What's The Use of a Motion Detector?

A Motion detector's primary application is as an interior trap; a catch all, if you will.  Back in the day, conventional wisdom said that a motion detector was all you needed.  The logic was that anything that moved inside the house would cause the authorities to come running.

In many cases this turned out to be a terrible inconvenience after a birthday party when a mylar balloon would go bobbing by when the furnace kicked on, or when the family pet went wandering in the middle of the night.  Maybe it is better to be safe than sorry, but having the police kick down the door in the middle of the night because a curtain moved could be inconvenient.   What about those midnight snack attacks?

Most Motion Detection Devices (also abbreviated PIR for Photo Infra Red) today are designed to discriminate between a person and other unidentified moving objects or smaller animals.  They do this by looking for heat and motion.  Still, to rely too much on motion detection is a mistake in most cases for the following and other reasons...

1. Their ability to distinguish the mortifying from the mundane is limited.  Hard wired sensors can ignore up to approximately 80Lbs, and wireless up to about 40Lbs, without getting into some very high tech, very expensive equipment.  Sometimes even a smaller pet can set off a motion detector depending on their activity, relative distance from the device and general position like going up stairs.
2. Deployed and programmed properly, the motion detectors are only reporting intrusion when the system is in an "away" mode- meaning that no one should be in the location.  When occupied, only perimeter protection would be reporting intrusion.  They don't protect you when you are home.
3. Motion detectors can give false alarms due to any number of failure modes, such as dirty lenses, low batteries, not to mention opperator error.

Still, what a great device!  Just a couple of well placed motion sensors can protect a warehouse or abandonned property.  Also, as many families have two wage earners, many homes are empty during the day.  Statistics show that most burglaries happen during the day.  As a back-up device, motion detectors cover a lot of area with just one sensor.  Many times you will see them in main hallways or centrally located, so that even if some type of spy were to dangle through the ceiling to rob your house, he wouldn't get too far before the motion sensor would sound the alarm. 

It's not a bad alternative if you hate to spend a lot of money to protect each individual opening in the home or office or barn or warehouse or...you get the picture.

Of course all opinions and suggestions manumitted here are subject to your own better idea. I'm not proud.