How Does a Door Sensor Work?

Door sensors are an essential component of your home security system: they let you know when someone is entering your home. These devices are made up of two parts, which form a circuit when they’re kept parallel to each other. When someone opens the door, the two parts separate and break the circuit, which triggers the control panel to sound an alarm.

Because door sensors are simple to install, it’s easy to take these helpful gadgets for granted. But the more you know how your home security system works, the better you can use and maintain them.

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The inner workings of a door sensor

While there are several different types and styles of door alarm sensors, most use a reed switch and a magnet to determine when a door is open or closed.

Reed switches are used in countless devices, from doorbells to laptops, and rely on a set of electrical connectors. The switch is closed when the two parts are sitting close to one another, and an electric current can flow. When the switch opens, the two parts separate, causing the electrical current to stop and the circuit to deactivate.

When you add a door sensor into your home security system, the device will come with both pieces: a reed switch and a magnet. One piece attaches on the door frame, and the other attaches parallel to the first piece on the door itself. The two parts create a closed circuit when the door is shut. As the door opens, the magnet and switch separate, breaking the circuit. When the circuit breaks, the sensor signals the central control panel.

Installing door sensors

Door sensors are easy to install. Using a strong adhesive, attach one piece of the sensor onto the door and the other to the frame.

The two pieces must sit directly next to each other with only a small amount of space between them. If they’re too far away or aren’t parallel to one another, the pieces can’t interact and the reed switch and magnet won’t create a circuit.

Garage door sensors

Though not as common as regular door sensors, garage door sensors can bring a lot to a home security system. There are two ways to detect a garage door opening: with a standard door sensor or with a pitch sensor.

The standard sensor works the same as on any other door. The two pieces are installed next to each other, one on the door and one on the wall right next to the door. When the garage door is raised, it breaks the circuit and sets off the alarm.

A pitch sensor is a bit different. Rather than working through two pieces, pitch sensors are standalone devices. They’re designed to detect changes in angle. When the garage door is closed, the sensor sits vertically; when the door is opened, the sensor will move up with the door, and that change in angle will set off the sensor to trigger the alarm.

Door sensor use and maintenance

Depending on the security system, you may be able to customize the type of alert you receive when a door opens. You may choose for an audible alarm to sound when a door opens. Or, you may prefer the alarm to trigger silently while alerting your security company and notifying you of a possible breach.

No door sensor lasts forever. The sensor may become damaged, the switch may wear out, and wireless sensors eventually run out of batteries. If a door sensor is setting off false alarms or malfunctioning, make sure to call your home security company to replace it immediately.

Now that you know the inner workings of door alarm sensors, it’s time to shop around and find one that meets your home security needs. Check out our guide to door and window sensors for comprehensive reviews of our top picks.

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Info current as of 05/27/2021. Offers and availability may vary by location and are subject to change.
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Celeste Tholen
Written by
Celeste Tholen
Celeste has dedicated her decade-long career to reporting and reviews that help people make well-informed decisions. She oversees editorial strategy and production for SafeWise, with a goal to help people find the information they need to make their homes and lives safer. Prior to SafeWise, she worked as an editor and reporter for KSL and Deseret News. She continues to report on local news as a volunteer with the community paper. For the last six years, she’s led a Girl Scout troop, teaching girls about safety and preparing for whatever life throws their way.

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