Presence detectors: types and where to use them

Motion and presence are not the same

• Motion detector: detects someone walking. Enough for corridors, stairs, garages and storerooms.
• Presence detector: far more sensitive, it detects someone sitting working or reading. Needed in studies, meeting rooms, bathrooms and living rooms.

Technologies

• Passive infrared (PIR): detects moving heat. Affordable and reliable, but can't see through obstacles and loses someone sitting very still.
• Radar or microwave: detects tiny movements, even breathing. Very sensitive; it can detect through light partitions, so it needs tuning.
• Dual technology: combines both to reduce false triggers.

Many detectors also include a light sensor: they don't switch on if there's already enough daylight.

Where to place them

• On the ceiling, centred over the area to cover.
• Clear of obstacles such as pendant lights, beams or tall shelving.
• Away from air vents and radiators, which confuse infrared sensors.
• In long corridors, two small ones rather than one at the end.

What they're for

• Lighting: automatic on and off, with a different level by day and by night.
• Heating and cooling: comfort when occupied, setback in empty rooms.
• Bathroom ventilation.
• Security: with the alarm armed, the same detector can raise an alert.
• Occupancy of meeting rooms in offices and guest rooms in hotels.

Settings that make the difference

• Hold time before switching off: short in corridors, long in a study.
• Light threshold above which it won't switch on.
• Sensitivity and zones, so it doesn't pick up the corridor from the bedroom.
• Pre-warning: the light dims before going off, in case someone is still there.

Where not to automate

In bedrooms and the living room at night, automatic switching on is usually annoying. Automatic off on absence, with manual on, works better there.