Get thee to Radio shack. They have "Security Combo" sets that include several little crap personal alarms, but what you want is the doorstop alarm in them. They run off a 12V battery and when a door opens, it hits the contact plate and the alarm is SHRILL.
I had to go babysit the cousins once two days after their house had been robbed. The thieves had kicked the door in and my uncle had only been able to get the doorknob lock working. I'm used to sleeping with an alarm system so there was NO WAY IN HECK I was going to worry about every bump in the night. I set those puppies on each door, stuffed my weapon and holster between the mattress and springs, and slept very well.
Those sets are $10 apiece, btw. The personal alarms that come with it are crap but fun to play with.
