Today I am working on a new project to keep potential intruders outside.
I have an attached garage, it has two regular doors a well as the overhead door. One door enters into the house, the other exits to the backyard. This is a comman setup, and a favorite of burglars. The outside door is often poorly maintained, has glass for light, and a cheap lock. Entry though this door into the garage allows thieves access to the home entry door, out-of-sight from neighbors. I secured my door with an inside hasp, and padlock.
Even if breaking down the home entry door (from inside the garage) trips a home security alarm... and even if the intruder immediately leaves. I would still be stuck with two damaged doors.
I wired a switch into the frame of my garage-to-backyard door. If the door is open - the contacts open. Door closes - contacts close. I have storm doors on all my outside doors (none with bars or anything... I just live in a cold climate). I am wiring the storm door storm door the opposite. Storm door opens - contacts close. the two switches will be connected so if the storm door is opened while the hasp-ed door is closed a connection will be made. This will be wired directly to a wal-wart and a siren. Anyone checking out the backyard garage entry, day or night, alarm system armed or not, will get a surprise.
What I really like about the setup, is its automatic. Nothing needs to set, and no damage is required to trigger it.
Now I am thinking..... if I use macro's, timers, and/or interior motion sensors I should be able to configure something similar for my other backdoor.