The smart home is at its best when it runs itself, and one of the core components of automation is a motion sensor. When you think about motion sensors, you’re probably thinking of the traditional type that powers security systems, activated by Passive Infrared (PIR).
Most likely, you’ve come across their major downside: when you don’t move, these motion sensors detect that there’s no motion, so activate a routine right when you don’t want it, such as turning the lights off because you’re sat on the sofa.
There is an alternative: mmWave-powered presence sensors, which are sensitive enough to pick up breathing, so they keep detecting presence even without overt movements. They bring their own quirks, and aren’t ideal for everywhere. Which one is right for you? I compare to the two technologies to help you choose. When you’re ready to buy, my guide to the best motion sensors has our top picks of both types.
What is a PIR motion sensor?
A passive infrared (PIR) sensor is the cheapest form of presence and motion detection. These sensors watch for changes in infrared energy (heat) across the sensor’s field of view. In simple terms, if something warm (a body) moves, the sensor registers the movement and activates.
As these devices are very simple, they can run on batteries, lasting years between changes. They’re also cheap and simple to set up detecting motion quickly. The Aqara Motion Sensor P1 is a typical example.
There are downsides. First, motion sensors can pick up any movement, including those of a pet, and trigger a routine. For security systems, there are pet-safe motion sensors, but they rely on clever placement, such as not putting a sensor covering a table that a cat can jump up on. Or, in my real-wold example, I find that if one of my cats sticks his head between the posts on the stairs, he can trigger the motion sensor in the hall.
Secondly, PIR sensors only detect movement. If you sit still to read, work at a desk or lie in the bath, the sensor sees no change and, once its timeout runs out, your lights switch off. Movement straight towards the sensor is also harder to pick up than movement across it.
What is an mmWave presence sensor?
An mmWave sensor uses millimetre-wave radio signals (radar), typically at 24GHz or 60GHz, and measures what bounces back. Because it measures reflections rather than heat, it can detect very small movements, down to the rise and fall of your chest as you breathe. For a deeper look at the technology, read our explainer, What is mmWave, and what does it mean for your smart home?
This technology means that mmWave can actually detect whether people are in a room, whether they’re moving or not. That’s great for keeping the lights on.
These sensors can be more flexible than motion sensors, too. Many presence sensors can measure distance, so you can limit detection range, split a room into zones or ignore an area such as a doorway. More advanced models can track where people are in a room.
With the Aqara FP400, for example, you can set different zones and have different things happen. For example, from the one sensor, I can have the heating turn on when I come into the room, but a reading light turn on only when I’m sat in my reading chair.
There are trade-offs. mmWave sensors need more power, so many are mains or USB powered, and they can be too sensitive. Fans, moving curtains, pets and even movement on the other side of a thin wall can cause false detections, so they can require careful tuning, although most models have AI setup routines that help.
Some mmWave sensors also have a PIR sensor: they can use that to pick up big movement first, and then switch to mmWave to check for continuing presence.
mmWave vs PIR: Comparison
| PIR | mmWave | |
|---|---|---|
| How it works | Senses changes in infrared energy (heat) across its field of view | Sends out radio waves (usually 24GHz or 60GHz) and measures the reflections |
| What it detects | Movement only | Movement and presence, including tiny movements such as breathing |
| People sitting still | Can’t see them, so lights can switch off while you’re in the room | Detects them reliably |
| Sensitivity | Best at movement across its view; weaker at movement straight towards it | Very sensitive, picking up movements of a few millimetres |
| Through materials | Blocked by glass and most objects | Can pass through thin, non-metallic materials such as plastic, fabric and plasterboard |
| Common false triggers | Radiators, direct sunlight, and pets | Fans, curtains, plants, pets and movement in a neighbouring room |
| Zones and tracking | No, just motion or no motion | Many models support distance limits and detection zones; some track position |
| Clear time | Needs a long timeout to cover gaps in movement | Clears quickly and accurately when a room empties |
| Power | Very low, so batteries can last a year or more | Higher, although battery-powered options are becoming more common |
| Setup | Works out of the box | Often needs sensitivity and zone tuning |
| Price | Cheap, often under £20 | Usually £25 to £60 or more |
| Best for | Hallways, stairs, and entrances | Living rooms, offices, bathrooms and anywhere people sit still |
Which should you choose?
A PIR sensor is the better choice for areas where there’s consistent movement:
- Hallways, landings and stairs
- Entrances, utility rooms and garages
- Security alerts, where you only need to know someone has walked in
An mmWave sensor is the better choice for areas where you need to check for presence without motion:
- Living rooms, where people sit still for long periods
- Home offices and desks
- Bathrooms
- Bedrooms, such as to know when someone is in bed
In short, use PIR where people pass through and mmWave where people stay.
FAQs
Can mmWave sensors see through walls?
They can pass through thin, non-metallic materials such as plasterboard, glass and plastic, so a sensor can pick up movement in the next room. Brick, concrete and metal block the signal. Point the sensor away from shared stud walls, or use distance and zone settings to cut out the area beyond the room.
Are mmWave sensors safe?
Yes. They transmit at very low power, within the exposure limits that apply to all consumer radio equipment, and they don’t capture images.
Do pets set off motion sensors?
Both types can be triggered by pets. Some PIR sensors have a pet-immune mode that ignores smaller heat sources. With mmWave, the best approach is to use detection zones to exclude areas such as a pet bed.
Can mmWave replace PIR?
In most rooms, yes. PIR still makes more sense where you need basic motion detection, as these devices are cheaper and easier to configure.
