"Occupancy sensing" covers several quite different technologies. Some only notice movement, some count people at a doorway, and some can tell that a person is present even when they are sitting still. Choosing the wrong one is a common reason projects disappoint.
This guide compares the main options and explains how to choose by the decision the data will support, the space and the privacy expectations of the people in it.
Start with the question you need answered
Different questions need different sensors:
- Is anyone in this room right now? (presence)
- How many people are in this area? (count)
- How many people came through this entrance? (flow)
- Is a cubicle or room in use? (state)
- Has someone stopped moving? (activity)
Write down the question first. It narrows the options quickly.
Passive infrared (PIR)
PIR sensors detect movement of warm bodies. They are inexpensive, low-power and well understood, which makes them a good choice for lighting control and simple occupancy.
Their main limitation is that a person who sits still may not be detected, so a room can appear empty during a quiet meeting. They do not count people.
Door contacts and lock sensors
A contact on a door or lock shows whether it is open, closed or engaged. For washroom cubicles and small rooms this is often the most reliable availability signal, and it captures nothing about the person.
It shows state, not presence. A locked door does not prove someone is inside, and an unlocked one does not prove the room is empty.
Low-resolution thermal sensing
Thermal array sensors see heat patterns at a very low resolution. They can detect people who are still, estimate counts within their field of view and show roughly where people are, without producing identifiable images.
Mounting height, field of view and nearby heat sources affect accuracy, so placement should be planned and tested on site.
mmWave radar and sensor fusion
Radar sensors can detect very small movements such as breathing, which makes them useful for confirming that a person is present but motionless. They can also work through some light materials.
Combining radar with thermal sensing can improve reliability for safety-related uses, such as alerts in accessible toilets or patient rooms, where both missed events and false alarms matter.
Compare on more than price
When comparing options, look at:
- Privacy: what the sensor could capture, not only what it is used for.
- Detection: whether it detects presence, movement, count or state.
- Coverage: field of view, mounting height and how many sensors a space needs.
- Installation: power, connectivity and how much work is needed in an occupied building.
- Maintenance: battery life, calibration and how faults are reported.
The best choice is often a combination, for example door contacts for availability and thermal sensing for safety in the same washroom.