Your lights turned off while you were sitting there. That's not a smart home.
- Wayne Du Bruyn
- Jun 17
- 5 min read
We've all been there. You're settled on the couch, maybe reading, maybe watching something. You haven't moved much. And then the room goes dark.
You wave your arm. The lights come back on.
That's not your house being smart. That's your house not knowing you're there.
It's the most common complaint I hear from people who've had budget smart lighting installed: "the sensors don't work properly." But the sensors are working exactly as designed. The problem is that motion sensors were never meant to detect presence. They were designed to detect movement, and those are two different things.
Motion is not the same as presence
A PIR sensor (passive infrared) picks up changes in heat. When you walk into a room, your body heat sweeps across the sensor's field of view and it registers that as motion. The lights come on.
Then you sit down. You stop moving. Your heat signature stays roughly constant. The PIR timer runs down. Lights off.
Every PIR sensor in the world works this way. They can't tell that you're still there, quietly reading. They only know something moved and now nothing is moving. For a hallway or a bathroom, that's fine. For a living room, a study, or a bedroom, it's maddening.
The smarter alternative is called presence detection.
What mmWave sensors actually do
Millimeter-wave radar sensors (mmWave) work on the same principle as radar. They emit radio waves and measure what bounces back. The resolution is fine enough to detect micro-movements: the rise and fall of your chest as you breathe, a hand turning a page.
A high-frequency radar presence sensor in a KNX system doesn't ask "is someone moving?" It asks "is someone there?" That's a different question, and the answer shapes everything else the home does.
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mmWave sensor tracks multiple people in the same space independently. If two people are in the lounge and one leaves, it knows. The lights stay on for the person still there. A PIR sensor has no concept of this; it just sees "motion detected" or "no motion."
Zone detection within a room is also configurable. You can tell the sensor to recognise a specific reading chair, a desk position, a corner of the kitchen. Sit in that chair, and the reading lamp activates. Move to the couch and the scene shifts. The sensor knows where in the room you are, not just that someone is somewhere.
A shower curtain blocks infrared. An mmWave sensor doesn't care. It still detects you. That bathroom scenario where the lights go off mid-shower is gone.
Bed occupancy is detectable too. Mounted above or below a bed, a quality mmWave sensor can tell whether the bed is occupied with reasonable reliability. That opens up automations that were previously impractical: HVAC adjusts when you turn in, the alarm only arms when the bed is unoccupied, morning light scenes trigger when presence clears the bedroom.
Why this matters in a KNX home
Presence detection in a KNX system does more than keep the lights on. It feeds energy management, HVAC, security, and scene logic across the whole house.
The HVAC stops heating an empty room the moment the sensor knows no one is in it, not after a timer runs down. In a large Joburg home with zone heating, that's real energy. During load shedding, when you're running off battery or solar, it means stored energy goes where the people are, not to rooms nobody is in.
Scene logic gets more interesting too. A "reading" scene can activate not because someone pressed a button but because the sensor picked up a person sitting still in a specific spot for more than 30 seconds. "Away" mode can trigger automatically when presence clears from every zone in the house, not when someone remembered to tap the keypad before leaving.
Security tightens up as well. If the alarm is supposed to arm when the house is empty, the system can verify that it actually is empty, not just that no PIR triggered in the last few minutes.
For architects and interior designers specifying KNX: presence sensors are ceiling-mounted and invisible to the occupant. No hardware on the wall, no visible device. Just a small disc in the ceiling that knows where everyone is.
The brief is where this gets planned. Or missed.
Most residential builds I see follow the same path. The lighting consultant specifies PIR sensors because that's what the spec sheet calls for. The automation contractor installs them. The client moves in and finds the lounge lights annoying. A year later someone calls me to retrofit presence sensors and rework the automation logic.
The rework is manageable. But it's easier and cheaper to specify it correctly from the beginning.
If you're an architect or a developer putting together a smart home brief, these are the questions to add to your scope:
Are all occupied zones served by presence detection, not just motion detection?
Is the HVAC zoning tight enough that presence data can drive it meaningfully?
Where are the sensors physically mounted, and are the ceiling positions accounted for in the architectural drawings?
Is the system commissioned to distinguish individual zones within larger rooms?
KNX handles the complexity of presence-driven logic across an entire building in a single, documented, commissioned system. The sensor data flows to the bus. All the automation logic lives in ETS6, properly programmed and backed up, not in an app on someone's phone that gets discontinued in four years.
A real example of what changes
A client I've worked with had a home office that used PIR sensors when I first visited. Smart lighting, but PIR. The lights went off roughly every eight minutes during long stretches of focused work. He'd taken to waving his arm at the sensor from his desk. Not how anyone should be living.
We replaced the ceiling sensor with a KNX mmWave presence detector, defined a desk zone and a couch zone, and set it up so that working at the desk held the office at task lighting, while sitting on the couch shifted to a softer reading scene. Lights now go off only when he actually leaves the room.
It's a small change in hardware. The difference in daily experience is not small.
The technology has matured
I want to be straight about where mmWave sits right now. It's not infallible. False positives can occur. HVAC airflow has been known to register on poorly calibrated sensors. Placement matters. The installer needs to understand detection angles and zones, and commission the sensor correctly.
But the technology has improved significantly over the past two years. High-frequency radar presence sensors with native KNX integration are now genuinely reliable in residential applications when properly installed. The experimental phase is over.
This is still a commissioning problem as much as a hardware problem. The sensor is only as smart as the logic built around it in ETS6. That's why the conversation I always push for is with the installer and the architect together, before the first conduit goes in, not after the client is living with lights that don't know they're home.
If you're planning a build or a renovation where this is part of the brief, I'm happy to walk through what the sensor layout and zone logic would look like for your specific project. No hardware recommendations until I understand the space.
Just a conversation about what the project needs.
Wayne du Bruyn
wayne@knxlogic.co.za | 082 564 3982 | www.knxlogic.co.za
KNX Logic | KNX home automation design and installation, Johannesburg
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