Step 1: Parts
I used four cereal boxes.
One or two sponges, 1/2 inch thick.
I got mine at the dollar store.
18-24 gauge should be fine, I used speaker wire.
A power source
Battery or battery pack, 6-12 volts, or an AC/DC adapter you can cut up, 6-12 volts. If you don't have one, Radio shack is a good place to get a harness for a 9V battery, and thrift stores are good for AC/DC adapters.
Something to insulate the wire
Tape, heat shrink tubing, etc.
An alarm (also called piezoelectric buzzers)
I got mine at Radio Shack, there were plenty to choose from. it's the little black box in the picture, and it's about 2 inches square. I think it was 4 or 6 dollars.
A soldering Iron
I suppose you could just twist the wires together, but that's sloppy. Or you could use butt splices.
A hot glue gun
Hot glue is best. Dries quickly.
Packing tape is good, but any wide tape will work.
Wire clippers (optional, could use scissors)
Some way to test it, like a volt meter with conductivity testing (optional)
You could just wait 'till it's assembled to test it.
Bait, if that's what you're using it for.
A pet, unless you have a different purpose for this device.
Step 2: Getting Started
Tape the cardboard together in the middle on both sides. Now trim the edges until they line up.
Step 3: The Foil
Cover one side of each layer, if you have a hole in the middle, cut that out. Hot glue it all to the cardboard. Careful, hot glue is hot.
Strip the end of one of your wires, strip about 2 inches off. Cut a 2 1/2 inch by 5 inch rectangle of foil, and put the stripped wire in the center. Put a bead of hot glue next to the wire and fold it up around the wire (don't burn your fingers!). Mage sure the foil makes good contact with the wire. Now glue the square to one layer of the pressure plate, on the edge somewhere. Repeat this process with another wire for the other layer.
Step 4: Sponges
Hot glue them in a grid as shown in the picture (about 3 inches apart). You will need to hot glue them to the top and bottom, closing up the pressure plate. If you rip some foil, cut out a square and glue it over the hole.
Step 5: Wiring
Run one end from the positive lead of the alarm straight to the positive end of your adapter/battery pack and solder.Run the wire from one pressure plate to the negative lead on the alarm, and run the lead from the other layer of the pressure plate to the negative lead of the adapter/battery pack.
Now put in some ear plugs and try it out.
In the picture I have split the triggering wires off to connect to another source. You don't need to do that.