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.

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.

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.

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.