Motion Sensor Security System

Short answer

• A motion sensor security system watches rooms, hallways and outdoor approaches for movement, then triggers an alarm, a phone alert, a camera recording or a light when it detects motion it was not expecting. The sensor is the detector, and the security panel, siren and phone app are what turn a detection into a response.
• Most home sensors use passive infrared, known as PIR, which notices changes in infrared radiation from warm bodies moving across its field of view. Microwave sensors send out microwave signals and read the reflections using the Doppler effect. Dual technology sensors combine PIR and microwave and usually alarm only when both detect motion at the same time, which cuts false alarms.
• Place indoor sensors 7 to 9 feet high in a corner, aimed so a person walks across the detection pattern rather than straight toward the sensor. Keep them away from windows, direct sunlight, radiators, fireplaces and heating and cooling vents, because sudden heat changes are a common cause of false alarms.
• Pet immunity is a real feature, but it is model specific and it has limits. Depending on the model, a sensor may ignore pets up to 40, 80 or 100 pounds, and only when it is mounted at the right height, set to the right sensitivity, and placed where pets cannot climb within 6 feet of it. It is not a promise that no pet will ever set the alarm off.
• Motion sensors detect movement, not identity. They cannot tell a burglar from a family member, they cannot see through walls or glass, and they work best as one layer of a system that also includes door and window contacts, and, where wanted, cameras. This article explains how each part works so you can judge what a system can and cannot do.

What a Motion Sensor Security System Is

A motion sensor security system is a set of detectors placed around a home or small business that watch for movement in spaces that should be empty, and report that movement to a central panel. When the panel receives a signal while the system is armed, it can sound a siren, send a notification to a phone, start a camera recording, switch on lights, or pass the alarm to a monitoring center, depending on how the system is set up.

The sensor itself does only one job: it notices movement and sends a signal. Everything else in the system decides what that signal means. That distinction matters, because most of the questions people ask about these systems, such as whether the sensor will catch a pet, whether it works in the dark, or why it went off when nobody was home, are really questions about how the sensor detects motion and how the system is configured.

A typical home setup has a control panel, one or more motion sensors, door and window contacts, and a siren or a phone app. The contacts guard the edges of the home: they report when a door or window opens. The motion sensors guard the space inside: they report when something moves through a room, hallway or stairwell. The two work together. A contact catches an entry at the perimeter, and a motion sensor catches movement deeper inside the home, including movement through an entry point that has no contact on it, such as a window left open or a door that was never fitted with a sensor.

Motion sensors are also used outside the alarm itself. The same technology switches on floodlights, opens automatic doors, and tells a smart home system that a room is occupied so it can adjust lights or heating. This article concentrates on security use, but the sensing principles are the same in every application, and the placement rules for security overlap heavily with the rules for lighting and automation.

One more point belongs at the start. A motion sensor system is a detection system, not a barrier. It does not stop anyone entering, and it does not identify who moved. It buys time, attention and evidence: a loud siren, a prompt alert, a recording, and a record of when and where movement happened. Judging a system means judging how reliably it detects real movement and how rarely it cries wolf, and both of those come down to the sensor technology and where the sensor is placed.

How Motion Sensors Work

Every motion sensor answers the same question: has something moved inside the area I am watching? The different technologies answer it in different ways, and the differences explain most of the strengths and weaknesses that follow.

PIR sensors

Passive infrared, or PIR, is the most common technology in home security sensors, and it is passive because it sends out nothing. It listens. Every warm object gives off infrared radiation, and a PIR sensor watches for changes in the pattern of that radiation across its field of view. When a person walks through the room, their body heat moves across the sensor’s detection zones, the infrared pattern changes, and the sensor registers motion.

The lens in front of a PIR sensor is a Fresnel lens, a flat plastic lens divided into segments that split the field of view into a fan of detection zones, with gaps between them. A person walking across those zones produces a sequence of changes as they pass from one zone to the next, and that sequence is what the sensor’s electronics recognize as movement. This is why installers talk about mounting a sensor so that an intruder crosses its pattern. A person walking directly toward the sensor crosses fewer zones, so the signal is weaker and detection can be slower. Honeywell’s installation instructions for its 5894PI detector state this plainly: for best coverage, mount the detector so that the likely direction of intruder motion is across the pattern.

PIR sensors have two properties that shape everything else about them. First, they work in the dark, because they sense heat, not light. Second, their sensitivity falls when the background temperature approaches body temperature. OPTEX, a sensor manufacturer, notes that in high temperature environments the difference between a person and the background shrinks, which can reduce detection sensitivity. A PIR sensor in a room that is already close to body temperature has less contrast to work with.

PIR sensors are inexpensive, use little power, and are easy to run on batteries for years, which is why they dominate wireless home sensors. Their weakness is that anything that changes the infrared pattern can look like motion: sunlight sweeping across a floor, a radiator warming up, a curtain moving in a draft from a vent, or a pet crossing the room. Good placement and sensitivity settings exist to manage exactly those problems, and they are covered below.

Microwave sensors

A microwave sensor is active: it sends out microwave signals and reads what comes back. When the reflected signal changes because something in the room is moving, the sensor detects the shift in frequency caused by the Doppler effect and registers motion. OPTEX describes this as detecting changes in reflected microwave signals using the Doppler effect.

Microwave sensors can see movement that a PIR sensor would miss, and they keep working when the room is hot, because they do not depend on a temperature difference. They also have drawbacks. OPTEX notes that they are more susceptible to environmental noise such as rain and reflections from metal surfaces, and that their detection area tends to be less clearly defined than a PIR sensor’s. Microwaves can also pass through some materials that block infrared, which means a microwave sensor can sometimes detect movement on the other side of a thin wall or a glass panel. That is a false alarm risk in a home, and it is one reason microwave sensors are often used together with PIR rather than alone.

The frequencies used in these sensors are in the microwave band used by many intrusion detectors. Bosch’s Blue Line Gen2 detectors, for example, operate at 10.525 GHz in the US models, with European variants at 10.510 to 10.610 GHz. Those numbers matter mainly for installers, who need to know that nearby microwave sources can interfere, and that the sensor’s range can be adjusted on many models.

Dual technology sensors

A dual technology sensor, also called dual tech or TriTech in Bosch’s product line, combines PIR and microwave in one housing and requires both to agree before it alarms. OPTEX states the rule plainly: its dual technology sensors cross verify the detection results from both sensors and only trigger an alarm when both detect motion simultaneously, using AND logic. Honeywell’s 5898 dual technology detector works the same way: both technologies must detect movement simultaneously within their fields for an alarm to be triggered.

The benefit is a sharp drop in false alarms. A burst of sunlight may fool the PIR element, but it will not fool the microwave element, so no alarm sounds. A vibration or a reflection may disturb the microwave element, but it will not disturb the PIR element, so again no alarm sounds. Only something that looks like a moving warm body to both technologies at once, which is what a person is, produces an alarm. OPTEX adds that dual technology offers high resistance to environmental factors that often cause false detections in PIR only systems, such as strong sunlight, vehicle headlights and sudden temperature changes.

Dual technology sensors typically cost more than PIR only sensors, and they can be slightly slower to respond, because they wait for confirmation from the second technology. In rooms that are prone to false alarms, such as rooms with large windows, heating vents, or pets, that trade is usually worth making. In a quiet interior room with no heat sources and no pets, a PIR only sensor is often enough.

Other sensor types

A few other technologies appear in security systems, though they are less common in homes.

Ultrasonic sensors send out pulses of sound above the range of human hearing and listen for the echo. Movement changes the echo, and the sensor registers it. They are sensitive and can cover a room well, but they can be disturbed by air movement and by sounds from machinery, and they appear more often in commercial and occupancy sensing than in home alarms. ADT’s guide lists ultrasonic sensors among the less common types and advises sticking with proven technologies for home security.

Video motion detection does not use a separate sensor at all. A camera, or software behind a camera, compares one video frame with the next and flags changes in the picture. It can tell where in the frame the movement happened and how large it was, which makes it useful for sorting people from passing cars or swaying trees. Its weakness is that it needs light or infrared illumination to see, and that changes in light, such as clouds passing or a light switching on, can look like motion. Video detection works best as a partner to a PIR sensor, not as a replacement for one.

Tomographic sensors use a mesh of radio signals between small nodes placed around a room and detect when a body disturbs the mesh. They are a newer technology and are rare in ordinary home security systems, so they are noted here for completeness rather than recommended.

The Parts of a Motion Sensor Security System

A sensor on its own is not a system. The parts below are what turn a detection into protection, and each one is a point where a system can be strong or weak.

The sensor is the detector. It watches its zone and sends a signal when it detects motion. Wired sensors send that signal along a cable. Wireless sensors send it by radio to the panel, and many use low power wireless links that run for years on a single battery. Honeywell’s SiXPIR, for example, is a wireless detector with a stated 300 foot transmission range and encrypted signalling, and our guide to Bluetooth Low Energy beacons explains the low power radio ideas that wireless sensors rely on. Battery life varies by model and by how often the sensor triggers, so a sensor in a busy hallway will need a new battery sooner than one in a spare room.

The control panel is the brain. It receives signals from every sensor and contact, knows whether the system is armed or disarmed, and decides what to do when a sensor trips. Panels are usually armed in two modes: away, in which all sensors are active, and stay or home, in which perimeter contacts are active but interior motion sensors are off, so people can move around the house at night without setting the alarm off. Knowing which mode you are in is essential, because most of the “my sensor did not go off” complaints turn out to be a system armed in stay mode while the owner expected away mode behavior.

The siren, bell or sounder is the immediate response. It makes the alarm audible inside and sometimes outside the home. Its purpose is to alert the people inside, to startle an intruder, and to draw attention from neighbors. A system with no siren and only a silent phone alert can still be useful, but it gives up the deterrent effect of noise.

The notification path is what tells you when you are not home. Most modern systems send a push notification or a text message to a phone app, and monitored systems pass the alarm to a monitoring center, which can call the owner and, where arranged, contact the police. This is also where the problems of wireless and cellular systems show up: a system that cannot reach its panel or its monitoring center cannot report an alarm. Our companion article on common pain points in wireless and cellular home security systems surveys those failure modes, including signal loss, interference, dead batteries and cellular outages. If your system is wireless or cellular, read that article alongside this one, because the sensor is only as reliable as the link that carries its signal. That link is flagged here as confirm live or hold, since the companion article is in the same wave and was not live when this article was written.

The power supply and the backup battery keep the panel running during a power cut. Wired sensors draw power from the panel, so they keep working while the panel has power. Wireless sensors have their own batteries, so they keep working during a power cut, but they will stop when their batteries run out, and a low battery warning that is ignored is a common cause of a sensor going quiet.

Sensor Types and Where Each One Fits

Not every motion sensor is the same shape, and the shape usually tells you where it belongs.

Wide angle PIR sensors are the standard indoor sensor. They cover a broad fan shaped area, typically around 90 degrees and up to about 35 to 40 feet deep, depending on the model. Honeywell’s 5894PI covers up to 35 feet, and Bosch’s Blue Line Gen2 TriTech models cover 12 meters by 12 meters, or 40 feet by 40 feet. They mount in a corner or on a flat wall and guard a whole room.

Long range or corridor sensors use a narrow beam to watch a hallway, a corridor or a long room. Their detection pattern is a thin wedge rather than a fan, so they can watch a long space without being distracted by movement in rooms off to the side. They are useful for hallways that lead to bedrooms, where a wide angle sensor would cover rooms that do not need watching.

Curtain sensors create a thin vertical plane of detection, like an invisible curtain, across a wall of windows, a sliding door or a balcony. Anything crossing that plane triggers the sensor, while movement behind it does not. They are the right choice for a wall of glass, where a wide angle sensor would either miss the windows or be blinded by sunlight.

Outdoor sensors are built for weather, temperature swings, insects and small animals, and they use different detection logic from indoor sensors. OPTEX notes that outdoor sensors face swaying vegetation, vehicle headlights, strong sunlight, rain and wild animals, while indoor sensors face pets, air conditioning and sunlight through windows. An indoor sensor used outdoors will false alarm constantly and will fail in rain. If you need outdoor coverage, buy a sensor rated for outdoor use and expect it to cost more and to need more careful aiming.

Ceiling mount sensors watch a room from above, usually in a 360 degree pattern. They suit large open rooms and commercial spaces. They are rarely needed in a home, but they are the right answer for a big open plan living area where wall mounting would leave blind spots.

Pet immune sensors are not a separate technology. They are PIR or dual technology sensors with a lens and signal processing designed to ignore movement close to the floor. How that works, and where it fails, is important enough to have its own section below.

Occupancy sensors look like security sensors and use the same PIR technology, but they are built for a different job: turning lights on and off and telling a building system that a room is in use. They are tuned for sensitivity rather than for security, and they are not a substitute for an alarm sensor. If a sensor is sold as an occupancy sensor, treat it as a lighting control, not as part of your security system.

Placement: Where to Put Sensors and Where Not To

Placement decides whether a sensor works. A good sensor in a bad spot will false alarm or miss an intruder, and a modest sensor in a good spot will do its job for years. The rules below come from the installation manuals of Bosch and Honeywell, and they agree with each other closely enough that they can be treated as the standard guidance.

Mount indoor sensors 7 to 9 feet high, on a wall or in a corner, level both horizontally and vertically. Bosch’s Blue Line Gen2 manual gives 2.2 to 2.75 meters, or 7.5 to 9 feet, with no adjustment needed inside that range, and Honeywell’s 5898 manual gives 7 to 9 feet. The height matters because the lens is designed to project its zones downward from that height. Mount too low and the zones reach too far into the room, so a pet on the floor can trigger them. Mount too high and the zones pass over a person close to the sensor, leaving a blind spot right under it.

Aim the sensor so that a person crossing the room walks across its detection pattern, not straight toward it. The best position is usually a corner that faces the room’s main entry points, so anyone entering has to cross the sensor’s view. A sensor facing a door directly will see a person coming toward it, which produces a weaker signal, and a sensor facing a window will see sunlight and movement outside.

Cover the routes an intruder would have to use, not every square foot of the house. The rooms that matter most are the main entry hallway, the living room, and the route to the bedrooms. A sensor in each of those, plus contacts on the doors and windows, covers most homes well. Bedrooms themselves usually do not need motion sensors, because people move in them at night while the system is in stay mode, and a sensor there would either be switched off or cause false alarms.

Keep sensors away from heat sources. Do not point a sensor at a radiator, a fireplace, a stove, or a heating vent, and do not mount it above a heat source. Do not point it at a window where direct sunlight will sweep across the floor during the day, and do not mount it where it faces an air conditioning outlet. Bosch’s installation guidance lists the same avoid list: direct sunlight, drafts, heat sources, moving objects, windows and uninsulated walls. Honeywell’s guidance adds fluorescent lights and moving machinery to the list of things to keep away from.

Keep the sensor’s view clear. Furniture, tall plants, curtains and stacked boxes can block the lower detection zones and create blind spots. A sensor that cannot see the floor near the far wall cannot see a person crouching there. Walk the room after installation and check that nothing stands between the sensor and the areas it is supposed to watch.

Outdoor placement follows the same logic with a wider margin for error. Mount outdoor sensors where they watch approach paths, driveways and the ground in front of windows and doors, and aim them slightly downward so they watch the ground rather than the sky. Keep them away from trees and shrubs that sway in the wind, and away from roads where passing headlights can sweep across the lens. Expect to adjust an outdoor sensor after a few days of use, because the first position is rarely the best one.

When the system is installed, test it with a walk test. Most panels have a walk test mode that lets you walk through each sensor’s zone and confirm that it triggers. Walk across the pattern at different speeds, walk along the edges of the coverage, and check that the sensor does not trigger when you move outside its intended area. Repeat the test after any furniture move or any change to heating and cooling, because a room that has changed can turn a good sensor into a bad one.

Pet Immunity: How It Works and Where It Fails

Pet immunity is the feature most often misunderstood in motion sensor security. It is real, it works when it is set up correctly, and it is not a guarantee.

A pet immune sensor ignores movement that stays close to the floor and below a certain size. It does this in two ways. First, the lens is designed so that the lower detection zones are aimed higher, or are masked, so that a small animal moving on the floor does not cross enough zones to trigger an alarm. Second, the sensor’s electronics compare the size and the heat signature of the moving object with what a person would produce, and ignore signals that are too small. Bosch’s Smart Home motion detector, for example, pairs its PIR sensor with a temperature sensor and an algorithm, and switches off its downward looking sensor when pet immunity is turned on.

The weight limits are model specific, and they vary widely. Bosch’s professional TriTech detectors are rated to ignore one or two pets up to 45 kilograms, or 100 pounds, or numerous rodents. Honeywell’s 5898 wireless dual technology detector offers pet immunity settings of 0, 50 or 100 pounds with its animal immune lens. Honeywell’s IS2535 PIR sensor allows pets up to 36 kilograms, or 80 pounds, on its lowest sensitivity setting, and up to 18 kilograms, or 40 pounds, on its low setting. Bosch’s Smart Home detector is rated for pets up to 25 kilograms. These are not interchangeable numbers. The limit belongs to the model, and the only reliable figure is the one in your sensor’s own manual.

Pet immunity also depends on the installation, and the manuals are strict about the conditions. The sensor must be mounted at the specified height, usually 7 to 7.5 feet for pet applications, and set to the right sensitivity. Pets must not be able to come within 6 feet of the sensor by climbing on furniture, boxes or stairs, because a pet that climbs close to the sensor looks large to it, and the immunity no longer applies. The sensor must not be aimed at a stairway that a pet can climb. Honeywell’s instructions for the 5894PI and the Aurora both repeat the 6 foot rule, and both state that the total weight of the animals must stay within the rated limit.

There are also conditions the manuals note that owners often miss. Honeywell’s 5894PI instructions state that pet immunity applies when the room temperature is above 50 degrees Fahrenheit. Bosch and Honeywell both note that pet immunity characteristics have not been tested by Underwriters Laboratories, which means the feature is a manufacturer rating, not an independently certified one. A large dog that jumps, a cat that climbs onto a shelf near the sensor, or two pets whose combined weight exceeds the rating can all defeat pet immunity, and the manuals say so.

If you have pets, the practical approach is to choose a sensor whose rated limit comfortably exceeds your pet’s weight, mount it exactly as the manual directs, keep climbing routes away from it, and then test it with your pet in the room while the system is in test mode. If the sensor trips, adjust the position or the sensitivity before you rely on it. And if your pets are large, or you have several of them, consider arming the system in stay mode at night, or using door and window contacts as the main protection while you are home, rather than depending on pet immunity alone.

False Alarms: Causes and Fixes

False alarms are the most common complaint about motion sensor systems, and they are also the most fixable problem, because most of them have a visible cause. The table of causes below follows the troubleshooting guidance in the Bosch and Honeywell manuals.

Sunlight is the leading cause. Sunlight moving across a floor, or reflecting off a window, changes the infrared pattern fast enough to trip a PIR sensor. The fix is placement: move the sensor so it does not face the window, or fit a blind or curtain that keeps direct sun off the sensor’s field of view. Dual technology sensors are much less affected, because sunlight does not trigger the microwave element.

Heating and cooling cause similar problems. A radiator warming up, a fireplace lighting, or warm air blowing from a vent can change the infrared pattern in the sensor’s view. Move the sensor away from the heat source, or aim it away from the vent. Do not mount a sensor above a radiator.

Pets are the next most common cause, and the fix is in the pet immunity section above: correct mounting height, correct sensitivity, the right lens fitted, and no climbing routes within 6 feet. If false alarms continue after those steps, the pet may be above the sensor’s rated limit, and a dual technology sensor or a different placement may be needed.

Moving objects inside the sensor’s view can trip it. Curtains moving in a draft, a balloon drifting across a room, a mobile turning in a breeze, or a plant swaying near a vent can all look like motion to a PIR sensor. Remove the object from the detection zone, or move the sensor. Insects are a smaller version of the same problem: a spider walking across the lens, or a fly inside the housing, can trigger a sensitive PIR sensor. Sealed optics and insect resistant housings help, and Bosch notes draft and insect immunity as a feature of its Blue Line detectors.

Microwave sensors have their own false alarm causes. Because microwaves can pass through some thin materials, a microwave sensor can detect movement in the next room, or a person walking outside a window. Water moving in pipes, a fan, or a fluorescent light can also disturb a microwave sensor. The fix is usually to reduce the microwave range, which many dual technology sensors allow, or to switch the room to a PIR only sensor if the microwave element is the problem.

Low batteries cause a different kind of failure. A wireless sensor with a dying battery may send false signals, or may stop reporting altogether. Replace batteries when the panel reports a low battery, and do not wait for the sensor to fail. A sensor that has gone quiet is worse than a sensor that false alarms, because nobody notices it until it is needed.

Finally, a sensor that has been knocked, moved, or mounted loosely can false alarm when it shifts. Check that the sensor is firmly fixed to the wall, level, and that its cover is properly closed. A loose cover can let in drafts and insects.

When a false alarm happens, do not just reset the system and forget it. Note the time, the sensor, and what was happening in the room: was the sun shining in, was the heating on, was a pet in the room, was a window open? A pattern across two or three false alarms usually points straight at the cause, and the manuals’ troubleshooting tables are built around exactly that kind of pattern.

What Motion Sensors Cannot Do

Knowing the limits of a motion sensor is as important as knowing how it works, because the limits decide what else your system needs.

A motion sensor cannot identify who moved. It cannot tell a burglar from a family member, a cleaner, or a guest. That is why systems have entry delays, disarm codes, and stay mode: the system needs a way to know that the movement is authorized, and the sensor alone cannot provide it.

A motion sensor cannot see through walls, solid doors or glass. A PIR sensor needs a clear line of sight to the area it protects, and anything that blocks infrared radiation blocks the sensor. A person standing behind a closed door is invisible to it. Microwave sensors can sometimes see through thin materials, but that is a flaw rather than a feature, because it produces false alarms rather than useful coverage.

A motion sensor cannot detect a person who does not move. Someone standing perfectly still in a room, or moving very slowly, may not produce enough change in the infrared pattern to trigger a PIR sensor. Honeywell’s manuals give a detectable walk rate of 0.5 to 10 feet per second for its detectors, which means movement slower than a slow walk can be missed. This is a real limit, but it is a narrow one: an intruder who wants to move through a house has to move, and crossing the sensor’s zones at any normal walking speed will trigger it.

A motion sensor cannot detect entry through a door or window by itself. It detects movement after entry, not the act of opening. That is why door and window contacts matter: they catch the entry at the moment it happens, and they work even if the intruder moves slowly or stays still once inside.

A motion sensor can be defeated by tampering. Covering the lens, spraying it, or placing an object in front of it can blind a PIR sensor. Higher grade sensors include anti masking detection, which reports when the lens is covered, and Bosch’s Professional Series TriTech+ models include active anti masking as a feature. A basic home sensor usually does not. If tampering is a concern, choose a sensor with anti masking, mount it where it is hard to reach without crossing another sensor’s view, and pair it with a camera that can see the sensor itself.

A motion sensor cannot protect a home on its own. It is one layer. Door and window contacts guard the perimeter, motion sensors guard the interior, cameras provide identification and evidence, and lighting deters and reveals. A system that relies on motion sensors alone has gaps at every entry point and no way to identify who set it off. Plan the system as layers, and judge each layer on what it actually does.

Wired, Wireless and Smart Home Integration

Most home motion sensors today are wireless. A wireless sensor runs on a battery, talks to the panel by radio, and can be mounted anywhere within range of the panel without running a cable. That makes installation easy and makes it simple to add a sensor later. The costs are batteries that need replacing, a radio link that can suffer interference, and a range limit: a sensor too far from the panel, or behind thick walls or metal, may not report reliably. Our guide to how to organize smart home wiring covers the planning side of a home with many connected devices, including where wireless links struggle and where a wired connection is worth the effort.

Wired sensors draw power from the panel through a cable and send their signal along the same cable. They never need a battery, and their signal cannot be jammed the way a radio signal can. The cost is installation: running cable through finished walls is disruptive and often needs a professional. Wired sensors suit a new build or a major renovation, where cables can be run before the walls are finished. Wireless sensors suit an existing home, where running cables is impractical.

Smart home integration turns a motion sensor into more than an alarm device. The same detection that triggers an alarm can switch on lights when someone enters a room, adjust heating when a room becomes occupied, or send a notification when movement is detected while the house is empty. Our guide to building a smart home explains how these devices fit together, and how to plan a system so that the alarm, the lighting and the heating do not fight each other. The important point for security is that a sensor used for automation and a sensor used for the alarm are doing different jobs with different reliability needs. A light that turns on a second late is an annoyance. An alarm that reports a second late is still useful, but an alarm that reports to a phone app instead of a siren is a different system from one that sounds a siren, and the owner should know which one they have.

Connected sensors also raise a network security question. A wireless sensor that talks to a panel over an encrypted radio link is hard to interfere with, but a sensor that connects through a home Wi-Fi network depends on that network being secure. Keep the home network protected with a strong password and up to date router software, and read our guide on how to protect your home computer for the basics that apply to every connected device in the house, including security sensors.

Choosing a Motion Sensor System

This article recommends no product, and no brand is endorsed here. The questions below are the ones to ask when comparing systems, because they are the questions that decide whether a system will work in your home.

First, what technology does each sensor use, and where will it be placed? PIR only sensors suit quiet interior rooms. Dual technology sensors suit rooms with windows, heating vents, pets or other false alarm risks. Outdoor sensors need to be rated for outdoor use, and a sensor that will watch a wall of glass needs a curtain pattern.

Second, what is the coverage of each sensor, and how many sensors does the plan need? A wide angle sensor covers a room up to 35 or 40 feet deep, but a long hallway needs a long range sensor, and a large open plan room may need two sensors. Ask for the coverage pattern of each sensor in the plan, and check that the plan covers the entry routes, not just the rooms that are easy to reach.

Third, what is the pet immunity rating of the sensors, and does it cover your pets? Check the rated weight, the required mounting height, and the sensitivity setting. If the rating is below your pet’s weight, the sensor will false alarm, and no amount of adjustment will fix that.

Fourth, how does the system report an alarm? Does it sound a siren, send a phone notification, call a monitoring center, or all three? Does it keep working during a power cut, and for how long? Does it report a low battery, a sensor fault, or a sensor that has gone quiet? A system that cannot tell you that a sensor has stopped working is a system that can fail silently.

Fifth, what standard does the equipment meet? Intrusion detectors are built to standards such as UL 639 in the United States and EN 50131 in Europe, and the European models of Bosch’s Blue Line detectors are rated compliant with EN 50131-2-4 Grade 2 (US models are UL 639 listed). A standard does not guarantee that a sensor is right for your home, but it does mean the sensor has been tested against a published set of requirements, and that is a better basis for comparison than a marketing claim.

Sixth, who installs and maintains it? A professionally installed system comes with placement expertise and a maintenance visit, and the installer carries the responsibility for getting the placement right. A do it yourself system costs less and can be fitted in an afternoon, but the placement decisions, the walk testing and the false alarm diagnosis become the owner’s job. Either route can work, but the owner should know which responsibilities they are taking on.

Maintenance and Testing

A motion sensor system needs little maintenance, but it needs regular testing. The routine below takes a few minutes and catches most failures before they matter.

Test the system every month with a walk test. Arm the system in test mode if the panel offers one, walk through each sensor’s zone, and confirm that each sensor triggers. Check that the panel reports each sensor by name, so you know which sensor you are testing. If a sensor does not trigger, check its battery, its mounting, and whether anything is blocking its view.

Replace batteries when the panel reports a low battery, and keep a note of when each sensor’s battery was last changed. A sensor that false alarms or goes quiet is often a sensor with a dying battery. Where a sensor uses a standard battery, keep spares in the house.

Clean the sensor lens gently with a soft dry cloth if it is dusty, and check that the cover is closed and the sensor is still level. Do not use cleaning fluids on the lens, and do not paint over a sensor. Check that no new furniture, plant or stored box has moved into the sensor’s view since the last test.

Review the false alarm history when it happens. Most panels log which sensor triggered and when. A sensor that false alarms at the same time of day is probably seeing sunlight. A sensor that false alarms when the heating comes on is probably seeing warm air. A sensor that false alarms only when a pet is in the room is probably a pet immunity problem. The log is the fastest route to the cause.

Finally, revisit the placement once a year, or after any change to the home: a new window blind, a moved radiator, a new pet, a rearranged room, or a new piece of furniture can all change what a sensor sees. A system that was well placed when it was installed can drift into false alarms as the house changes around it.

FAQ

Do motion sensors work in the dark?

Yes. PIR sensors detect infrared radiation from warm bodies, not visible light, so darkness makes no difference to them. Microwave sensors also work in the dark, because they use reflected microwave signals. Video motion detection is the exception: a camera needs light or infrared illumination to see, so it needs a light source or night vision to work in the dark.

Can motion sensors detect pets?

They can, and that is the problem pet immunity is designed to solve. A pet immune sensor ignores movement close to the floor and below a rated size, but the rating is model specific: examples in this article range from 25 kilograms for a Bosch Smart Home detector to 100 pounds for Bosch professional TriTech models and Honeywell dual technology detectors. Pet immunity only works when the sensor is mounted at the specified height, set to the right sensitivity, and placed where pets cannot climb within 6 feet of it.

Why does my motion sensor keep going off for no reason?

The most common causes are sunlight sweeping across the sensor’s view, warm air from a radiator or vent, a pet in the room, curtains or plants moving in a draft, or a low battery. Check the time pattern: sunlight causes alarms at the same time of day, heating causes alarms when the system warms up, and pets cause alarms when the pet is in the room. Move the sensor away from the cause, fit a dual technology sensor if the room is prone to false alarms, and replace the battery if the panel reports it as low.

How high should a motion sensor be mounted?

Indoor sensors are usually mounted 7 to 9 feet high, on a wall or in a corner. Bosch’s Blue Line Gen2 manual gives 7.5 to 9 feet, and Honeywell’s 5898 manual gives 7 to 9 feet. The height matters because the lens projects its detection zones downward from that height. For pet immunity applications, follow the manual’s specified height exactly, usually 7 to 7.5 feet, because the immunity depends on it.

Do I need a motion sensor in every room?

No. Cover the routes an intruder would have to use: the main entry hallway, the living room, and the route to the bedrooms. Door and window contacts guard the entry points themselves. Bedrooms usually do not need motion sensors, because people move in them at night while the system is in stay mode. A sensor in every room adds cost and false alarm risk without adding much protection.

Are wireless motion sensors reliable?

Yes, when they are within range of the panel and their batteries are fresh. Wireless sensors use low power radio links, and many use encrypted signals. Their weak points are battery life, radio range through thick walls, and interference, and a wireless system also depends on the link between the panel and the phone app or monitoring center. Test sensors regularly, replace batteries promptly, and read our companion article on common pain points in wireless and cellular home security systems for the failure modes to watch for. That link is flagged as confirm live or hold, since the companion article is in the same wave and was not live when this article was written.

Sources & references

• OPTEX, "What Is a Motion Sensor? Principles and Technical Features", optex-asean.com, published 26 May 2025, updated 25 May 2026. PIR, microwave and dual technology principles, AND logic, and indoor versus outdoor false alarm factors.
• ADT, "A Complete Guide to Motion Sensors: What You Need to Know", adt.com. Sensor types including PIR, active, combined and ultrasonic, detection range, and integration with alarms, cameras and lighting.
• Bosch Security Systems, Blue Line Gen2 TriTech Motion Detectors user manual, 2021. PIR and microwave Doppler radar combination, pet friendly models rated to 45 kilograms or 100 pounds, mounting height 2.2 to 2.75 meters or 7.5 to 9 feet, coverage 12 meters by 12 meters, and EN 50131-2-4 Grade 2 compliance.
• Bosch Security Systems, DS820i and DS820iT Series TriTech PIR and Microwave Intrusion Detectors installation instructions. Pet immunity for a dog up to 100 pounds or two 60 pound dogs, mounting height 7.5 to 9 feet, and guidance to avoid direct sunlight, drafts, heat sources and windows.
• Bosch Smart Home, Motion Detector instruction manual. PIR with temperature sensor and algorithm, 90 degree and 12 meter detection range, mounting height 2.20 to 2.70 meters, and pet immunity for pets up to 25 kilograms.
• Resideo Honeywell Home, 5898 Wireless Dual-TEC Motion Detector user manual. PIR and X-Band microwave dual technology with AND logic, mounting height 7 to 9 feet, and pet immunity settings of 0, 50 or 100 pounds with the animal immune lens.
• Honeywell IntelliSense, IS2535 Passive Infrared Motion Sensor instruction manual. Mounting height 2.3 to 2.7 meters, pet immunity up to 36 kilograms or 80 pounds on lowest sensitivity and 18 kilograms or 40 pounds on low sensitivity, and the 6 foot climbing rule for pets.
• Honeywell, 5894PI Passive Infrared Motion Detector installation instructions, 2004. Wide angle coverage up to 35 feet, mounting across the intruder's path, pet immunity up to 80 pounds, and detectable walk rate 0.5 to 10 feet per second.
• Honeywell Aurora and Aurora-T Passive Infrared Motion Detectors installation instructions. Split zone optics, pet immunity up to 40 pounds, mounting height 7 feet, and the 6 foot climbing rule for pets.
• AlarmClub, "SiXPIR: Resideo Honeywell Home Wireless SiX-Series Encrypted Pet-Immune PIR Motion Detector". 300 foot transmission range, AES 128-bit encrypted signal, 40 foot by 56 foot detection pattern, and pet immunity for pets up to 80 pounds.