Sensors
Motion Sensors vs Presence Sensors: Which Should You Use?
Compare PIR motion sensors with presence sensors for lighting, air-conditioning and room occupancy automations in a Singapore home.
Motion and presence sensors both help a smart home understand whether a space is being used, but they detect different things. A typical passive infrared motion sensor is excellent at noticing movement. A millimetre-wave presence sensor can detect much smaller motion and may divide a room into zones.
Neither is universally better. The right choice depends on the room, the action and how still people are likely to be.
Use motion sensors for clear movement
Motion sensors suit corridors, entrances, store rooms and other spaces where someone moving through the detection area is the main event. They are compact, often battery powered and straightforward to position.
They do not continuously prove that a person remains in the room. Someone reading on a sofa or working quietly at a desk may stop producing enough motion, so a short turn-off timer can switch lights off too early.
Passive infrared, or PIR, sensors react to changes in infrared energy across detection zones. Movement across the sensor’s view is usually easier to detect than movement directly towards it. Mounting height, aiming and obstructions therefore matter as much as the quoted range.
Use presence sensors for occupied rooms
Presence sensors are better suited to living rooms, studies and other spaces where people stay relatively still. Supported models can use zones, direction or distance to make automations more specific.
Positioning and calibration matter. Fans, moving curtains, reflective surfaces, pets and activity beyond an open doorway can affect detection. The sensor should be tested in the actual room before its automations are treated as final.
Millimetre-wave sensors transmit radio energy and analyse reflections. They can detect small movements such as breathing or hand movement that may not retrigger PIR, but greater sensitivity creates more setup work. Detection zones are software boundaries within the sensor’s field; they are not solid walls and should be validated against the actual room geometry.
| Characteristic | PIR motion | mmWave presence |
|---|---|---|
| Best at | Clear body movement | Small movement and continued presence |
| Typical power | Often battery | Often continuous power |
| Setup | Aim and timeout | Position, zones and sensitivity |
| Common issue | Misses very still occupants | Detects fans, curtains or adjacent areas |
Position sensors around movement and obstructions
For PIR, aim across the path of travel and avoid placing the lens behind glass, inside enclosed cabinets or where strong heat sources dominate its view. For presence sensors, confirm the required power location, mounting orientation and whether the product is intended for ceiling, wall or corner placement.
Open doorways can allow a presence sensor to see motion in a corridor. Ceiling fans, moving fabric and some robotic appliances can create unwanted detections. Start with conservative sensitivity and a smaller active area, then expand it after observing missed detections rather than accepting an entire room at maximum range.
Match the sensor to the consequence
The more disruptive the action, the stronger the evidence should be. Turning on a low passage light is low risk; unlocking a door or shutting down an occupied room is not.
Use asymmetric timing. Turning on a light after a confident entry can be immediate, while turning it off should require sustained vacancy. Air-conditioning may need an even longer delay because cycling equipment for a brief absence is uncomfortable and inefficient.
- A passage light can use a motion sensor and a short timeout.
- A study light should not turn off just because the occupant is typing quietly.
- Air-conditioning should use conservative delays and manual overrides.
- Security alerts need different logic from convenience lighting.
Combine sensors when the room needs context
A door sensor, light level, time condition and occupancy sensor can work together. For example, entering a dark bathroom can turn on a light, while continued presence keeps it on and an extended vacancy turns it off.
Start with simple rules, observe real use and add conditions only when they solve a known problem.
Sensor fusion is most valuable when each input answers a different question. A contact sensor confirms that a door changed state; PIR confirms movement; mmWave supports continued presence; illuminance indicates whether additional light is useful. Two sensors that make the same uncertain observation do not necessarily create certainty.
Test occupancy with real room behaviour
Do this before linking occupancy to disruptive actions. A week of observation with notifications or low-risk lighting will reveal more than repeated hand-waving during installation.
- Walk across and directly towards the detection area.
- Sit still at every normal desk, sofa and bed position.
- Run ceiling fans and air-conditioning and move curtains.
- Open adjoining doors and check for detection outside the room.
- Test pets and robotic cleaners if they use the space.
- Verify manual override and recovery after the room becomes vacant.
Choose a sensor from the room pattern
Use PIR where movement is obvious, battery operation is valuable and a modest vacancy delay has little consequence. Use presence sensing where people remain still, zones add useful context or premature shut-off would be disruptive. Use both only when each provides distinct information required by the automation.
A product specification cannot decide placement from a floor plan alone. Furniture, partition materials, fans, door positions and the occupant’s normal posture change the detection problem. Treat commissioning and a period of observation as part of the sensor cost, particularly for mmWave products with detailed zones and sensitivity controls.
Official references
Product and standards information was checked against these primary sources. The article above is original Aqara Singapore editorial content.
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