Why the Lights Go Off While You're Still in the Room

A PIR sensor detects movement, not people. Sit still and, as far as it is concerned, the room emptied.

Texas Instruments opens its own paper on this with the scene everyone recognises: working late, “typing out some last emails”, when “the lights in your ‘smart office’ turn off, and you find yourself standing up and waving your arms so that the motion sensor will trigger the lights back on.”

Its verdict on the whole category is blunter than anything a retailer will tell you — today’s motion sensors are “notoriously bad at detecting when indoor spaces are occupied and are often useless when a person is relatively still”.

What the sensor can actually see

Passive infrared works by watching for changes in heat across its field of view. Infineon puts the mechanism plainly: PIR sensors “work by detecting changes in infrared radiation and converting these changes into a voltage signal.”

The word carrying the weight is changes. A person walking past moves a warm shape from one zone to another, and that change is the signal. A person reading in a chair produces no change, so there is no signal, so there is nobody there.

TI states the limit directly: PIR and ultrasonic sensors “do not have the fidelity to detect finer motion, such as when someone is sitting still at their desk, typing, breathing or talking on the phone.”

They measured it, and the result is absolute

This is the part that settles the argument. TI put a PIR sensor and a millimetre-wave radar module in a 5.1m by 4.4m conference room and tested three categories of motion — walking, typing, and breathing — from seven chair positions between 2.6m and 5.1m from the sensor.

Motion	PIR	mmWave
Walking	Detected at all 7 positions	Detected at all 7
Typing	Failed at all 7	Detected at all 7
Breathing	Failed at all 7	Detected at all 7

In TI’s words, the PIR sensor “was able to detect walking across the entire conference room, but was unable to detect typing or breathing in any location.” Not worse at distance — failing everywhere, at every distance.

Distance was never the variable. The sensor is not weak at seeing you type; it cannot see you type at all.

The same blindness runs the other way

The complaint is usually about lights going off. The same sensor also turns them on for things that are not you.

Infineon notes that PIR sensors “are susceptible to false detections when non-human objects enter the sensor’s view” — a radiator coming on, sun moving across a floor, a pet. It also flags a maintenance problem nobody expects: the sensor needs “a semi-transparent plastic radome or cover to allow infrared energy to pass through, which can be prone to dust and dirt accumulation, affecting performance”, and outdoors, “snow, dirt, and other outdoor conditions, can significantly degrade the performance of PIR sensors.”

So an outdoor sensor that has stopped working may need a cloth rather than a battery.

Why raising the timeout is the wrong fix

The instinct is to set the delay to fifteen minutes. It works, in the sense that the lights stay on, and it destroys the point of the automation: lights now burn for a quarter of an hour in every empty room, and the corridor that should go dark behind you does not.

You have converted a sensitivity problem into a waste problem. The sensor is still blind to you; you have just told it to take longer to act on being blind.

What to actually do

• Match the sensor to the room’s behaviour. Corridors, stairs, entrances and cupboards are pure movement, and PIR is genuinely good at those — cheap, battery-friendly, and reliable for exactly that job.
• Buy a presence sensor for rooms where people are still. Desks, sofas, reading chairs, home offices, bathrooms. These are sold as “presence” or “mmWave” or “occupancy” sensors rather than “motion”, and the wording is the specification, not marketing.
• Expect to pay more, and to power it. Radar sensors generally need mains or USB power rather than a coin cell, because unlike PIR they are actively transmitting. Plan the socket before you buy.
• Put the presence sensor where the stillness is. Aimed at the chair, not the doorway. A radar sensor pointed at an entrance gives you an expensive PIR.
• Keep the timeout short once the sensing is right. A long delay is a workaround for a sensor that cannot see you. Fix the seeing, and you can go back to thirty seconds.
• If lights also fail during an outage, that is a different fault entirely — what stops working when the internet drops covers which half of your setup depends on a server.