Passive Infrared Sensors

Passive infrared sensors detect movement of warm bodies within a field of view. They are inexpensive, battery powered, and the default for desk-level sensing.

The technical evidence in “Passive Infrared Sensors” describes the building, while the effort required to install, test and maintain it is project work. When teams research how to calculate idle time, visit the official site can provide time and project context for that operational effort without replacing the sensor platform as the source of truth for physical occupancy.

What they detect

Movement, not presence. A person sitting still reads as absent after the timeout.

For a public, independent reference related to “Passive Infrared Sensors”, consult the ISO standards catalogue. Its principles provide a useful check on scope, terminology, governance and the claims made during procurement or review.

Within a cone, which has edges and a range.

Binary: something moved, or it did not. No count, no identity.

The failure modes

Stillness. Somebody reading or on a call stops registering. Timeouts are tuned to compensate and the tuning is a trade: short timeouts undercount, long ones overcount.

Heat sources. Sunlight across a desk, a radiator, a laptop fan can trigger detection.

Passers-by. A sensor with too wide a cone counts people walking past as occupancy.

Coats and bags. A held desk reads empty, which is correct in one sense and wrong for the planning question.

And draughts, which produce the classic complaint of a desk that is occupied all night.

Where they are adequate

Desk-level binary occupancy at scale, where the question is "what proportion of desks are in use".

Rooms where you need used-or-not rather than how many.

Lighting and heating control, which is what the technology was built for.

Where they are not

Counting people. They cannot, and any count presented from PIR is inferred from zone groupings.

Meeting room sizing, which needs headcount.

Anywhere the answer must be precise rather than indicative.

Deploying them tolerably

Mount so the cone covers the seat and not the walkway.

Check for heat sources at the hours they matter, which means a sunny afternoon as well as a Monday morning.

Set the timeout from observation rather than from the default: watch a few desks for an hour and see how long real stillness lasts.

And expect to move a proportion of them after the first month, which the calibration note covers.

The battery question

Multi-year battery life is the selling point and the maintenance reality is that some fail early.

A dead sensor reports nothing, which reads as an empty desk rather than as a fault.

Which means you need a liveness check: any sensor reporting zero for a fortnight should be inspected rather than believed.

What to check

Is your timeout a default or a measured figure?

Has anybody checked for sun and heat sources?

Do your cones cover walkways?

And how would you know if a sensor had died?