What You Need to Know about Door and Window Sensors

Open glass door with sensor bar on top next to tall rubber tree and side table

The Spruce / Christopher Lee Foto

Door and window sensors make up the backbone of any home security system. Most systems come with a set number of sensors, and then you can add more for an additional fee. Ideally, the sensors get placed on every door and window in the home, though some people only use them on the lower level of the house to save money. If a door or window gets opened or breached when the alarm is on, the sensor sends a signal to the main control panel, triggering an alarm.

There are different types of door and window sensors, though they all perform basically the same function. Understanding how your sensors work helps you keep them well-maintained. This keeps your system performing optimally, so your risk of home invasion remains reduced.

How They Work

Door and window sensors come in two pieces. One fits onto the door or window itself, while its counterpart attaches to the frame. Adhesive usually keeps the sensors in place, though sensors can be screwed directly into the frame. Position the two pieces of the sensor right next to each other; when the two pieces are separated, such as when the door or window is opened, they send a signal to the alarm panel.


Sensors are either wired directly into your alarm system, or they have a battery to power them so they can function wirelessly. The wired sensors require more time and effort to install, while the batteries on wireless sensors need frequent checking to ensure they don’t run low. The connection between the two sensor pieces is formed in one of several ways. Some sensors rely on magnets to create the connection. When the two parts of the sensor are separated, the magnetic field is broken and the alarm goes off. Others rely on a light beam, with one piece of the sensor generating the light and the other receiving it. If the light field is breached or turned off while the system is on, the alarm activates.


Your alarm panel often alerts you when your sensors aren’t functioning properly, but conduct weekly checks anyway to discover and solve problems before they become severe. The adhesive holding the sensors in place loses strength over time. This process accelerates if your home suffers from higher humidity. If a sensor starts slipping off a door or window, you’re likely to experience multiple false alarm activations. Physically inspect the sensors as well to look for signs of wear. If someone accidentally hits a sensor, such as while moving furniture or something else heavy, the sensor’s function can be impeded.

Check your alarm panel regularly as well to check for error messages or warnings indicating a sensor problem. Notify your alarm company when this occurs, so they’re aware of your system problems and can help you with a solution. While sensor repair is possible, replacement is often the cheaper and safer option.

Other Types of Sensors

Door and window sensors should not be your sole line of defense in your home security system. A shatter sensor detects the sound of a window breaking, alerting your alarm system in the process. Likewise, it detects strong vibrations that a regular window sensor might not pick up on. Motion sensors often rely on infrared energy detection. As someone moves through a room, the sensor detects the changes in infrared energy and activates an alarm. It works from a distance, whereas door and window sensors solely work for the door or window they’re attached to.

Count the number of doors and windows in your home when planning for a home security system. Include the garage door as well. While reducing the number of sensors you need saves money in the short term, a single unsecured or unmonitored door renders your entire security system useless. Shop around when looking for a new system or when replacing your existing sensors to make sure you get the best deal possible. Ask about any special discounts currently available. A dealer is likely to cut you a price break if you ask for one because the home security industry is so competitive and he wants your business.