Because door sensors are simple to install, it's easy to take these useful gadgets for granted. But the more you understand your home security system works, the more you can take advantage of these amazing tools. Door sensors are a vital element of your home security system: they tell you when someone is entering your home. These devices are made up of two components, which form a circuit when they're kept parallel to one another. When someone opens the door, the two parts separate and break the circuit, which triggers the controller panel to sound an alarm.