Wireless Home Security 101 – How Motion Sensors Work

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There is definitely some mystery surrounding motion sensors. They are a core element of most alarm systems, and it’s rare that I see an alarm system without at least one motion sensor. So, let’s dig into what they really do.

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 my recent post for a reminder on “Stay” vs. “Away” arming).  It’s usually not practical to put a sensor on every window, and you usually don’t need to, since you can use motion sensors and glass break detectors 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” given to the device. The smart detectors look for objects warmer than the normal background temperature, using a special lens to create “beams” of passive energy, and then look for motion: when the sensor detects a “warm” object moving across several beams within a specified time frame – that trips the alarm. If you want more of the science, here is a great link for the specifics.

Where do you use them?

The standard range 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 hallways, living rooms with big-screen TV, etc. The sensors work better when people 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 (going back to my previous blog post) 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 the critters.

There is a lot more I could say on the topic, but this is a good start. If you want more detail, you can check out the home security equipment video and podcasts at www.frontpointsecurity.com – or talk to one of our professional Security Consultants. They are trained to customize your alarm system appropriate to your home and lifestyle, or business – and all the reviews say they do a great job. This level of service is getting harder to find, but it’s a hallmark of the FrontPoint approach.  And when it comes to understanding motion sensors, give yourself a gold star; you’re ready for the next installment of Wireless Home Security 101!

Comments (9)

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  1. Dart Humeston

    Is there a way to put the motion detector in “test mode” so I can walk in front of it and determine what sets it off?

    Thanks much.

    • Jamie Botzer

      Hey Dart, there is! It’s pretty simple; here are the step-by-step directions:

      1. Disarm your system
      2. Press the ridged button on the top of the motion sensor. The front of the sensor will fall off
      3. Put the front of the sensor back on, you will hear a clicking noise
      4. A red light will illuminate on the front of the sensor for a couple moments
      5. Once the red light turns off, walk in front of the motion sensor — make sure you move across the field of vision of that sensor. The light will flicker red when it senses your movement
      6. If the red light does not flicker when you move through the field of vision for the motion sensor, please call one of our Customer Support Specialists (1-877-602-5276) and they’ll be able to troubleshoot with you further

      • EricTheRed

        If you press *6, enter the master code, and use the > icon to scroll to “Walk Test” you can safely set off the motion detector without alarming ADT.

  2. Brian

    Do motion sensors really need to be 4-5 off the ground, or could they be higher? Feels like an awkward hight to install them at. In other homes I have lived in the motions were pointed downward where the wall meets the ceiling.

    • Peter M. Rogers

      Brian – thanks for your question. They can be a bit higher. but these sensor look both “out” and “down.” If they are placed too high, then the infrared “beams” that detect an object in motion (based on a different heat signature compared to the ambient background temperature) will be less effective. In over twenty years in the business, I am not familiar with seeing them placed as high as you describe. It’s not that they would not work at that higher elevation: they just may not work as well.

      • Karen

        We are running into the same issue. With our previous alarm company, sensors were placed in the corner of the living room, angled down. The FrontPoint motion sensor we just received doesn’t seem to detect most of the motion. As another poster noted, there is no obvious spot I can think of to place the motion sensor to cover the entire room. The room is not that large…maybe 450 square feet.

        • Jamie Botzer

          I’m sorry you’re having issues, Karen! That’s not what we want at all. I’m going to have a Customer Support Specialist contact you to figure out the best placement of your motion sensor so that it covers the entire room. Thanks for bringing this to our attention.

  3. Chad

    I have read some blogs where they suggest installing some motion detectors upside down, 4-5 feet high, so they detect “out and up”. The idea is they would not detect pets (unless they’re climbers), but they would detect people walking around.

    Would this work as intended?

    Does anyone use “dual-tech” motion sensors any longer? Some claim they have a lower rate of false alarms. Any truth to that?


    • Peter M. Rogers

      Chad – thanks for the excellent question. The motion sensor that we sell is designed to “ignore” pets up to 40 lb. – although we have heard that some very athletic and acrobatic cats have set them off, jumping around! For homes with larger pets, our customers may limit where a pet can go, or use glassbreak sensors instead of motion sensors, and/or more door/window sensors, etc. There is usually a workaround, and we are very good at helping you through all the options. What you suggest – mounting the motion sensor upside down, but at a lower height, would negate the sensor’s “look down” capability, making the sensor less effective in detecting an intruder moving directly beneath the sensor.

      As for dual-tech sensors, they usually employ passive infrared technology (which is what our sensors use) in combination with micro-wave technology – and to activate the sensor, both technologies must sense motion. This is rarely used in a residential setting, but is used with some frequency in commercial settings, which are considered “harsher” environments and more in need of false alarm reduction through use of two technologies in combination.

      Thanks again.

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