How Does a Door Contact Sensor Work?

A door contact sensor works using a sensor and a magnet. The sensor has an internal reed switch that is closed when it is in direct proximity with a magnet. When the door is opened, the magnet is separated from the sensor. This releases the reed switch, and the sensor alerts the panel.

Door contacts are some of the most popular sensors used with security systems. These are the best devices for setting a system know whenever a door has been opened. Since doors are the primary method an intruder could use to gain entrance into a building, maintaining functional door contacts is of the utmost importance for any alarm system. Door contacts are also very easy to understand, making them great for DIY users who want to troubleshoot their own equipment.

The average door contact sensor consists of two parts. These parts are a sensor and magnet. The sensor is typically installed on or inside the stationary frame of the door. The magnet is then mounted on or inside the moving portion of the door, usually within a half-inch of the sensor. The magnet needs to be in very close proximity to the sensor when the door is closed, or else the device will not work properly. It is very common for the sensor and magnet to be in direct contact when the door is closed. Hence the name, "door contacts".

Once the door is opened, the magnet will separate from the sensor. This will cause a reed switch inside the sensor to be released. The sensor will then send an alert to the alarm panel to let it know that the door has been opened. Wireless door sensors will alert the panel using wireless RF signals, while wired door sensors will send the alert through a direct hardwired connection. Once the signal is received, the system will respond based on the programming settings for the zone.

Besides being wireless or hardwired, door contacts can also be classified as either surface-mounted or recessed. Surface-mounted contacts are mounted on the outside of the door and frame, and they are visible from the outside. Recessed contacts are installed inside the door and frame, and they are hidden from view. Although some users appreciate the fact that recessed contacts are hidden, these sensors are more difficult to install since holes need to be drilled.

There is also another special type of recessed door sensors called plunger switches. These devices have a switch that is pressed-in when the door is closed. When the door is opened, the switch will pop up, and the sensor will alert the system. An example of a door sensor that operates this way is the Honeywell 5800RPS.