Getting away from all the legal discussion, yes these "12 Gauge Perimeter Alarm System" thingies could come in handy for a variety of reasons. However $30 each is a bit steep when, as Jerry pointed out, one can construct something that works as well from a mousetrap. The motion sensing electronic gizmo SuperiorDG posted above costs closer to $10, so somewhat more economical, although you have to worry about the batteries dying.

I'm more intrigued about how does one protect the perimeter without such devices, barring having someone keeping watch? Ideally it would be something that alerts you not only of the direction of approach of the intruder (so effectively, which part of the perimeter has been breached), but does so in a way that alerts you and not the intruder. Also needs to have a little bit of range to it, because what good is perimeter alerting if you don't have time to react to a breach?

The closest thing I've been able to come up with is some sort of a laser-"tripwire" system broadcasting to an alarm console. I'm sure someone builds these, but they're probably crazy expensive, and/or I haven't found them yet. Another possibility is some "game cameras" that sense motion then send an alert and picture via wireless signal to the base camp receiver. I've heard these are available, but again, have not had a chance to do much research. And of course there's always wireless CCTV cameras, assuming you have someone monitoring them at the base camp.

But as with all technology-based solutions to a problem, you run the risk of batteries dying and/or it failing just when you need it most. So I'm interested in any ideas people have that could use a low-tech approach.

Thoughts, anyone?