Motion Sensor Lights: How They Work, How to Choose and Fit Them

A motion sensor light switches itself on when someone moves through its detection zone, then off again after a delay you set. Most use a passive infrared sensor that picks up body heat on the move, and a light threshold stops them coming on in daylight. They replace a switch in a hall, on a stairs, at the front door or along a path.

It is the simplest way to stop fumbling for a switch in the dark and to stop leaving lights on for nobody. A sensor works with most fittings, indoors and out: from a porch light by the back door to the wall lights in your hall. This guide explains how the sensor works, how to choose between a motion and a presence detector, how to set it up so it behaves, and who can wire it in an Irish home.

How does a motion sensor light work?

Most lighting sensors use a passive infrared sensor, usually called a PIR. It is passive because it sends out nothing: it measures the infrared radiation, or heat, given off by people and objects in its field of view. The plastic window on the front is moulded into facets, each one a small Fresnel lens that splits the area into separate beams. When a person walks from one beam to the next, the sensor sees a rapid change in its signal, and the electronics switch the light on through a built-in relay.

Two settings do the rest. The time delay sets how long the light stays on after the last movement detected. The light threshold, sometimes marked LUX or with a sun and moon symbol, stops the light coming on when there is enough daylight. Every new movement restarts the time delay, so the light stays on as long as someone keeps moving about. To give an idea of the ranges, a surface-mounted Legrand wall sensor (reference 048945) offers a time delay of 10 seconds to 10 minutes and a light threshold of 10 to 1,275 lux.

Because a PIR reacts to movement across its beams, it spots someone walking past it sooner than someone walking straight towards it. Keep that in mind when you choose where it goes.

Motion detector or presence detector?

Both use the same kind of sensor, but not with the same sensitivity. Hager, which makes both, describes the difference like this:

Criterion	Motion detector	Presence detector
What it picks up	Clear movements, such as someone walking	People sitting or standing still at a task too
Where it goes	Indoors and outdoors	Indoors
Daylight	Adjustable threshold for switching on	Can also switch off when daylight is enough
Typical use	Hall, landing, stairs, porch, path	Office, room where people sit

In a room where you sit still, a motion detector can switch the light off while you are still there: that is the job of a presence detector. A third option is the vacancy sensor: you switch the light on by hand, and the sensor only switches it off once the room is empty.

How to choose a motion sensor light

The first choice is where the sensor goes. A sensor switch replaces an ordinary light switch, in the existing box. A separate sensor fixes to a wall or ceiling and controls one or more fittings. A light with a built-in sensor puts both in one product, which is common outdoors. And a sensor bulb, with the sensor built into the cap, needs no wiring at all: check that it comes with the B22 bayonet cap used in most Irish light fittings.

Next come the angle and the range. The Legrand sensor above covers 140° and up to 8 metres in front of it, fixed at the recommended height of 2.5 metres. Common PIR sensors with several lenses reach about 10 metres. Mounted on a ceiling, some models cover a full circle: Hager gives detection angles from 140° up to 360° across its range. Finally, check on the data sheet that the sensor is compatible with LED bulbs and with the wattage of your fittings.

Outdoors, the fitting and the sensor must be made for the weather: look at the IP rating on the data sheet before you buy.

How to connect a motion sensor to a light

A sensor switch goes in place of the switch that controls the light, using the same cable to the fitting. There are two versions:

• 2-wire: it connects like the switch it replaces, with the live in and the switched live out to the light.
• 3-wire: it also needs a neutral in the switch box. The Legrand sensor above is one of these. Not every switch box has one, so check before you buy.

If the light is controlled from two places today, for example at the top and bottom of the stairs, check that the sensor you choose is designed for that layout.

In Ireland, fitting a sensor switch is not just a practical question. Safe Electric explains that most electrical work in a home is Restricted Electrical Works under S.I. No. 264 of 2013, which only a Registered Electrical Contractor may carry out and certify. That covers any new circuit, for example a new supply to a garden light, and any work in a bathroom, which is one of the special locations of the National Rules for Electrical Installations, I.S. 10101. The exemption for minor works covers like-for-like replacements, such as a switch for an identical switch; a sensor is not one. Have it fitted by a Registered Electrical Contractor, who will give you a Completion Certificate.

Outdoor sensor lights: front door, path and garage

Outside, a sensor does two jobs: it lights your way when you come home in the dark, and it saves leaving a light on all evening. Aim it at the path or the front door, not at the road or the footpath, or it will come on for every passer-by. Set the light threshold so it only works after dusk, and choose a short delay for a path and a longer one at a door where you look for your keys.

The sensor can be separate from the light or built into it. A separate sensor leaves you free to choose any outdoor wall light for the porch. Where no cable reaches, a solar wall light needs no connection at all: our large waterproof solar LED wall light charges during the day and lights a side passage or a shed door without an electrician.

Motion sensor lights indoors: hall, stairs and landing

Indoors, a motion sensor light earns its place wherever you pass through with your hands full: the hall, the landing, the stairs, the utility room, the downstairs toilet or the garage. At night, a low light on the landing that comes on by itself also saves hunting for a switch half asleep.

Any ordinary fitting can be controlled by a sensor switch, so you are not limited to lights sold with a sensor. In a hall, a globe wall light gives a soft, even glow: the ceramic enamel wall light with glass globe takes a G9 LED bulb and is rated IP44. Over a stairwell, a staircase pendant light lights every step. On a landing, the colourful designer aluminium wall light, with its frosted glass globe and IP44 rating, gives a gentle light that is pleasant at night. For more ideas, browse our LED wall lights.

Sensors also save energy where lights tend to be left on. A meta-analysis of 88 studies on lighting controls in commercial buildings, published in 2012, put the average saving from occupancy sensing at 24% of lighting energy. At home the figure depends on your habits, but the principle is the same: the light is only on when someone is there.

Settings and small faults: what to check first

Most disappointments come from a setting, not from the sensor. If the light comes on in daylight, lower the light threshold. If it goes off too soon, lengthen the time delay. If it stays on, shorten the delay, look for something moving in the zone and check that the sensor is not in a forced mode: many models have a button or a setting that keeps the light permanently on or off.

False triggers have known causes. A strong infrared source, such as car headlights or direct sunlight on the lens, can set a PIR off, and so can warm air from a vent or a heater blowing across it. Pets can trigger it too, which is why some sensors let you raise the sensitivity threshold so they ignore a cat or a dog. Outdoors, branches moving in the wind in front of the sensor are a classic cause: trim them or move the sensor.

Motion sensor light: frequently asked questions

Sources

• Restricted and Controlled Electrical Works, and Completion Certificates, guidance for customers. Safe Electric
• Passive infrared sensor, and Occupancy sensor, encyclopaedia articles. Wikipedia
• 140° motion sensor, reference 048945, technical data sheet. Legrand
• Motion and presence detectors, lighting and controls catalogue. Hager
• Williams A., Atkinson B., Garbesi K., Page E., Rubinstein F., Lighting Controls in Commercial Buildings, LEUKOS, 2012. doi.org