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Home Security 101: Motion Sensors

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March 24, 2014

Home Security 101: Motion Sensors

Today we’ll cover the second specific alarm equipment device that we’ll be examining in this educational series on home alarm technology and systems: the wireless motion sensor. As was the case when we discussed the door & window sensor, we’ll explain how the motion sensor works, and why it’s an important component of just about any home alarm system. So, let’s start with the basics.

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!

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June 28, 2014
What's the 'optimal height' for the motion sensor? Should I put it closer to the ceiling (as shown on the picture) or "3 to 5 feet from the floor" as I once heard? Or it doesn't really matter? Thanks, —Alex.
Jamie Botzer
June 30, 2014
Hi Alex, great question. The optimal height for your motion sensor is 4 to 5 feet from the floor. The picture is a bit misleading, sorry about that. I’ll see if we can’t find a better picture for this post to clear up any confusion. Thanks for your comment!
June 28, 2014
What's the 'optimal height' for the motion sensor? Should I put it closer to the ceiling (as shown on the picture) or "3 to 5 feet from the floor" as I once heard? Or it doesn't really matter? Thanks, —Alex.
October 9, 2014
what if I place my sensor facing an open window with direct sunlight how do I place it in a square room with a window on every side?
Gilbert Cho
October 10, 2014
Hi Nirvana! We advise that you do not place your Motion Sensor so that its faces a window (unless it’s covered). This can cause a number of problems because the sensor can detect things that you may not want it to – like a mailman passing by. Placing a Motion Sensor in a room with windows on every side is an interesting case, and a number of factors should be considered. Are the windows covered? How big are they? Is it a room that sees a lot of activity? It might turn out that a Motion Sensor isn’t the best answer for that room and that a Door and Window Sensor or Glass Break Sensor is. We’d be happy to help you further, either about placement of your Motion Sensor or going with an alternative option. Please feel free to call us at 877-602-5276, or email us at Thanks!
May 9, 2015
Hello, I just put in an order for a frontpoint system. I was wondering how I needed to place my motion sensors. I have a living room with an opening (about 3ft wide) into my kitchen. Would it be better to place the sensor on the molding of the opening between the two rooms or to place it on the far wall of one room (say the living room) but looking into the kitchen ? I see disadvantages in both since the former would cause the areas falling behind the "plane of the sensor" to be ignored (partially) while the latter would cause blockage from the walls on either side of the opening. Any advice (besides buy one more sensor -- my budget is pretty stretched thin as it is). ?
Valerie Saponara
May 11, 2015
Naveen, these are all great questions! It sounds like you have a very specific need and we want to make sure you're completely covered. We're going to have one of our Support Specialists reach out to you to go further into all the details about your current home's layout to best figure out where that sensor should go.
August 26, 2015
How sensitive are the motion sensors to ceiling fans? We live in south central Texas and leave our ceiling fans on all day to keep the air moving - will it ignore the ceiling fan movement? Thank you, Jess
Gilbert Cho
August 26, 2015
Hi Jess, don't worry, the Motion Sensor will ignore your ceiling fans! The way Motion Sensors are placed (using the recommended placement), ceilings fans will usually be out of their "vision." However, even if the ceiling fans were in the sensor's vision range, there should be no reason that they would trigger an alarm.
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