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.

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

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.

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.

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.

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.

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

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?

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.