Motion is not the same as presence
A motion sensor reports that something moved. A presence sensor reports that something, usually a person, is there, even if they are sitting still. The difference explains a familiar annoyance: office lights that switch off while someone is reading quietly at a desk. Early and inexpensive sensors are mostly motion detectors, while newer types attempt to detect presence more directly.
Both kinds feed many connected systems, including lighting, alarms, heating and ventilation, and elevator and door controls. The best choice depends on what must be detected, where, and what a mistake costs.
Passive infrared (PIR)
PIR sensors are the most common motion detectors in homes and offices. They contain a sensing element that responds to infrared radiation in the heat range emitted by warm bodies. A lens divides the field of view into zones. When a warm object crosses from one zone to another, the sensor sees a change and reports motion.
Because it is passive, the sensor emits nothing and uses very little power, which makes it well suited to battery devices. It is also cheap.
Typical weaknesses:
- Stationary people. Without movement across zones there is no change, so a person sitting still may be missed.
- Direction. Movement straight toward the sensor can create a smaller change than movement across its zones.
- Heat sources. Sunlight through a window, a heater, an oven or hot air from a vent can trigger false alarms.
- Pets. Animals are warm and move, so they can set it off. Some models use lens designs or thresholds to ignore small animals, with mixed success.
- Temperature similarity. When the room is close to body temperature, contrast falls and sensitivity can drop.
- Obstructions. Infrared does not pass through glass or most walls.
Ultrasonic
Ultrasonic sensors emit sound above the range of human hearing and listen for echoes. If the pattern of the reflected sound changes, something has moved. The same principle, measuring echo time, is used for distance measurement.
Compared with PIR, ultrasonic coverage can fill a room more evenly, including areas around obstacles, and it can detect small movements. It uses more power because it transmits.
Typical weaknesses:
- Air movement. Fans, drafts and heating or cooling vents can disturb the echo pattern.
- Soft surfaces. Curtains, carpets and upholstery absorb sound and reduce range.
- Interference. Other ultrasonic devices, and some noises, can confuse the sensor.
- Pets. Some animals can hear ultrasound and may be disturbed by it.
Microwave and radar-style sensing
Radar sensors emit radio waves and analyze what bounces back. Simple microwave modules detect the Doppler shift caused by movement. More advanced millimeter-wave radar can measure distance, speed and fine motion such as breathing, which allows detection of a person who is almost still. This is the approach behind many newer presence sensors.
Advantages include sensitivity, the ability to work in darkness, and the fact that radio waves can pass through some thin materials, such as plastic housings and sometimes drywall.
Typical weaknesses:
- See-through coverage. The ability to pass through walls can cause detections of activity in the next room or corridor.
- Moving non-people. Fans, curtains swaying in a draft, plants and even rain on a window can create a signal.
- Cost and power. Radar modules cost more and usually draw more power than PIR sensors.
- Tuning. Sensitivity, range and zone settings often need adjustment, and the right values depend on the room.
Combining sensors
Because every method has weak points, many products combine two. A dual-technology detector might require both PIR and microwave detection to trigger an alarm, which reduces false alarms, and use either one to keep the lights on, which reduces missed detections. Other approaches blend a door contact, a light sensor or a thermal array. The design principle is that independent methods tend to fail in different ways.
Other ways of sensing presence
Beam-break sensors detect a person crossing a line. Pressure mats and chair sensors detect weight. Thermal array sensors produce a low-resolution heat image. Cameras can count people but raise greater privacy questions, which are explored in the article about image sensors. Even network signals, such as a phone joining a Wi-Fi network, can hint at occupancy, though they say little about who is actually in a room.
Reducing false alarms
- Place sensors away from heaters, vents, windows with strong sun and moving curtains.
- Aim and mount them at the height and angle recommended in their documentation.
- Adjust sensitivity and time-out settings instead of leaving defaults in every room.
- Consider the pets and children in the building.
- Test by walking through at different speeds, directions and times of day.
- Use a delay before turning things off, so a person sitting quietly does not get left in the dark.
These practical steps overlap with the guidance on sensor placement, since a well-placed cheap sensor often outperforms a badly placed expensive one.
Where it matters
A false alarm from a lighting sensor is a nuisance. A missed detection in a safety application is more serious. For alarms and occupancy sensing used to control heating, ventilation or lighting, think about how much error is tolerable, and keep a manual override. Where life safety is involved, rely on certified equipment and professional installation instead of general-purpose connected sensors.