See what your presence sensor covers before you buy
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Motion detectors and presence sensors look alike but work differently. One notices that something is moving, the other that someone is there. In a smart home where the light should stay on while you read, this difference is crucial.
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 works
A radar presence sensor emits weak radio waves (24 or 60 GHz) 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 a proven choice for range and presence in a room, with distance measurement and distance-based zones.
- 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 masked out |
| 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 |
Radar sees every movement in the room, not just people. Interference sources in a fixed place, such as a fan, can be hidden with a zone. Anything that moves through the whole room cannot be limited to a zone. A lower sensitivity helps, but makes the detection of people sitting still less reliable. Alternatively, the automation ignores such signals.
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.
Using the values: Adding a presence sensor to Home Assistant
Frequently asked questions
Does a radar presence sensor have a camera or a microphone?
No. It only measures how radio waves come back: distance and the slightest movements. It does not record images or sound. None of the Senvolon presence sensors has a camera or a microphone.
Is the radar radiation dangerous?
No. The presence sensor transmits at low power, comparable to a Wi-Fi device.
Can a radar presence sensor detect through walls?
Radar waves partly pass through light materials such as plasterboard, wood or glass. Set range and sensitivity so that movement behind the wall does not count; on the KL MZ, the boundary and interference zones also help.
Does a presence sensor need the internet?
The Senvolon presence sensors work locally: they send their values via Wi-Fi and MQTT to Home Assistant, ioBroker, openHAB or Node-RED in your network. Only the optional cloud for remote access and push notifications needs the internet.