Recessed has the magnet placed in a 3/8″ hole drilled into the edge of the door,and the switch in another hole drilled into the door casing, wiring concealed in the wall.
Recessed Magnetic Door Sensors are typically used on closed circuit alarm systems. These devices work by magnets. These magnet sensors are mounted inside of the door and inside of the frame. When the door is closed and the magnets are lined up, the switch is closed. These devices are made to be ascetically pleasing, since they cannot been seen and also to be less prone to tampering. Below are the pros and the cons of using this type of door contact.
Less Risk of Tampering: since these devices are installed inside the door and frame they are far less likely to be tampered with since they cannot be seen when the door is closed. Invisible when door is closed: This makes for the devices to be more aesthetically pleasing than the surface mounted door contacts. These are the standard type of contacts used in residential settings for this reason.
Harder to Install than surface contacts: Since they have to be installed the door and the frame, they are more difficult to install than the surface contacts which stick right onto the outside.
The other alternative to using recessed door contacts is to used surface door contacts. These are typically a little cheaper and easier to install than the recessed door contacts, but they are visible and more prone to tampering.
Install Intruder Alarm Prevent a Crime Before it Happens.