I'm mainly just sticking this here in case anyone else would benefit from it. I wouldn't regard it as in any way interesting.Use the anti-burglary devices which set off an alarm when the windows on which they are stuck vibrate strongly as "doorbells". Stick them to glass on a front door so that when someone knocks, it sets off an alarm.
Boring but practical.-- nineteenthly, Apr 17 2009 Glass Alarm http://www.allprodu.../kwd/23-ld-26b.htmlNot just for breaking glass [csea, Apr 17 2009] I'm pretty sure glassbreaks work based on the sound of breaking glass. Just knocking won't do it, I don't think.-- phoenix, Apr 17 2009 It will, i've tried it. It even works with a loud noise near the glass.-- nineteenthly, Apr 17 2009 The one i've used turns itself off after a few seconds. I was wondering about the traffic noise, but it's been OK so far.-- nineteenthly, Apr 18 2009 random, halfbakery