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How Do Motion Sensors Work?

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How Do Motion Sensors Work?
By: Peter M. Rogers
January 31, 2020
45 comments

How Do Motion Sensors Work?

This post was orginially published on September 2, 2010. It was updated on January 31, 2020 with new product information.

Motion sensors detect and report motion. When incorporated into your home security system, motion sensors can help identify when someone is moving around inside your home.

As with other home security devices, the trend has been moving toward wireless alarm 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, adjust, and even move with you if you are fortunate enough to have a DIY alarm system.

I will explain how a motion sensor works and why they are an important component of any home alarm system.

How motion

sensors work

Motion sensors can detect activity—in most cases, when nobody is home. That’s why they are normally not “awake” when you arm your system for the night. Motion sensors are an important part of a home alarm system because it’s not always practical to put a door or window sensor on every entry point. Motion detectors and glass break detectors can help you get the protection you need more affordably.

Several different types of motion detectors use different technology to function:

  • Passive infrared (PIR)
  • Active (Microwave)
  • Ultrasonic
  • Multi-technology

Passive infrared (PIR) motion sensors

PIR motion sensors are among the most common type of motion detectors used today in home security systems, including Frontpoint’s. When armed, these devices detect heat given off by people and animals. They look for objects warmer than the normal background temperature using a special lens to create beams of passive energy. They also look for motion. When the PIR motion sensor detects a warm object in motion across several infrared beams within a specified time from, it trips the alarm.

These passive motion sensors are also pet-friendly. They ignore body heat from small aminals (less than 40 pounds) which minimizes false alarms.

Active (microwave) motion sensors

Microwave motion sensors use microwaves to “see” what is happening around them. They send out pulses of microwaves, which bounce off of objects and reflect back to the sensor. If an object is moving, the measurement between it and the sensor will change, which trips the alarm. Many of the early motion detection devices used microwave technology, and there are still some microwave sensors being installed in commercial spaces today. Though they cover a larger area than PIR devices, microwave motion sensors are typically more expensive and are subject to interference from electrical pulses.

Ultrasonic motion sensors

Similar to microwave motion sensors, ultrasonic motion sensors send out pulses of ultrasonic waves, which bounce off of objects and reflect back to the sensor. The device measures the distance between it and objects, and if the measurement changes, it trips the alarm. Ultrasonic motion sensors were more popular when motion detectors were first invented but aren’t widely used today.

Multi-technology motion sensors

Also called dual-technology motion sensors, multi-technology motion detectors use aspects of more than one type of device. By combining “passive” and “active” technologies, like PIR and microwave, for example, multi-technology motion sensors reduce the likelihood of false alarms.

Things to

note about motion sensors

Though motion sensors can be a useful part of a home alarm system, intruders may be able to fool them. By knowing the ways someone can do this ahead of time, you’re better able to protect your home.

Some ways to avoid tripping a motion sensor include:

  • Moving extremely slowly
  • Walking directly underneath a detector
  • Walking and hiding behind large pieces of furniture
  • Crawling or crouching to a pet’s height
  • Covering a motion sensor with a piece of paper, foil or fabric

Many modern motion detectors are designed to withstand these evasion tactics. For example, the motion sensors Frontpoint uses project beams down and out to detect someone who is crawling. You can also prevent someone from fooling your home security motion sensors by placing them in the following areas:  

  • Hiding amongst decorations, like on a mantle or bookshelf
  • Behind (but not completely blocked) by your valuables, like a TV or computer
  • On the ceiling

To maximize the protection of your home, you should also consider using multiple types of devices. By using window and door sensors and glass break sensors with motion sensors, you’re more likely to catch an intruder.

Motion sensor placement

Motion sensor placement

The standard range for motion sensors is about 30 feet. The coverage area is shaped like a teardrop, with the skinny part at the detector. If placed strategically, motion sensors can maximize security coverage in your home or business.

The ideal spot to place a motion detector is in a high-traffic area that an intruder would cross if moving about in your home. To maximize “catch” performance, beams project down and out to pick up anyone trying to avoid detection by crawling.

Some examples of motion sensor placement include:

  • Near major home entry points, like the front door, back door or patio door.
  • A hallway that a person must walk through to access your home
  • A living room or waiting room with a big-screen TV
  • In the corner of a room with multiple entry points

For more information on where to place motion sensors, read our more detailed article on What is the Best Motion Sensor for Home Security?

Number of motion sensors

You might be asking yourself, “How many motion sensors do I need?” The answer depends on your space and the other intrusion sensors in your home. Motion sensors work better when intruders move across the beams, as opposed to approaching the sensor directly. Depending on the layout of your home, you may need to install sensors across from each other so there’s no chance an intruder wouldn’t cross the beams. If you need help determining how many sensors you need and where to put them, talk to our customer support representatives.

Thinking you want to add more motion sensors to your home security system? Check out the Frontpoint Motion Sensor.

Frontpoint keeps homes safe whether families are there or not. We've been revolutionizing the home security industry for over a decade. And we're just getting started. To shop DIY home security systems, check out our Security Packages. If you have questions or would like to discuss a quote, contact us at 1-877-602-5276.

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Comments
Chad
May 9, 2013
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? Thanks!
Reply
Peter M. Rogers
May 15, 2013
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.
Reply
Chad
May 9, 2013
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? Thanks!
Reply
Peter M. Rogers
May 15, 2013
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.
Reply
Brian
August 17, 2013
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.
Reply
Peter M. Rogers
August 19, 2013
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.
Reply
Brian
August 17, 2013
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.
Reply
Peter M. Rogers
August 19, 2013
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.
Reply
Karen
July 16, 2014
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.
Reply
Jamie Botzer
July 16, 2014
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.
Reply
Dart Humeston
September 20, 2014
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. Dart
Reply
Jamie Botzer
September 24, 2014
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
Reply
Dart Humeston
September 20, 2014
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. Dart
Reply
Robin E Haldeman
January 10, 2015
I purchased Alarm point Sensors with the condition that they be installed by a professional and they were in Early 2014. We have a 3 story house with a walk out on the bottom level and nothing has changed. Recently the Alarm went off and we had our shades up so we blamed it on ourselves and closed the shades. This morning we were woken by a Policy officer saying he was dispatched by your company and we heard no alarms and we wake up easily. I called Alarm Point and requested a call from someone to check out our system and the Alarm Point guy did not call me he called my husband even though I was the first phone number to be called. So my husband explained that he walked through a test on the phone and said everything was fine. Everything is not fine when there is no explaination as to why the sensor set the alarm off and why we didn't hear anything. Special note we don't use alarms to wake us up to go to work because we walk up at the least noise and when Alarm Point Alarms go off they can wake the dead. As you say 40 lbs or an overheated area. The area is cold all of the time we don't heat it because we do not use that level of the house and we have no pets. My husband said the guy gave no explaination as to how this could happen My underlying concern is that this will cause a boy cried wolf issue and when we are actually being robbed we will not recieve the appropriate response. My husband stated the Front Point has zero technical staff for onsite visits when this type of thing happens, is a true statement.?
Reply
Gilbert Cho
January 14, 2015
Hi Robin, I apologize for the delayed response, but I hope you'll still find it useful. It's true that we do not employ a technical staff for onsite visits, but it's for a number of reasons. To begin with, we personally don't like having strangers in our home (even if they're technicians), so we'd rather not subject our customers to having one in their home! But most importantly, we have a team of knowledgeable and helpful Support Specialists who we're confident will do everything in their power to work with you to solve the problem. The do-it-yourself style of our system lends itself well to this "teamwork" solution, where you act as our eyes and ears as we guide you. As for your concern regarding a boy-who-cried-wolf scenario, let me reassure you that there is no chance of this happening. We treat every single alarm very seriously - even if it's the tenth occurrence, we'll treat it like it was the first. Again, sorry for taking so long to respond to you. Hope this helps. Please feel free to ask further questions if you have any!
Reply
raphael
January 12, 2015
I have two ge motion sensors at minimal cost from Walmart. When set there is just one steady beam but pressing alarm there is a steady signal, is this the way they work. One does not become active at all. What is the problem with this sensor? Are ge sensors reliable? My problem is with staff in this senior residential bldg.,that for years illegally enter when I am not at home-harassment to vacate. As a last resort outside of a combo pad locked locker I have notice activity. Are these sensors reliable?
Reply
Gilbert Cho
January 12, 2015
Hey Raphael, While we use Interlogix (GE Security) sensors and are very confident in their reliability, we also have the benefit of having a superb and knowledgeable team backing them up. Since you purchased your sensors from Walmart, I'd recommend directly contacting the manufacturer using the phone number that came with the sensor.
Reply
Bob
January 16, 2015
why are 2 motions activated and 1 idle Thanks Bob
Reply
Gilbert Cho
January 20, 2015
Hi Bob, I'm not exactly sure what that's going on with your Motion Sensors, but I'll have a Support Specialist reach out to you soon to troubleshoot.
Reply
RB
January 31, 2015
Hi we bought frontpointbsystem and have been very happy for about 6months While we were asleep the motion sensor goes off at night. I did receive a call but they were not technical experts ! They had no clue which sensor tripped! We panicked as nothing of this sort happened before. While on phone I looked all around the house and noticed the living room motion fallen down. Can alarm trip if motion sensor falls off when armed ?
Reply
RB
January 31, 2015
Can motion sensor go off if it falls off when armed?
Reply
Patrick S
November 16, 2015
I just purchased Frontpoint and installed this weekend. Everything seemed to work fine until I tried testing the motion sensor. In test mode it works fine and detects motion. However when not in test mode and I arm the motion detector then I walk by it, nothing happens. The alarm should sound but there seems to be issues with the motion sensor connection to control panel.
Reply
Valerie Saponara
November 16, 2015
Patrick, I think what you are describing is our motion sensor lock-out period. Meaning that if you test your sensor you will see it works and detects motion. After running this test you need to give that motion sensor two minutes to switch over off that test mode. Then once you arm that sensor and walk by it you should see that alarm go off. Please try testing your motion sensor again, then leave it alone for a couple minutes, then arm your system and walk by that sensor. If your system isn't detecting your movement at that time let us know we can have someone give you a call to figure everything out!
Reply
Mary Nobida
April 17, 2016
Just installed security system. Does the red light on motion sensor only light up when testing? Thought it should light anytime we walk by it? Also, when arming will it always beep for 60 seconds until armed?
Reply
Katie Rynex
April 18, 2016
Mary, great questions! The red light on the Motion Sensor will only light up when the sensor is in test mode. As for the arming, you can silence those beeps by pressing the "silent" button on the Control Panel or by selecting the "silent arming" box on your smartphone app. We hope this answers your questions and ask that you please let us know if you have any other questions going forward.
Reply
Frank
May 20, 2016
Hello, I am about to purchase a used system from a Frontpoint client who is moving out of country. Will it be easy to obtain monitoring and how to proceed. I think his service contract is ongoing at this point.
Reply
Katie Rynex
June 7, 2016
Frank, great question! All you would need to do is give our Sales Team a call so they can set up the account in your name. Thanks for reaching out and please let us know if you need anything else!
Reply
David
May 22, 2016
My motion detector is making a high pitched sound. Is it broken? Does it need batteries?
Reply
Katie Rynex
June 7, 2016
David, we want to make sure that you are entirely taken care of which is why we are going to have one of our Support Specialists reach out to you to investigate that Motion Sensor. Thanks for reaching out and we look forward to speaking with you soon!
Reply
Josh
June 23, 2016
I am installing some z-wave light switches. Can I use a motion sensor outside to activate them through my frontpoint control center rather than having the alarm go off?
Reply
Courtney
August 18, 2016
the quote I received did not include window sensors and I was advised that they aren't needed due to motion sensors. But what about window intrusion when I'm at home and set to stay mode??
Reply
Katie Rynex
August 19, 2016
Courtney, Thank you for reaching out! We would love to assist you by having one of our Support Specialist reach out but we are having trouble locating your account based off of the information here. Please send us an email to Wecare@frontpointsecurity.com with the name on the account, or a phone number to reach you with and you reach out to you!
Reply
destiny
August 22, 2016
can I put my motion sensor on the ceiling ?
Reply
Katie Rynex
August 24, 2016
Great question Destiny! We would advise not to put the motion detector on the ceiling as this would make its coverage area smaller. We recommend putting it 4-5 ft off of the ground, on a wall, as this is the best way to maximize your protection. Please see our resource center (http://frontpointsecurity.com/support-overview/resource-center/user-guides) and chose our motion detector guide as this will walk you through best practices for mounting this particular sensor. Thanks for reaching out and we hope this was helpful!
Reply
Mikey
September 15, 2016
I would like to disable the red light on the sensing part of the device so that a passerby will not realize they have set off an alarm. Is that possible to do while still having the alarm work?
Reply
Katie Rynex
September 21, 2016
Thanks for reaching out Mikey! If you are referring to our Motion sensor then the red light should not be visible. This could indicate a tamper within the sensor. We'd like to have someone from our Support Team reach out to help you but we are not able to locate your account with the information provided here. Please send us an email to Wecare@frontpointsecurity.com with the name on the account so that we can have someone assist. Thank you and we look forward to hearing from you soon!
Reply
Manning
October 10, 2016
Like Bob asked on Jan. 16, 2015 at 6:29 PM, why are all of my motion sensors indicating that they are idling when the system is armed? I asked a friend to walk pass one and the alarm did NOT go off. I turned the system off and then armed it again using the keypad and all of the motion sensors were shown as "activated". I asked my friend to take another pass and the system seemed to be working properly, i.e. the alarm went off. However, shortly thereafter, while the system was armed again, all the motion sensors went back to the idling mode. I asked my friend to walk pass one one more time and the alarm went off. Strange. How do I know if my family and I are being protected if this system is not working properly. I called customer support for help but they were closed for the evening, which is strange because I never heard of burglars taking off like that. And after trying to get through to technical support on three separate occasions on the same night, I never got a call back . . . that evening or even the next day as I was told by "[your] answering service". If I'm paying for a product like this, then someone should be around if the system begins to fail. I can understand not having someone come by my residence but Frontpoint doesn't even have someone, or enough people, available to answer questions over the phone! I find that to be a little ridiculous. Currently, as I look at the Frontpoint app on my cellphone, there are two motion sensors idling and two that are shown as activated, even though the system is NOT alarmed. Sincerely, Manning
Reply
Katie Rynex
October 10, 2016
Manning, thank you for sharing your experience. Because your security is our top priority, we would like to have one of our Support Supervisors reach to you and assist with the testing of your motion sensors. As we are not able to find your account with the information provided here, please send an email with the name on the account to WeCare@frontpointsecurity.com and we will have someone reach out to you as soon as possible. Thank you again and we look forward to hearing from you soon.
Reply
Rob
October 23, 2016
Is there a way to put a delay on the motion censor? I would think you would be able to use a 30 sec delay just like the door censors, but I can't figure it out.
Reply
Katie Rynex
October 28, 2016
Rob, great question! There is an option to add a delay to your Motion Sensor so we ask that you contact our Support Team at your earliest convenience so they can help you set that up. Thanks for reaching out and we hope to hear from you soon!
Reply
Steven
November 6, 2016
Peter, We're approaching our 4 year anniversary of having a FrontPoint system. For the first time earlier this week we received a late night call from FrontPoint that a motion detector had been activated. (We were out of town at the time.). The police were called, investigated, and quickly followed up ( as did FrontPoint) with the info that nothing looked amiss. So, great service and follow up. Returning back home the other night I noticed a framed photo that had been on the same or nearby shelf as the motion detector was on the floor. Aha - I figured that it must have somehow fallen past the motion detector and set it off. Mystery solved I thought. Relaying the story to a friend tonight he thought that it wouldn't have set it off because he believed the motion sensors are heat (IR) sensitive, which you also indicate in your blog, and would not be set off by an object falling by it. . So...we are stumped. The photo was not on the floor when we left home, and this is the first time we have had an alert. So what do you think could have set off the alarm?
Reply
Steven
December 3, 2016
I wrote this a month ago - disappointed in FrontPoint that no response. I'd like to understand how our system works. First time I've experienced poor customer service. Hope it's not a sign of change at FrontPoint.
Reply
Katie Rynex
December 5, 2016
Steven, thanks for bringing this to our attention. We have one of our Support Supervisors looking into that alarm event and will have them reach out to you shortly to discuss everything in detail. Thanks again and we look forward to speaking with you soon!
Reply
Nicole
December 13, 2016
My upstairs motion detector is active but my downstairs is idle. What does this mean? Says that if I'm armed away or armed stay. Is it still working??
Reply
Katie Rynex
December 27, 2016
Nicole, when a Motion Sensor is listed as "active" it means that the sensor has detected motion. It can take up to an hour for your Motion Sensor to refresh and list as "Idle", which means there has been no motion detected. Your Motion Sensor is definitely still working properly and would trip if someone were to walk in front of the sensor. We hope this answers your question and ask that you please let us know if you have any other questions going forward.
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