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.

Types of door sensors

SimpliSafe door sensor. Image: Katie McEntire, SafeWise

Traditional door sensor

  • Visible on the door and door frame
  • Sold by all home security providers
  • Costs an average of $27.50
  • Installs with adhesive
  • Indoor use only
  • Available as standalone products (see Best Door and Window Sensors)

Frontpoint recessed door sensor. Image: Cathy Habas, SafeWise

Recessed door sensor

Ring outdoor sensor. Image: Ring

Outdoor door sensor

Frontpoint's garage sensor activates when the magnets tilt away from each other.

Frontpoint garage tilt sensor. Image: Cathy Habas, SafeWise

Garage tilt sensor

  • Visible on the garage door and door frame
  • Sold by fewer brands, including Vivint, Frontpoint, ADT, Ring Alarm, Brinks Home, and Link Interactive.
  • Costs an average of $33.00
  • Installs with adhesive or screws
  • Temperature-resistant for use with garage doors

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.

Recessed door sensors are a little trickier to install since you'll need to use a large drill bit to create cavities for the sensors. Hold the drill as level and straight as possible and go slow.

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.

What's the difference between a door sensor and a window sensor?

Window sensors and door sensors are both called entry sensors or contact sensors. They work the same way and are usually interchangeable. Window sensors may be thinner or come with an extra-long magnet so you can get a bit of a breeze without sacrificing security. Learn more about how window sensors work.

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Cathy Habas
Written by
Cathy Habas
With over seven years of experience as a content writer, Cathy has a knack for untangling complex information. Her natural curiosity and ability to empathize help Cathy offer insightful, friendly advice. She believes in empowering readers who may not feel confident about a purchase, project, or topic. Cathy earned her Bachelor of Arts degree in English from Indiana University Southeast and began her professional writing career immediately after graduation. She has contributed to sites like Safety.com, Reviews.com, Hunker, and Thumbtack. Cathy’s pride and joy is her Appaloosa “Chacos.” She also likes to crochet while watching stand-up comedy specials on Netflix.

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