It was pretty useless for urban area where your mailbox is a 1/2 mile away, but it detected the light from someone opening the mailbox and sent a signal to a receiver that would have a light and alert that the mail had come.

I have one at home, and it may work perfectly to hack into a similar modification.
The bonus is the connection to the phone would be wireless and compact.

Here is a similar product:
http://www.grfrequencies3.com/other.htm

Also after owning several “ring detectors” that work off radio, I can ensure you they are worthless. The pen I owned didn’t work with my cellphone, but it randomly vibrated at anytime due to other RF signals.
The flashing model I bought for my car would flash randomly also, but did at least work with my cellphone.

anyways, thats all imho of course… and kudos for the couple of weeks worth or hacks, they have been cool.

What would be much more interesting (and much less practical… in the useful sense of the word) would be if someone built something sensitive enough to be able to detect phone calls in… for example a movie theatre — and, for the purposes of practical jokes, made certain that the whole theatre would know…