A motion sensor answers a simple question: did something move. A presence sensor is sold as answering a harder one: is somebody here. The marketing distance between those two questions is about a metre and a half. The engineering distance, we found out over two evenings in September, is the whole width of the living room.
There are three millimetre-wave radars in the flat. Two of them are the subject of this page; the third is in the bedroom and has its own job on the night score. Both of ours say presence on the box. In the space of two evenings one of them failed in both possible directions: it saw a person who was not there, and then, once we had corrected for that, stopped seeing a person who was. The other has not put a foot wrong, and the difference between them turned out to be the instructive part.
The phantom on the couch
One night the living-room lights were still on after everyone had gone to bed. That in itself is a small thing. What sat behind it was not: the sleep flag, the one that tells the house we have retired, had been off all day, and the wind-down that should have raised it had never started.
The living-room radar had reported presence in an empty room for 41 minutes. Its own state said move, its distance readout said 0 m. Both sensitivities, move and presence, were at 10, the maximum, and the range was the full 6 m, which in this flat reaches into the hallway. Something in the room was moving as far as the radar was concerned, and our best suspect is the air purifier two metres away, ioniser on, pushing air about. We have never proved it.
Nothing raised an alarm, because nothing was wrong in any way a machine can see. A presence sensor said on. Everything downstream took it politely at its word: the lights, the sleep detector on the detection page, the half-hour "somebody has moved somewhere in the flat" flag that the colour alerts check before speaking. A lie from a presence sensor is not loud. It is the opposite of loud, which is how it survives.
We lowered both sensitivities to 7 and the range to 4.5 m, and within sixteen seconds the room was declared empty, the wind-down came on, and the evening tidied itself up. We wrote down that distance = 0 was the signature of noise, that a real person would appear at a real distance, and went to bed pleased with ourselves.
The person on the couch
The next evening one of us lay still on the couch, reading. The radar declared the room empty and the lights went out over the reader, which is a remarkably effective way to report a sensor fault. A presence sensor that cannot see somebody reading on the sofa is a motion sensor with a better vocabulary.
We watched the raw numbers this time. A person lying still produced state = move, distance = 0, which is exactly the phantom's signature. On reflection, the diagnosis of the night before had been wrong in the most useful possible way: distance does not separate a phantom from a person, and we should not have concluded anything from it. We keep a whole page of things we verified wrong, and this one joined it the same evening.
The settings went back up: presence sensitivity 10, move sensitivity 7, range 6 m. We checked the outcome this time rather than the settings screen — twelve samples over two minutes with a still person on the couch, presence held on throughout. Our working hypothesis is that the move sensitivity alone drove the phantom, and that stillness and range are what a lying-down person needs. It is only a hypothesis, because we changed all three at once on the first night and cannot honestly credit the cure to any one of them. The proper test is an empty room at night with move at 7. We are waiting for one.
The other radar
The office has a different unit, and a more talkative one. Instead of a single presence signal it reports three: big movement, small movement, and a third tier for breathing and fidgeting that reaches 2 m. That third one is the entire reason it appears here. It also has a built-in siren, which we have never used and regard with the suspicion one reserves for a smoke alarm that also makes toast.
It sits directly above the monitor, so the desk chair is well inside the 2 m static range. The still person, the one the living room lost, is this radar's easiest case, because a whole tier of the device exists for nothing else. We have not raised its static range and do not intend to; the living room taught us what a generous range buys.
Which of the two is better has no clean answer. The office one has more settings, finer classes and a dedicated tier for people who sit, which is most people most of the time. If we had to choose a single property, it would be that tier. Sitting still is what humans do in rooms.
What a presence sensor actually gates
The expensive property of a presence sensor is not that it can be wrong. Every sensor can be wrong. It is that its off is consulted by everything, and consulted silently. Lights, sleep, the flag that says somebody is home when they are not carrying a phone, the office screen: each of them asks "is anyone there", takes the answer and moves on. Eight automations read the office radar alone. When the answer is a phantom they all wait. When it is a false empty they all switch off, sometimes over a person with a book.
There is a quieter trap in the same place, and it is arithmetic. A radar holds its own state for a while after a room empties, and the automation reading it then waits its own delay before acting. Nobody ever adds the two together, and the sum is always longer than either number looks on its own. The monitor page has ours, and what it cost us.
So the rule we came away with is small and slightly humbling. Motion is not presence. Presence is not a person. And distance = 0 is not a diagnosis, whatever we said just before midnight on the fifteenth.