Pet-immune PIR's--how do they work

[lugnut]

Anyone know of a description on what exactly these types of pet-immune PIR's are doing? They are sold as "for pets under 40 lbs" or "pets under 80 lbs" etc, but they can't really be detecting weight.

If they are detecting cross sectional area, how do they account for the variation in that with distance? and couldn't an intruder simply ball up and crawl across the floor?

Also, on very lean dogs like sighthounds, or dogs with very long fur, it seems that would skew the use of cross section as an estimate of weight (if that's what they are doing). Reason for my question is I have a dog in this category that is a bit under 35 lbs actual weight, and I want to try and figure if a 40 lb pet immune PIR is going to give occasional false alarms. I'd rather not go to an 80lb PIR if it's not necessary.

 

[paulw11]

Hi,

There are a few of different techniques used to provide pet immunity.

The first is to configure the lenses in the PIR so that it is less sensitive to infrared close to the floor where animals move and/or require more sensing "zones" to be triggered at the same time. A thief could attempt to defeat the detector by crawling on the floor, but as they have a larger body mass they will emit more infrared and so will probably still be detected. The pet immunity won't work where the animal can climb and therefore be visible in the more sensitive zone. Some sensors also have a "look down" lens that can detect movement directly under the detector but this is not compatible with pet immunity.

A variation on this uses dual detector elements where in order to trigger the alarm both elements need to see IR movement. The idea is that a pet is physically too small to be in enough zones at once and therefore won't trigger the detector.

You will also find "dual-technology" detectors. These combine a microwave sensor with a PIR sensor and require the simultaneous triggering of both detectors types in order to signal an alarm. The idea here is to adjust the microwave sensitivity so that pet movement is not detected while human movement is.

Paul

 

[CollinR]

Get a true dual technology motion like the Honeywell DT7235, its a regular PIR and microwave in one unit. It knows how much mass the object has regaurdless of how close or far away it is. These aren't that much more then normal "pet immune" motions and work way better.

 

[lugnut]

Get a true dual technology motion like the Honeywell DT7235, its a regular PIR and microwave in one unit. It knows how much mass the object has regaurdless of how close or far away it is. These aren't that much more then normal "pet immune" motions and work way better.

Thanks for the DT7235 reference. I hadn't come across that one before. C&K/Honeywell has some pretty interesting products. Do you have any suggestions for suppliers that carry the full line and have good prices to the general public (or suppliers that allow the general public to create essentially a dealer account)?