I've installed a mains-switching infrared sensor unit, Cybertech LFMS-WH. It doesn't say passive sensing anywhere, but I think it is passive. The lamp switching part of the circuit works properly and the detection part is problematic.

Cybertech LFMS-WH

The sensor is on the wall of a exterior corridor that opens onto a public sidewalk six meters away. The goal is for it to light up when someone enters the corridor, not when someone walks by outside. I get the impression that heat doesn't reflect off surfaces, but I do wonder if the shape of the corridor is related to the problem. It's under an overhang, so the sensor location is darker than the area of the motion.

The device has two levers of adjustment: its aim and the sensitivity ("RANGE") control. In my tests it behaved the same aimed downwards in any direction, even away from the street. With a tiny turn at around 20% of the adjustment range the device goes from seeing nothing at all to seeing me out on the sidewalk. The circuit lights up way too often. I have tried mounting an aluminum shield on its side to block the street angle, but the problem persisted.

Is this hopeless? Should I switch to a ultrasonic sensor?

Photograph of the corridor

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    Brand I've never heard of, located 10 miles from the Port of Long Beach. Implying they go over there and buy stuff out of catalogs and private label it. I would try subbing a decent Lutron unit, probably same country of manufacture but their own design and with good QA. Sep 7, 2017 at 19:47
  • My experience with an el-cheapo one mounted in a hallway is that it won't turn on until it sees a neck/face (uncovered) or an uncovered torso (summer days). Clothes seem to fool it off, even a T-shirt. I can't remember the brand though. So, yeah, something even cheaper (or more expensive but well adjustable) might be what you need. Sep 7, 2017 at 19:59
  • Turn the range to a lower setting depending on the model it may not be able to be turned down to less than 6m
    – Ed Beal
    Sep 7, 2017 at 21:57
  • I bet that passersby cast a shadow down the corridor, so when someone walks by, the IR light levels change for the whole cove, triggering the sensor. try to use a few IR LEDs (or even red) to repel the IR shadow around the area the sensor sees, it shouldn't take much, and a line of sight to the corner of the device should preserve entrant triggering functionality once re-calibrated.
    – dandavis
    Sep 8, 2017 at 13:46

1 Answer 1


The problem is simple. These are residential motion detectors. Yours probably ran you somewhere between $20 to $30 dollars. Manufacturing costs on this was probably $7 - $8 material inclusive, let's say 50%. You can't expect a highly sophisticated piece of electronic equipment that you can calibrate to spec from these things. They are what they are, a basic low grade and inexpensive motion detector for general residential use.

Many years ago we added these things in our no warranty list with light bulbs and ballasts due to the fact that most people were disappointed in their results. Even though they selected them and they were performing, just not the way they wanted them to. We were completely able to supply them with a more sophisticated motion detector, but most people considered it cost prohibitive.

In short it's a cheap product and it performs in a cheap manner.

  • Actually you can. It requires excellent design and top QA in manufacture, even Apple builds over there, they just use top components and tight QA. And that costs a few bucks more. Meanwhile indigenous firms make what are best called "knock-offs", designed to look right but be as cheap as possible and still power up. Look at any of the Youtube teardown vids. US importers shop these catalogs and slap their own name on, pretending to be the manufacturer. Big Box stores have several "house brands" which are this. Sep 8, 2017 at 17:39

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