How Modern Intrusion and Motion Sensors Work—and Make Homes Safer

According to research from the Bureau of Justice Statistics, most home burglaries don’t involve a break-in. From 2003 to 2007, more than half of these crimes were categorized as unlawful entries, meaning that thieves didn’t need to break down a door or shatter a window. Open or unlocked doors and windows gave most of these criminals the opportunity they needed to get in, loot, and get out.

Intrusion detection systems are designed to help households deter and catch these criminals—whether they walk in or break in. Today’s home security systems are powered in large part by sensors designed to detect shattering glass, opening doors or windows, and unauthorized motion.

In this article, we explain how these sensors provide consumers with a wide range of approaches to making their homes safer, smarter, and more convenient. We explain how motion sensors work, what makes glass break and door/window sensors so effective, and why these wireless devices work so well with monitored smart-home security systems.

Home security systems have evolved a great deal in a short time. Studies of New Jersey neighborhoods and stories from convicted burglars reveal that alarms and cameras go a long way toward preventing burglaries (or stopping one in progress). While social media platforms and local news outlets have recently focused on the benefits of video surveillance—by sharing scary or amusing encounters with ne’er-do-wells—far simpler devices also provide effective forms of intrusion detection.

Some of these sensors, like motion detectors, are “active,” which means that they emit energy—like ultrasonic waves or vibrations. But many modern sensors use “passive” technology, which doesn’t require this sort of energy discharge. These passive sensors activate in response to infrared rays, audible sound, or other activity. Because they don’t need to constantly generate energy to detect something, they can use it sparingly, making it possible for a battery-powered device to last years without replacement. Let’s look at the history and design of three of these devices:

• Glass break sensors
• Door and window sensors
• Motion sensors

Glass break sensors—used to secure windows and paned doors—work either by “hearing” or “feeling.”

Some active glass break sensors hit a window with ultrasonic waves; the sensor activates when the reflections of those waves change—as they do when the glass breaks.

Passive glass break sensors rely either on built-in weights or microphones that respond to broken glass. Vibration-type sensors, which mount against the glass, feel for pulses: a broken window will vibrate several times as shards of glass collide. Acoustic glass break sensors, on the other hand, detect noise using a microphone. While different types of glass sound different as they break—with laminated, tempered, and plate glass making distinct noises—these sensors are well-suited to a wide range of glass types. These sensors don’t need to be placed directly on the glass they protect. In fact, some acoustic sensors can operate at a distance of 20 feet in any direction.

Another intrusion detection device can sound the alarm when burglars enter through windows or their other favorite point of access: doors. These door and window sensors typically use two pieces that essentially sit adjacent to each other. When the door or window opens, the components are pulled apart, and a signal is transmitted.

Many door and window sensors are a type of contact sensor, meaning that both pieces need to touch while the door or window is closed. Others, however, are magnetic sensors. These offer the same essential features using a magnet and a transmitter but can be placed with a slight gap between them. They can detect activity on doors with molding, sliding glass doors, double doors, and a range of other objects. And the applications extend beyond intrusion detection, with consumers using these devices to monitor liquor cabinets, safes, mailboxes, and more—basically, anything that opens and closes.

The earliest known motion sensors used radar, the radio-wave-based detection technology created in the late 1800s. By bouncing these waves off solid objects, aircraft and ocean vessels could uncover hidden hazards—including, famously in the case of World War II, enemy ships. This technology formed the basis for the first active motion alarms. But by the 1980s, passive technologies, including infrared sensing, began to displace active models.

Now, motion sensors are ubiquitous, integrated into security cameras, motion-activated lights, and as standalone units. And one of the most common passive types detects the presence of heat. These passive infrared (PIR) motion sensors are some of the most effective varieties and are common in the home security industry. They work because all objects emit invisible rays. A human being, for example, releases about as much heat as a 50-watt lightbulb. Those rays vary with the temperature of the object from which they emit, allowing PIR sensors to distinguish between living creatures with body heat and inanimate objects in motion.

Passive motion sensors cost less than active sensors—such as microwave, ultrasonic, and vibration types—and they may also be less susceptible to false alarms. However, careful placement is critical. Heat sources, direct sunlight, and pets over 40 pounds may lead to unwanted (if cute) incidents:

Intrusion and motion sensors have a long history—but today’s home security devices have vastly improved on old designs, extending their core benefits and adding new layers of convenience and function.

One feature common to modern door/window sensors, glass break sensors, and motion sensors is their wireless capability. Similar to WiFi or Bluetooth, Z-wave is a wireless technology that allows electronic devices to send and receive data without a hard-wired connection. But Z-Wave has a longer range than Bluetooth and they are more energy efficient than Wi-Fi when communicating with a control panel. Sparing energy use allows battery-powered Z-Wave sensors to last for years, leading to battery lifespans like:

• Glass break sensors: approximately 5 years
• Motion sensors: 6 to 8 years
• Door and window sensors: 6 years

Each Z-wave sensor can also transmit data over distances up to 100 meters in open spaces, allowing them to be placed far from the central hub of a home security system. And alarm dealers can ship new wireless sensors pre-programmed, meaning that they’re ready to connect to a specific security system as soon as they’re turned on.

It’s the meeting of powerful, energy-efficient, and pre-programmed technology that makes it easy to install and maintain intrusion detection systems without professional help.

Let’s illustrate how home a well-arranged home security system makes use of motion sensors, door and window sensors, and glass break sensors.

Glass break sensors, along with door and window sensors, are boundary-penetration sensors. When used effectively, they create an enclosed space that can’t be entered without an electrical signal being sent to the security system. There are limits, however, to what these sensors can do. Door and window sensors only work when an intruder opens a door or window. They’re less likely to work if an intruder smashes through. Similarly, acoustic glass break sensors can be made less effective by anything that stifles sound—like curtains or blinds—and won’t work when thieves carefully cut through the glass or open the window normally.

Motion sensors, on the other hand, are often installed as interior sensors. They work best when supplementing the efforts of boundary-penetration sensors. These are typically deactivated when people are home and awake so that residents can move freely without triggering an alarm. If they are on and an intruder finds an unprotected point of entry, or if a different sensor fails, the motion sensor can detect them.

Note that placing a motion sensor at a standard height and orientation can work in most circumstances—but young children or pets may generate false alarms if the detector’s field of view is too large or aimed too low. Fortunately, some motion sensors don’t pick up pets weighing less than 40 pounds. But when that doesn’t work, or when dealing with larger dogs or cats, motion sensors can be installed in specific ways to minimize false alarms.

These sensors do an excellent job of detecting movement at an angle to the sensor. But objects moving directly toward a motion sensor may go undetected, as may intruders who crawl below the sensor’s field of view. Basically, no home—not even a small one with a single point of entry—can rely solely on a single type of sensor. The different types work together to provide comprehensive protection.

Intrusion detection systems protect a home from threats inside and out. But when they’re paired with the features powering today’s smart homes—namely, remote control, mobile notifications, automated activation, and professional monitoring—they’re able to deliver that protection in a much more user-friendly way.

One big benefit is that sensors can be armed or disarmed on-demand or on a schedule. Smart home security systems feature “stay and away modes.” In away mode, every sensor is active; in stay mode, only boundary-penetration sensors (and environmental sensors, like smoke and heat detectors) remain active. Even better, consumers can bypass only some sensors, disabling them while a window or door remains intentionally open. These settings can be triggered remotely, using any electronic device with internet access and the Frontpoint app.

And, as part of a monitored home security program, these sensors can be designed to promote swift police response while helping reduce the number of false alarms. Here’s how intrusion sensors and monitoring work together:

• Any “hot” sensor can trigger an alarm. The signal is sent immediately to the monitoring center.
• All cameras in the home begin recording.
• Users have a window of time to disable any false alarms. They can use their smartphone and compatible cameras to take stock of the situation. At this point, the alarm monitoring company also calls.
• If the user provides the correct verbal password (or disarms the system from a device inside the home), that’s the end of it. If not, monitoring professionals get in touch with emergency dispatch.

In addition, intrusion detection systems also make use of powerful rules that allow sensors to turn other devices on and off through a central hub. For example, consumers can program their intrusion detection sensors to turn off when they unlock the front door, or they can use motion sensors to turn on the lights after dark. These customizable features even allow intrusion sensors to protect against concerning (but not necessarily threatening) events, such as a liquor or medicine cabinet unexpectedly opening.

In short, consumers can use these electronics to easily assemble an intrusion detection system that’s as aggressive or as flexible as they need it to be. For buyers just entering the do it yourself security market—or for anyone fed up with the shortcomings of some home security monoliths—this new generation of equipment has a lot to offer.

Frontpoint keeps families safer and more connected in their everyday lives. We've been revolutionizing the home security industry for over a decade. And we're just getting started. To shop monitored intrusion detection systems, check out our Security Packages. If you have questions or would like to discuss a quote, contact us at 1-877-602-5276.