Some images for a response on the blog.

You can make an improvised switch by pressing two thumb tacks into the jaws of a clothespin, and soldering wires to the nails that stick through.

Then put something between the jaws to break the contact.

For example, grab the plastic tab that holds a loaf of bread shut and tie some fishing line to that. Place the plastic tab between the jaws, run the line across a doorway or path, and anchor the end to the wall somehow.

When a person enters the room/crosses the path, their leg snags the fishing line and pulls the tab out from the jaws of the clothespin, completing the circuit.

(Note: The particular buzzer used is really, *really* loud!)

(From the article, you can put an aspirin between the jaws to detect moisture.)

The version below includes a battery and a transistor socket. Stick the wires of a flashbulb into the socket and use the plastic tab as described above to make a flash-bulb warning of intruders.

Also of note: It's hard to see from the photo, there are 2 thumb tacks sticking out from the base of the battery. Take the whole unit and press it into the side of a tree or wooden molding and run the trip wire across the room to another thumbtack.

Takes about a minute to install 1 intruder sensor.

I probably shouldn't mention this, but mount a 35mm film canister on the clothespin, put the flashbulb *inside* the canister, and fill the canister with gunpowder.

This makes a very satisfying flash *and* bang for unwanted intruders!

It's also reasonably safe, assuming you use a plastic film canister and not a metal one.

I couldn't immediately find one of those or I would have posted an image.