What is a PIR sensor?

Glossary · Alarms, lighting and home installs

How a PIR sensor works

PIR stands for passive infrared. The sensor does not send anything out. It detects the infrared radiation given off by objects, and a person’s body gives off heat that the sensor picks up.^{[1]National Protective Security Authority, Intrusion detection} When a warm body moves across the sensor’s field of view, the sensor triggers the device it is connected to, such as a light.

The NPSA says the detection pattern looks like fingers radiating out from the central sensor, and that sensitivity is better across the fingers than along them.^{[1]National Protective Security Authority, Intrusion detection} That is why a light mounted facing a path can see someone walking across it sooner than someone walking straight towards it. Where you fit and aim the sensor matters as much as the sensor itself.

Electrical Safety First lists PIR and motion sensors as popular choices for switching on security lights on dark paths or entrances.^{[2]Electrical Safety First, Electrical safety advice for outdoor lighting} Our own installation page describes the same use: PIR lights that trigger on movement at doors and driveways.^{[4]Martin Day Electricians, Security lighting installation in Leeds}

What triggers a PIR and what can go wrong

The NPSA says PIR sensors can give false alarms when the general temperature changes, and when excessive infrared light reaches the sensor.^{[1]National Protective Security Authority, Intrusion detection} For a home that means a sensor that faces a bright window or a heat source may switch on more often than you want. It is why the set-up of range and sensitivity matters.

Our service sets the sensitivity and range on site so the light triggers on someone approaching the house, not on a neighbour’s cat or the road outside.^{[4]Martin Day Electricians, Security lighting installation in Leeds} Outdoor lights also need to be weatherproof, with a suitable IP rating.^{[2]Electrical Safety First, Electrical safety advice for outdoor lighting}

On safety, Electrical Safety First says to have mains outdoor lighting installed by a registered electrician in line with the manufacturer’s instructions and BS 7671. It says to switch off the power supply before cleaning, adjusting or checking outdoor lighting, and before changing any lamp. If you find a fault, switch the supply off straight away and have it repaired by a registered electrician.^{[2]Electrical Safety First, Electrical safety advice for outdoor lighting}

PIR or dusk-to-dawn

A PIR light is one way to switch on a security light. The other is a dusk-to-dawn light, which comes on as the light fades and goes off at daybreak. ProtectUK says lights can be activated by motion sensors or left on through the night, and that some lights suit constant illumination while others suit use with passive infrared detectors.^{[3]ProtectUK, Security lighting}

It adds that studies have shown a lower level of continual light is more effective as a deterrent than sudden bright lights. A good compromise, it says, is high-efficiency low-energy lighting on a photo-electric cell, sited out of reach.^{[3]ProtectUK, Security lighting} We fit both kinds and sometimes both on the same property.^{[4]Martin Day Electricians, Security lighting installation in Leeds}

Example

Linda in Meanwood has a dark side passage and a back gate. Her old floodlight comes on every time a cat walks past. She wants lighting at the front door and the side gate that works when someone approaches.

A registered electrician walks the property with her. They choose a PIR light for the front door and a dusk-to-dawn light for the passage and gate. They set the PIR sensitivity and range so it triggers on someone approaching, not on the cat.^{[4]Martin Day Electricians, Security lighting installation in Leeds} The electrician wires the lights to a weatherproof outdoor circuit and checks the IP rating.^{[2]Electrical Safety First, Electrical safety advice for outdoor lighting}

What people get wrong

Many people believe a bright light that snaps on is the strongest deterrent. ProtectUK says studies have shown that a lower level of continual light is more effective as a deterrent than sudden bright lights.

A PIR light still has its place at a door or a path, and ProtectUK says some lights suit use with passive infrared detectors.^{[3]ProtectUK, Security lighting} The other mistake is expecting a PIR sensor to work in any position. Aim, range and a heat source in its view all change how it behaves.^{[1]National Protective Security Authority, Intrusion detection} If your light misbehaves, ask an electrician to adjust the settings rather than reaching up a ladder to it yourself.

The bottom line

A PIR sensor detects infrared heat from a moving person and switches a connected light or device on. It is sensitive across its detection pattern and can be fooled by temperature changes or strong infrared light, so aim and settings matter. Pair it with a dusk-to-dawn light where continual low light suits the property, and have any mains outdoor lighting installed by a registered electrician.