So, a guy I work with told me a couple years ago that motion detectors don't like mirrors, shiny, bright, etc. Thinking about it, I came to the conclusion that they may not like them because they could fail to recognize motion. My experience since then seems to back that up. I mentioned it to another coworker, though, and he made a comment that it wasn't a big deal because our motion detectors (not used for security system reasons, only to detect human presence) wouldn't be calling the police or anything for false detection of motion. So...that's kinda (very) contradictory to what I've believed for a while.

So which is it? Will pointing a motion detector at mirrors, bright lights, shiny things, cause FALSE motion alarms? Or will it potentially cause the motion detector to NOT pick up motion (which is the bad thing in our application).

Thanks!

J

It picks up motion. Had a Lutron occ sensor in a pantry area with a stainless steel fridge that picked up reflected motion from a hallway. Had to mask it slightly to keep it from happening.

Some sense heat, some sense motion, some are PIR, some are not. They vary on what triggers them.

The generic DSC PIR detectors I last installed state to avoid aiming them in bright sunlight, and facing areas with rapid temperature changes or air ducts with substantial air flow. It also states it "detects motion crossing the beam; it is less sensitive detecting motion towards the detector".

The marketing materials mention "sharp analysis of body dimensions and differentiation from backgrounds and pets" and "unprecedented levels of immunity against visible light".

So in this instance, I don't think mirrors or shiny would make any difference. Not sure about bright light that's not the sun.

This is the motion detector we're using and I'm having a problem with it NOT picking up motion when it should:

[Link: resolutionproducts.com]

Thanks for the info on the exact model. See my signature.

Now that I can look at the sensitivity patterns, I'm reminded to ask whether these are poor at responding to lateral motion across the front of the sensor. or motion toward or away from the sensor. Do they respond well up to, say, six feet, but you're trying to cover twenty feet away and they won't do that? More detail, please.

Are there any Danish Wolf Hounds or other small horses at the address? Or cats, even?

The google response I got as to how these work says, referring to a pair of "slots" as shown in the Honeywell manual,

Walking laterally across slots causes multiple imbalances of IR info, so the thing alarms. Walking up to it, especially slowly, would make much more minor disturbances, perhaps not triggering an alarm.

So... does your positioning of things work with this kind of operation? Sit in the room where you can look at the room while you study the slot diagrams and think about it. Get your eyes up to where you've put the sensor and see if there's a bright light shining on the sensor, even though that's not cited as a potential problem.


Please give us some detail on "when it should" exactly as these are mounted.

By the way, when you do all the things I'm suggesting, you learn some things and it's not unusual to solve the problem as you go along.

Thanks for the response. Yes, I know details are important but I'm not as seasoned as many of you so I don't always know what to ask.

Based on your response, I think that our placement hasn't been that great. It works most of the time, but sometimes, there are just units that have chronic problems with motion detection. Most of the time, the placement is pointing directly at the main entry of a unit from an angle in a way that there isn't a lot of lateral movement. In some cases, the fact that it (the PIR motion detector) is described as having a 90 degree field is misleading in that we've placed some of them close to/in inward corners, blocking half of the field. It also makes sense that the performance has been much better when I've moved them into hallways where people would pass in front of them at basically a 90 degree angle (just a few feet).

You know, I bet someone at the company (or one of the, was it four? companies) that claims this product as their own could clarify some things for you over the phone. If you could do as little as make a sketch of the places where you're having problems, and have this in front of you when talking with someone from the company, you might get a LOT of insight as to what's going on.

Really. Call someone high up in the company that made these. Act ignorant. People often love to help a dummy. And don't take that personally.

PIRs work by effectively taking a thermal image of the room and analysing the amount of IR radiation being received on each of its detection zones. Detection zones are determined by a fresnel focusing lens on the front of the case, or on higher end devices by a built-in reflective mirror. These focus the received IR onto a Pyro element on the detector board.

The detector works by looking for a temperature change that moves logically across adjacent detection zones The temperature change needs to be above a certain threshold to be noticeable. This is why they don't like being hit by bright lights, installed in drafty locations, or mounted above heat sources. All of these will cause rapid temperature change across adjacent detection zones and generate false alarms. Shiny surfaces may reflect sunlight or other IR sources into the detector causing the same problem. Also when ambient room temperature gets close to body temperature, an acclimatised body in a room may be significantly harder to detect, especially for cheaper PIRs whose processing will not be so good.

Detection patterns typically have many more detection zones in the horizontal plane than they do in the vertical one. Usually most PIRs will only have three in the vertical plane, and will have a dead spot directly under the detector. Better detectors have an additional "look down" zone to cover that dead spot. They may have 6-10 (or more) zones in the horizontal plane and will cover a ninety degree angle. For best results place the detector in the corner of the room at a 45 degree angle. There is usually a diagram of the detection pattern characteristics include in the PIR instructions.

Because of that zone pattern the detector is most sensitive when a body moves across its field of view i.e. left to right or right to left. Much greater movement will be required to trigger two adjacent detection zones when walking directly towards it ,so this is its least sensitive pattern. Note that the further away you get from the detector the greater the distance becomes between the edges of the detection zone pattern, and therefore the further you will need to move to cause an alarm.

For best results use a quad zone PIR with mirrored optics. If the environment is IR hostile you can use a dual technology detector where a PIR is coupled with a Microwave detector in the same housing and the two technologies will check with each other that they have both been triggered before sending an alarm out over their relay contacts. Microwave is more like radar and works on Doppler shift principle to detect a moving body. Note a Dual Technology detector is not a fix for bad PIR placement!

That's a pretty good article you wrote there, Nick.