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.html
Not 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