Motion Sensors - What Motion Sensor Are and How They Work

What Motions Sensors Do

As the name implies, motion sensors detect and report motion – in most cases when nobody is home. That’s why they are normally not “awake” when you arm your system for the night (see a previous post that explains “Stay” vs. “Away” arming).

It’s not practical to put a door/window sensor on every window, and you usually don’t need to, since you can use motion sensors and/or glass break detectors (we’ll cover those in a future post) to get the protection you need more affordably. Plus, motion sensors have come a long way from the original models – much more reliable, and less prone to false alarms.

How Do They Work? 

The early motion sensors were considered “active” devices, because they emitted energy (microwave or ultrasonic) to see what was happening around them. There are some still some microwave sensors being installed in commercial spaces. Today the most common motion sensor uses Passive Infra-Red energy to detect heat given off by people (and animals!) – hence the name “PIR” often given to this device. 

Smart Technology

These are intelligent devices: they look for objects warmer than the normal background temperature, using a special lens to create “beams” of passive energy, and they also look for motion: when the sensor detects a “warm” object in motion across several infrared beams within a specified time frame – that’ what trips the alarm. If you want more of the science, here is a great link for the specifics.

Where Are They Placed?

 The standard range for motion sensors in the home is 30-35 feet, and the coverage area is shaped like a large water droplet, with the skinny part at the detector. The ideal spot for a motion sensor is in a high-traffic area that an intruder would cross if moving about in your home or business: think front hallways, living rooms with big-screen TV, etc.

Maximizing “Catch” Performance

Motion sensors work better when intruders move across the beams, as opposed to approaching the sensor directly. The beams project out and down, to pick up anyone trying to avoid detection by crawling.

Do not point them at a heat source like a stove or fireplace, and avoid pointing at windows that get direct sun. And, don’t use them in a space that gets so hot that the sensor can’t tell an intruder from the ambient temperature – like a garage in a warm climate.

Motion Sensors and Pets 

Today’s sensors are usually “pet-friendly” up to 40 pounds, which means they “ignore” cats and small dogs – unless your Siamese is downright acrobatic! That means that large dogs with the run of the house all day and night make it harder to use motion sensors – except for when you go on vacation, and board your critters. We’ll touch on this in more detail on Friday.

Wireless is the Name of the Game

As with other home security devices, the trend has been moving toward wireless technology for years. That means no drilling, no running wires, and a faster (and cleaner) installation process. It also means that these sensors are easier to troubleshoot, to adjust, and even to move with you if you are fortunate enough to have a DIY alarm system. See this previous post for the rundown on why wireless sensor technology is the new norm.

It’s true that every home alarm system I’ve seen over the past 25 years has at least one motion sensor. They work, they are reliable, and they should not cost a lot. Now you’ve got two alarm devices under your belt – the wireless door & window sensor, and the wireless motion sensor – and that brings us closer to being able to design a complete home security system. But we have a few more sensors to cover first!

Stay tuned for future posts in this series, and you’ll be in great shape when we put it all together. For now, keep focused on staying safe, and we’ll see you next Monday!