Why does my outside light come on in daylight or stay on all night?

The three little dials on a security light — TIME, LUX and SENS — are the whole control system, and most complaints come from them being left at the factory position. Understanding what each one measures makes the fault obvious in most cases, and turns a light that annoys the neighbours into one that does its job.

Safety

Step by step

1. Learn what the LUX dial does.It sets the ambient light level below which the sensor is allowed to trigger. Usually marked with a sun at one end and a moon at the other. If it is at the sun end, the light will trigger in broad daylight, which is the single commonest cause of daytime operation.
2. Learn what the SENS dial does.It sets how much moving heat is needed to trigger, which in practice sets the effective range. Turned up high, a PIR will see a cat at twenty metres and a car on the road behind.
3. Learn what the TIME dial does.How long the light stays on after the last detection. If it is set to ten minutes and something crosses the view every eight, the light appears to be on permanently. Reducing time-on is often the fix for "it never goes off".
4. Set the lux correctly using the test method.Most PIRs have a walk-test mode, or you can turn TIME to minimum. Set LUX to the moon end, then in the evening at the light level where you want it to start working, turn LUX slowly towards the sun until it triggers. That calibrates it to your actual conditions.
5. Look at what is in the detection field.PIRs see heat crossing their field, not heat approaching. Things that trigger them at night: cars on a road, a boiler flue plume, a tumble dryer or extractor vent, a warm wall cooling unevenly, a security light from next door heating a surface, cats and foxes, and branches moving in front of a warm background.
6. Re-aim rather than re-adjust.Most sensor heads pivot. Aiming down and away from the road, and away from any vent or flue, solves more cases than any dial. A PIR is most sensitive to movement across its view, so aiming along an approach path rather than at it reduces false triggers from beyond.
7. Mask part of the field if aiming is not enough.Many sensors come with a small mask or lens cover, and a strip of electrical tape on the lens works. Blocking the upper part of the lens removes distant triggers while keeping the doorstep.
8. Suspect water and heat for erratic behaviour.A sensor that behaves randomly, comes on and off in seconds, or works only in dry weather has water in it or a failing photocell. Isolate the circuit and inspect. If the fitting has condensation inside, it needs replacing and the reason for the ingress found.

Tips

• A dusk-to-dawn light with no PIR uses a photocell only; if it stays on all day, the photocell is either covered, faulty, or seeing the light's own output reflected back.
• A light seeing its own reflection in a window or a wet wall will latch on and never go off. Moving it a foot solves what no dial adjustment will.
• LED security lights that stay warm can, oddly enough, confuse an adjacent PIR. Keep the sensor away from the lamp head where the design allows it.

Common mistakes

• Turning sensitivity up because it is not detecting you — Range problems are usually aim problems. Higher sensitivity extends the range in every direction including towards the road, and produces exactly the all-night triggering people complain about.
• Assuming a light on in daylight means a fault — It is nearly always the LUX dial. Check the setting before condemning the fitting.
• Working on an outdoor fitting isolated only at the switch — The switch breaks the line to the lamp; the sensor and the fitting's supply may still be live. Isolate at the consumer unit.

If it doesn't work

Questions people ask