Posted by Nimabine Okay, I found a couple threads from 2003 on how to defeat them, but I am still interested in any info available and where I could obtain a switch to practice with.
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There's 2 basic forms used in door alarms. When the magnet (door) is near, the circuit will be either held open or closed (Pulled towards the door). As the door opens, its magnetic field's pull weakens, so the circuit will close or open, respectively, setting off the alarm.
In order to use a magnet to trick a reed switch, you'd have to know which of those two types it is, AND the polarity of their magnet. You'd want to know exactly where along the door frame the switch is. And if it's even behind a thin steel sheet (Door or door frame!) you can forget about using any reasonably-sized magnet to manipulate it. This isn't because steel blocks magnetic fields, but basically redirect them along the plate.
Cheap things that probably contain reed switches to play with:
http://shop.ebay.c...ed+alarm&_osacat=0 You could also check Family Dollar for any kind of "home alarm" with a niblet you stick on the edge of a door and a large box for the door frame. Those also contain a simple magnetic reed switch, and it will tell you very insistently if you fail to trick it.
It's pretty much a crapshoot. 50% chance for the polarity and switch mechanism, and a low chance your magnet has a chance to push the reed (Correct location and field penetration). Good luck, and blow a few bucks to see if you can do it despite my pessimism.