Magnetic 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. Inner Workings of a Door Sensor Door Sensor Switch: 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 to 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.