What's this cardboard square you ask? It's a pressure plate that triggers an alarm when stepped on. It's designed for the weight of a pet like a cat or a dog, I used it to teach my cats to keep off of the counter. The hole in the center is for bait (meat) and there is an alarm and power brick. I'm sure it could be used to teach pets to stay out of any area, maybe if they like to go there and do their business. You can make it as large as you want.
Step 1: Parts
You will need:
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
First cut up the cereal boxes. cut at one of the corners the long way and, of course, open them and take out the cereal if you haven't already eaten it. You'll also want to cut the flaps off. If you're making a pressure plate that uses bait, cut cut half a hole on each piece so that, when put together, there will be a hole in both layers that lines up.
Tape the cardboard together in the middle on both sides. Now trim the edges until they line up.
Step 3: The foil
Pull out some pieces of foil that will fit your pressure plate. For me it too two sheets to cover each layer.
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
Next cut up your sponge. I found that the pieces didn't need to be big, maybe 3/4"/1/2'.
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
So now you're done with the pressure plate itself. Time to wire it up.
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.
Step 6: Set the "trap"
Put the pressure pad where you need it, and cover it with a towel so your pet won't be suspicious. Power it up and wait. I set up a webcam to capture the moment. My cat's response wasn't that dramatic, but he's the brave one.