Motion detectors and presence sensors look alike but work differently. One notices that something is moving, the other that someone is there. For a smart home in which the light should not go out while you are reading, this difference is decisive.
In short
A motion detector reacts to motion, usually via infrared (PIR). A presence sensor also detects people who are sitting still, working, or sleeping. Radar presence sensors, often called mmWave sensors, can do this because they register even the movement of the chest while breathing.
How an infrared motion detector (PIR) works
A PIR sensor measures heat radiation that moves through its field of view. When someone walks through the room, the heat pattern changes and the detector switches. If the person sits still, nothing changes anymore: for the sensor, the room is empty. That is why motion detectors work with a timeout and switch the light off after a few minutes without motion.
How a radar presence sensor (mmWave) works
A radar presence sensor emits weak radio waves in the millimeter range and evaluates their reflection. With FMCW radar the frequency changes continuously; the difference between the transmitted and the received wave gives the distance. The smallest changes, including breathing, reveal that someone is there.
- 24 GHz is proven for range and presence in a room, with distance and zones by distance.
- 60 GHz resolves more finely: the presence sensor locates people on a floor area, separates multiple people, and allows freely drawn zones.
And HF motion detectors?
HF or high-frequency motion detectors also work with radio waves, usually at 5.8 GHz, and detect motion even through lamp glass and thin covers. As a rule, however, they only report motion, much like an infrared detector. A radar presence sensor with 24 or 60 GHz additionally evaluates the finest movements and therefore also detects people who are sitting still.
The comparison
| Motion detector (PIR) | Radar presence sensor | |
|---|---|---|
| Detects motion | yes | yes |
| Detects people at rest | no | yes, down to breathing |
| Distance, zones | no | yes |
| Reacts to sun and heat from radiators | can trigger | no |
| Reacts to fans, curtains | rarely | yes, can be hidden |
| Through thin walls | no | yes, limit the range |
| Power supply | often battery | usually a power adapter, because it measures continuously |
| Good for | hallway, stairs, outdoors | rooms where people spend time |
To radar, a robot vacuum is motion too, and it is detected as long as it is moving. Because it travels through the whole room, it cannot be hidden with a zone the way a fan can. A lower sensitivity helps but makes the detection of people sitting still less reliable. Alternatively, the automation ignores the signal while the robot is cleaning.
When is a motion detector enough?
Wherever you only pass through: hallway, stairwell, storage room, outside at the entrance. What counts there is that the light comes on when you enter; nobody sits still in the stairwell.
When do you need a presence sensor?
In all rooms where you stay longer: living room, study, bedroom, bathroom, office. Here the light should stay on as long as someone is there, the heating should only warm occupied rooms, and the alarm should know whether really nobody is home.
What to look for in a radar presence sensor
- Adjustability: range, sensitivity, and zones should be adjustable so that movement in the next room or a fan does not trigger it. It is convenient when the presence sensor learns such sources of interference in the empty room by itself.
- Integration: do the values reach your smart home locally, for example via MQTT in Home Assistant, or only through a manufacturer app?
- Additional sensors: brightness for lighting control, and temperature, humidity, and air pressure for heating and ventilation save further devices.
- Mounting: wall, corner of the room, or ceiling, and whether the device is surface-mounted. Mounting a presence sensor correctly
The Senvolon presence sensors are radar presence sensors with Wi-Fi and MQTT. The KL MZ learns the room at the push of a button: if a fan is running in the empty room, it detects the movement and creates an interference zone around it by itself. More on this: Setting up zones on the KL MZ presence sensor.