http://www.zen22142.zen.co.uk/Circuits/Alarm/5zalm.html
http://www.electronicsforu.com/efylinux/efyhome/cover/nov2002/ci_knock.pdf
If you can't hear your doorbell when you are in your basement try this circuit.
http://www.imagineeringezine.com/PDF-FILES/doorbel.pdf
Very handy for student flats. Gives alarm after taking beer out of the fridge wihout putting a score on the beer scoring list.
http://margo.student.utwente.nl/el/my_own/beerbuzz.zip
emit an audible alarm whenever the device’s power cord was unplugged from the wall for a period of time. The device was designed to snap onto the outside of a power cord, where it senses the AC electric field emitted. No direct connection to the internal
http://www.discovercircuits.com/PDF-FILES/cordalm1.pdf
For some medical equipment it is important for an operator when power is lost to the machine. The beeper is powered from a 9v battery and requires the machine to have a power switch with a third set of contacts
http://www.discovercircuits.com/PDF-FILES/powerlos.pdf
http://www.discovercircuits.com/PDF-FILES/dooralm2.pdf
allows it to operate for many years from one set of batteries.
http://www.imagineeringezine.com/PDF-FILES/dooralm2.pdf