Motion Detectors: How They Work, How To Choose—& Why You Need One

Posted June 19th, 2013 by SimpliSafe

Motion Detector Diagram

What's the best way to make sure no one's sneaking around your house at night? That's a no-brainer—a motion detector would have that creep behind bars in no time. But what kind of motion detector is right for YOUR home? That one's trickier. With so many types and technologies, you might be tempted to throw your hands up and try your luck with a baseball bat—but don't give up just yet! Here's a handy breakdown to help you get the most effective protection for your lifestyle, without getting bogged down by tech specs.


1. Driveway Motion Detectors

Get a lot of visitors? Mount one of these sensors outside and it will let you know if someone is walking, biking, driving, or riding a dinosaur up toward your house. Though mainly used for driveways, these alerts are also good for watching over other outdoor structures, such as stables, pools, or sheds. There's a wide array of driveway motion detectors available, and if the idea sounds right for you, there are a few things you should consider before settling on one:

  • Range:


    If the sensor is too far from its accompanying receiver, it won't help you at all. Shorter driveways will be fine with a basic model (most have around a 400 ft. range), but if your house is far from the street — or you're trying keep an eye on private hiking trails or a warehouse complex — you'll want to go for a special long-range one, which can beam its signal up to half a mile.

  • Response Type:


    Different driveway motion detectors also trigger different responses — some set off a sonic alert inside the house, effectively acting like a long-range doorbell ("it gives you time to throw some pants on," as one user put it). Others are more security-focused, allowing you to network multiple sensors into a walkie-talkie, which then tells you which section of your property has been compromised. If you're really ambitious, you can hack one so that it makes  an axe-wielding clown fall out of a tree. Whatever works.

2. Motion Flood Light

A motion-triggered light works like a driveway alert, but instead of just letting you know when it senses motion, it turns on a light for the world to see. If the illuminated figure is a burglar, he'll skedaddle, but if it's you, you'll be able to make it to your front door without tripping. Win/win. There are all different kinds of lights available, from "intimidating helicopter-style floodlight that pins you in place" to "welcoming, but still revealing, electronic lantern." Some units let you choose your own light. Look for one that has a timer or a photoelectric cell so that it automatically switches off during the day, when it would be useless anyway.

3. Motion Detector Guard Dog

Want all the protection of a snarly German Shepard without any of teeth? Put one of these near your front door and it will "bark" angrily when someone approaches. Many models sense movement through doors and windows, so you can keep them snug inside. Some are fancy and change the speed, volume and type of bark depending on how far away the visitor is. Others can be set to less-fraught sounds, like bells or rainforest noises, when you're expecting friendly visitors. It's the most flexible pet you'll ever have.

4. Security Alarm Systems

Indoor motion sensors are an integral part of most home security systems. When combined with other sensors, they help to give a home full coverage — if an intruder sneaks into the home in some unexpected way, his movements will get caught the second he strides towards your flatscreen TV. There are many, many types available, but before you get into design decisions (black or white?) or technical specs (how big do you want your area of coverage?), there are a couple of basic decisions to make:

  • Wired or Wireless:

    Wireless motion detectors are particularly useful because they allow you test different placements to find the most effective position. Set one too high on the wall, and your burglar might find it easy to sneak by; set it too low, and your dog will be setting off your home’s alarm every two minutes. With a wireless detector, it's easy to find the sweet spot that nabs the burglars and nothing else. But keep in mind, most wireless motion sensors have a maximum range of 400 ft—so if you’re trying to protect Brangelina’s 5,000 square foot Los Angeles beach house, a wired sensor will be a better fit.

  • Pet-Safe: 

    If you have a four-legged friend in your home, you'll want to make sure he or she isn't mistaken for an intruder. Some sensors are designed to be "pet immune," and will ignore pets up to a certain weight. Others can be made that way by placement, testing, and masking.


There are four types of motion-sensing technology available, and each has its ups and downs. Here's an overview:

  1. Passive Infrared [PIR]

    All warm-blooded creatures (yes, even that lizard of a burglar is warm-blooded) emit infrared radiation. PIR motion sensors contain a thin film of pyroelectric material, which reacts to infrared radiation by generating electricity. A PIR Motion Sensor will trigger your burglar alarm when this influx of electricity occurs. PIR sensors are inexpensive, don't use much energy, and last forever. They're very common in indoor alarms.

  2. Ultrasonic

    Ultrasonic sensors can be active or passive. Passive ones listen for specific sounds, like glass breaking or metal on metal. These are very sensitive, but the downside to this is that they're often expensive and prone to false alarms. Active ones emit pulses of ultrasonic wave (sound waves pitched higher than humans can hear) and then measure the reflection of the waves off a moving object. Dogs, cats, and some fish can hear ultrasonic frequencies, so an active ultrasonic alarm might unsettle them.

  3. Microwave

    Microwave sensors emit microwave pulses and then measure their reflection off of objects, in order to tell whether or not those objects are moving. These are also very sensitive, but they can sometimes "see through" nonmetallic objects and so might detect moving things outside of the target range. They also use a lot of power, so sensors are often designed to cycle on and off. This makes it possible to get past them, if you know the cycles. Electronic guard dogs use microwave sensors.

  4. Tomographic

    Tomographic sensors emit radio waves, and sense when those waves are disturbed. They can "see" through walls/objects, and are often positioned in a way that creates a radio wave net that covers large areas. They are extremely expensive, so they're usually used in warehouses, professional storage units, and in other situations that require a commercial level of security.

  5. Combined Types

    Some motion detectors combine sensor types in order to reduce false alarms. However, dual sensors are only triggered when both types detect motion. For example, a dual PIR/Microwave sensor will start out on the PIR setting, because that takes less energy. When the PIR is tripped, the microwave part will turn on; then, if the microwave is ALSO tripped, the alarm will sound. This “double trigger” is great for avoiding false alarms, but runs the risk of missing real ones.


  • Don't point your indoor motion sensor towards a window. This is a false alarm waiting to happen — a PIR sensor won't catch a human through glass, but it could be fooled by a car headlight or concentrated burst of sun.

  • Don't put your indoor motion sensor in a particularly drafty place, or near an HVAC vent. Too much hot or cold air blown onto the sensor could trigger a false alarm.

  • Motion sensors have blind spots. For example, a PIR sensor might not detect something that is moving in a straight line directly toward it. Do some testing, and if possible, build in some redundancy — for example, two sensors at perpendicular angles would make up for each others' limitations.

  • Make sure your outdoor motion sensors stay clean! Dirt or grime on the front window might inhibit them from being able to sense motion.

What's the craziest thing you've ever done with a motion sensor? Tell us in the comments!