We have a light connected to a motion detector in our backyard. It detects motion and turns on the light for 2-3 seconds, then the light turns off again, and won't detect further motion.

A single switch inside the house powers both the motion detector and a patio light. The patio light turns on and off with no issues.

Interestingly, if I leave the patio light on for several hours, the motion detector will stay on longer. It will continue detecting motion & stay on for several minutes.

Do you have any advice for troubleshooting why it won't stay on? It is about 20 feet off the ground, so anything we could do to figure this out without needing to rent a giant ladder would be helpful. I tried to get close enough to determine the brand but could not. Generic white security lighting.


Motion sensors don't like having power cut

You may think of a motion sensor as a stateless device: ”detect motion, turn on, how hard is that?" But actually, there's a lot more going on.

To start with, how often do you see motion sensors coming on by daylight? You don't. That's because they have light sensors so they don't come on in the day.

Day. Night. How hard is that? The human eye has a complex iris that allows adaptation to a wide variety of light levels. Because of this auto-compensation, day is just day, and night is just night. But to a machine, it sees only absolute lux to the sensor, and only on the light frequencies it detects. Turns out a shady north facing location can have less absolute lux by day then a location with a lot of artificial lighting by night. So the sensor must "see" a reasonable chunk of a day-night cycle to see what light levels will look like here. And after having spent much of a day learning that, it will store that data for future use.

However, if you cut its power, you wipe its memory and it must start over.

So expect substandard performance from your motion sensor as long as you have it on a switch. Nature of the beast.

  • Interesting! I am a little confused though, because it can detect motion at night and turn on... it just immediately turns back off. Also, after it turns off, I can do jumping jacks in front of the sensor and it won't go on again... – Kevin Burke Jan 28 '19 at 20:53
  • @KevinBurke in the initial minutes after initial power-up, motion sensors are in a diagnostic mode where they will help you troubleshoot the sense zone. That's probably what it's doing. I wouldn't seek deep inner meaning in that. – Harper - Reinstate Monica Jan 28 '19 at 22:00

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