Unseen door sensors
NOTE: This is a post copied from my old website. This content was posted back in 2012, but the idea is still a good relevant one.
I did some testing with a theory for a hidden door sensor. I had a pile of magnetic reed switches and got to thinking. Neodymium rare earth magnets are the strongest magnets you can get. I had one that I pulled from an old hard drive that was very strong. I connected my continuity tester to the switch and set it on top of my desk which is made of 3/4 inch press board. I held the magnet under the desk top and was able to activate the reed switch with the magnet even 1/2 inch or more away from the bottom of the desk top. This told me that it should be able to activate the switch through the frame of a door. The distance from the hinged side of the door to where the magnet and reed switch are mounted will determine the sensitivity of the switch action. The switch would be most sensitive at the furthest point from the hinged side. Below is a diagram of the concept.