The Beginner’s Guide to Motion Sensors

A motion sensor (or motion detector) is the linchpin of your security system because it detects when someone is in your home when they shouldn’t be. A motion sensor uses one or multiple technologies to detect movement in an area.

When a sensor detects motion, it sends a signal to your security system’s control panel, which connects to your monitoring center. This alerts you and the monitoring center to a potential threat in your home.

Motion sensors stand guard, ready to react to various situations, such as movement in your living room, windows or doors opening or closing, or a window breaking.

Here are some common uses of motion sensors:

• Alert you in the event that your teen breaks curfew
• Trigger a doorbell when someone approaches the front door
• Alert you when kids enter restricted areas in the home, like the basement, workout room, or medicine cabinet
• Save energy by using motion sensor lighting in unoccupied spaces
• Notify you if pets enter areas where they’re not supposed to be

Each sensor type operates in different areas of the spectrum (ranging from passive to active). Dual technology motion sensors are not as likely as other types to cause a false alarm, because both sensors need to trip in order to sound an alarm. However, this does not mean that they never cause false alarms.

• Area reflective sensors emit infrared rays from an LED and use the reflection of those rays to measure the distance to the person or object, allowing for detection when the subject moves within the designated area.
• Ultrasonic motion sensors measure the reflections off of moving objects via pulses of ultrasonic waves.
• Vibration motion sensors detect small vibrations that people cause when they move through a room. While you can buy them, they're easy to make at home too. A homemade vibration sensor uses a small mass on a lever, which activates an alarm switch when it vibrates. Homemade motion sensors can work, but they can also be unreliable.

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While the best thing you can do to increase the effectiveness of your sensors and prevent false alarms is to read the instructions that come with them, here are some placement tips to help leverage the sensor's strengths:

1. Keep PIR sensors 10–15 feet away from heat sources like radiators, heating vents, and sunny windows.³ Motion sensors can trip upon detecting a swift change in heat, even that of a cloud passing quickly over direct sunlight in your living room.

3. Assess where intruders are most likely to enter and what path they would take. Most burglars enter the home through a front or back door, patio door, or garage door, so it’s advisable to place the sensors near those areas.

For example, in a hallway you tend to walk parallel to the walls, not directly toward them.

With wireless motion sensors becoming standard, DIYers have it easier than ever when it comes to installation. If you know how to use a screwdriver, you can install a motion sensor. Correct installation gives you optimal coverage for motion detection and helps avoid false alarms.

You should consider the most effective sensor placement before mounting your motion detector:

• Corners are an ideal location, because you can position infrared sensors to cover the most area. Most motion sensor designs have angled edges with screw holes so they fit nicely into the corner of a room.

• Mount your motion detector high on the wall to get the best coverage—but avoid putting it over a large piece of furniture, like a bookshelf or entertainment center, because it will limit the passive infrared energy range.
• Mount your motion sensor opposite a main entrance—this applies in every room or hallway where you place these sensors so they can detect intruders right away.

Passive infrared sensors are lightweight, so you won't have to worry about drywall anchors or studs. A standard screwdriver should do the trick, but an electric screwdriver or drill can speed up the process.

Most motion detectors have a mounting bracket that pops off the main body of the device so you can screw it into the wall first, then clip the motion sensor back in. This also makes it easier for taking the motion detector off the wall during maintenance. Other infrared sensors may require a full disassembly for mounting.

Follow the manufacturer's instructions to connect your motion sensor to your system. Most DIY systems walk you through this process, often using the main keypad or mobile app to set up and adjust your motion detectors.

Most motion detectors have three main settings when you arm your system:

• In instant mode, all motion triggers an alarm.
• In entry delay mode, the sensor operates on a delay—even if it detects motion, you have roughly 30–60 seconds to disarm the system before an alarm.
• Interior follow-up mode works on an entry delay, but only when door contact triggers first—though it sounds an instant alarm if it detects motion in the home without a door contact triggering.

Over time, dust and debris can gather on the screen of your motion sensor and interfere with the infrared energy, making it less effective at motion detection. Clean it at least once every couple of months with a dry or slightly damp microfiber cloth.

If you decide to paint a wall near your motion sensor, be sure to remove the device first. If you get any paint on a passive infrared motion sensor, you need to replace it.

When installing LED motion sensor lights or cameras outside, keep in mind that installing them under an overhang (like a carport) can reduce their range.

Just like your field of vision decreases when you're wearing a hat with a brim or a visor, an overhang can block the motion detector light's 180-degree sensing angle, turning it into a 90-degree sensing angle.

Motion sensors are like flashlights sending out a beam of light but with motion-detecting infrared energy waves instead of light waves. Just like a light is brighter closer to the bulb, the infrared radiation is denser nearer to the device and it spreads out farther away.

A motion detector's waves can't penetrate through walls or other hard objects like furniture. When you set up your motion sensor, imagine it like a light on the wall. Anything that creates a shadow from that light's position can also block the motion sensor's ability to cover the shadowed area.

When choosing motion sensing light switches for indoor use, remember that not all motion detection switches work the same.

• An occupancy sensor turns on automatically when you enter a room and off again when you leave.
• A vacancy sensor turns off the lights when a room is empty, but you have to turn it on.
• Dimmer sensors are customizable for turning on lights at different levels of brightness.

Brands like Lutron have motion sensor light switches that come in all three options so you can choose the one that's right for you.

Motion sensors aren’t just for home security. Many industrial fields use them on assembly lines to keep track of the number of products and to shut down dangerous equipment if a person gets too close.

Here are a few other uses for motion sensors

• To open and close automatic doors
• To turn on and off automatic water faucets and toilets
• To turn on lights when a person enters a room
• To control ATM displays
• At automatic ticket gates
• For some parking meters

Motion sensors aren’t just an additional feature of a security system—they are essential. Without sensors, there wouldn’t be a way to detect intruders.

Hopefully our guide taught you the motion sensor’s role in your home’s security, the various technologies employed in motion detection, and how to properly install your sensors so they can do their job. For help researching your home security options, check out our article on the best home security systems.

• How Does a Motion Detector Work?
• Best Motion Sensor Security Cameras
• Cheap Motion Detectors
• Best Driveway Sensors and Alarms

1. Tom Harris, HowStuffWorks, "How Burglar Alarms Work," February 2002. Accessed March 7, 2023.
2. Tim O'Leary, Security Info Watch, "Making Sense of Motion Sensors," October 2008. Accessed March 7, 2023.
3. Digikey, "Panasonic Electric Works Design Manual," January 2005. Accessed March 7, 2023.