Where Do I Install a Sensor Contact on a Door?
The place where you should install a sensor contact on a door is on the stationary frame. Then, mount the magnet on the moving part of the door as close to the sensor as possible. Most door contacts need to be within a half-inch of the magnet when the door is closed for proper operation.
Most door contacts function using a sensor and a magnet. When the door is closed, the sensor should be in close proximity to the magnet, often within a half-inch. Many sensors will be in direct contact with the magnet, which is how these sensors got the nickname "contacts". However, there are some door contacts on the market that allow for a wider magnet spacing gap than the half-inch standard.
Door contacts are typically mounted on the outside surface of the door and its frame. These are referred to as "surface-mount contacts". Surface-mount contacts can be secured using either screws or double-sided foam tape. Many DIY installers prefer to use double-sided foam tape. This is because it eliminates the need to drill any holes. There are also some door contacts that are installed inside of holes that are drilled into the door and its frame. These are called "recessed contacts". Some users prefer recessed contacts since they are hidden from plain sight. However, recessed contacts are more difficult to install since holes must be drilled.
Inside the a door contact sensor is a reed switch that responds to magnetism. The reed switch will remain closed when the sensor is in close proximity with its magnet. This is the state the sensor remains in when the door is closed. That is why door contact sensors are said to be "normally closed".
When the door is opened, the magnet will become separated from the contact sensor. This will cause the reed switch inside the sensor to be released. The sensor will recognize this, and it will send an alert to the alarm control panel to let it know that the door has been opened. The security system will then respond based on the programming settings for that zone. Some door contacts communicate with the panel wirelessly, while others do this through a hardwired connection. Wireless door sensors are usually easier to install, but the downside is that they will need to have their batteries replaced every few years.
The reason that the sensor itself is typically installed on or inside the stationary door frame is to prevent damage to the sensor. A slammed door could cause the sensor to experience damage. Keeping it on the stationary portion is the best way to prevent this from happening. However, the magnet is less valuable, and it can often be easily replaced. It's not as big of deal if the magnet becomes damaged. That is why is is place on or inside the moving portion of the door.
Below is an example of what a door contact sensor will typically look like after it has been installed. In this case, the sensor is a 2GIG DW10 that was mounted using double-sided foam tape. Please note that the battery tab at the bottom would need to be removed for normal operation.
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