Presence detection vs motion sensors: why your lights keep turning off.

You're reading on the sofa. Nothing has moved for four minutes. The lights go out, and you wave your arm at the ceiling like you're flagging down a rickshaw.

This single experience is why most people conclude that smart lighting is a gimmick. It isn't. It's the sensor.

What a motion sensor actually sees

The little white sensor in most homes is a PIR — passive infrared. It doesn't see people. It sees changes in infrared heat across its field of view. Walk past, and the pattern shifts: "motion". Sit still, and nothing changes: silence.

So a PIR can't answer the question you actually care about. It answers "did something just move?" when the room needs to know "is anyone in here?"

Installers paper over this with timers. Ten minutes, twenty minutes. Now your lights stay on in an empty room for twenty minutes — you've traded annoyance for waste, which in a country with expensive electricity is a poor trade.

A motion sensor detects movement. A presence sensor detects a person. Almost every frustration with smart lighting lives in that gap.

How radar presence detection works

A millimetre-wave (mmWave) presence sensor emits a low-power radio signal and measures the reflections. It's sensitive enough to pick up micro-movements a PIR could never register: the rise and fall of your chest as you breathe, the shift of your hand on a keyboard, the small settle of your weight on a sofa.

The result is a sensor that reports continuous occupancy, not a flicker of motion. The room knows you're there while you read, work, eat, watch television — or sleep.

That changes what's possible:

• Lights stay on because you're present, and go off within seconds of you actually leaving — not twenty minutes later.
• Air conditioning shuts down almost immediately in a genuinely empty room, which is where most of the energy savings come from.
• You stop reaching for your phone, because the room is never wrong about whether you're in it.

Radar alone isn't the answer either

Honest engineering means naming the failure modes. mmWave sensors have them:

• They can see through thin walls. Badly placed, a sensor detects your neighbour in the next room and holds the light on forever.
• They notice fans, curtains and pets. A ceiling fan on high can read as motion unless the zone is configured properly.
• They need tuning. Sensitivity, detection zones and hold times differ for a bedroom, a stairwell and an open-plan lounge.
• They cost more than a PIR — worth it in rooms where people sit still, wasteful in a corridor.

Anyone who tells you a single sensor solves everything is selling a sensor, not designing a home.

Sensor fusion: how it's actually done properly

A room that never gets it wrong combines signals, each covering the others' blind spots:

1. mmWave radar for room-level presence, including stationary people.
2. PIR as a fast, cheap trigger for instant response the moment you walk in.
3. Door and window contacts to know whether anyone could have left the room at all.
4. Light-level sensors so lights only turn on when it's actually dark enough to need them.
5. Phone or presence on the network — used only for the coarse question of home vs away, never for room-level decisions.

Then the logic layers on top: a corridor holds for thirty seconds, a study holds for five minutes, the master bedroom refuses to turn the lights on at 3 AM and offers a dim floor-level glow instead.

Why this is the whole point of an intelligent home

Presence is the foundation everything else stands on. Get it right and the property finally has the one thing it needs to act on your behalf: an accurate answer to "is anyone here?"

Once it knows that, it can do the rest without being asked — warm the light as evening falls, stop cooling rooms you left, arm itself when the last person walks out, and tell you something useful when a room is occupied at 11:43 PM and it shouldn't be.

Get presence wrong, and you're left doing what most "smart home" owners do: opening an app to turn on a light. Which is a slower version of a switch.

What we do about it

At IntelliHome we place radar sensors where people sit still, PIR where people pass through, and we tune every zone on site rather than shipping defaults. The lighting adapts to the hour, the climate follows occupancy, and the phone stays in your pocket.

That's the difference between a house with sensors in it and a home that knows you're home.