metres.
Indoors, the job changes shape entirely — you are covering a volume rather than an approach, and the sensor wants to be overhead.
4. Indoor and ceiling sensors
Hallways, landings, WCs, stores and corridors are the classic wins. The light comes on when someone is there and goes off when they are not, and nobody ever leaves the store-room light on for a fortnight again.
The Knightsbridge OS0016 is the wide-coverage option.
The OS0016 is a 360° wide-range recess-mounting PIR for indoor use, with adjustable lux and time and a 2000W maximum load. Where the ceiling cut-out needs to be small and discreet, the OS0019 is a mini 360° ceiling sensor with a separate power module, IP20, 8-metre detection range, 2000W, 16AX.
Trade tip: check the load rating against what you are actually switching, and check it against the inrush, not the running watts. A run of LED drivers draws a hard spike on switch-on that a mechanical relay handles happily and a cheap triac does not. If a sensor is dying every few months on an LED circuit, that spike is why — and the fix is a sensor with a relay output, not another identical sensor.
5. Alarm PIRs, and the pet problem
An alarm PIR reports to a panel, so the only sensible question is which panel. Protocols do not mix — an 868MHz detector for one manufacturer's system will not enrol on another's, and no amount of wishing changes that.
Within the Pyronix Enforcer family, the Pyronix KX10DP-WE is the standard wireless detector where a dog or cat lives in the protected space.
The KX10DP-WE is a wireless pet-immune PIR detector with a 10-metre range on 868MHz. Pet immunity is a lens and processing trick — it discounts a small mass low in the field of view — so it only works if the pet stays low. A cat on the back of a sofa is at human height and will be treated as one. Browse the wider burglar and intruder alarms range for panel-matched detectors, and see how intruder alarms work for how detectors and panels fit together.
6. Fixing a PIR that misfires
- Turn lux down first. Most "faulty" sensors are simply set to work in daylight. This is the single most common fix.
- Look at what the sensor looks at. Stand where it is and follow its gaze. A road, a footpath, a neighbour's door or a south-facing wall in the field of view will all trigger it, and no setting cures a bad aim.
- Move it away from moving air. Extract-fan outlets, boiler flues and air-conditioning discharge all push changing warm air across the lens.
- Shorten the timer. A ten-minute hold makes three separate events look like one continuous fault.
- Mask a segment. A small piece of tape over the part of the lens that faces the road is crude, free and works.
PIR Sensors, Summarised
| Application | Type | Coverage | WME pick |
|---|---|---|---|
| External wall, drive or path | Lighting, IP44 | 180° / 12m | OS004B |
| Exposed external wall | Lighting, IP55 | 200° / 12m | OS001B |
| Room or corridor ceiling | Lighting, indoor | 360°, 2000W max | OS0016 |
| Discreet ceiling, small cut-out | Lighting, IP20 | 360° / 8m | OS0019 |
| Alarm zone with a pet | Alarm, 868MHz | 10m, pet-immune | KX10DP-WE |
Where DIY stops
Swapping a like-for-like sensor on an existing switched circuit is straightforward. Adding a new circuit, or any work in a special location, is notifiable under Part P and needs a registered electrician. Alarm detectors on a graded system should be enrolled and walk-tested by the installer who signed the system off, or the grading no longer means anything.
Expert Help in the West Midlands
Villa Road, Birmingham, since 1975. Bring us the panel model or a photo of the wall and we will tell you the sensor, the angle and the height — and, more usefully, we will tell you when the sensor you already own is fine and just needs its lux dial turning down. We keep both lighting and alarm PIRs on the shelf, which is why we are quite firm about not confusing them.
Browse PIR and motion sensors, or the full burglar and intruder alarms range. Get in touch for a spec.
Frequently Asked Questions
What is a PIR sensor?
A passive infrared sensor. It detects the change in infrared heat as a warm body crosses the segments of its lens. It emits nothing itself — it only watches, which is what "passive" means.
How does a PIR sensor work in hot weather?
Less well. Detection depends on the contrast between body heat and background temperature, so on a very warm day the effective range shortens. This is normal physics, not a fault.
Can I use a PIR light switch on LED lights?
Yes, but check the sensor's load rating and prefer a relay output. LED drivers draw a brief inrush current on switch-on that can shorten the life of an electronic switching sensor even when the running wattage is tiny.