My homemade version of a "stomp pad" automatic mat switch made with foil, cardboard, and lots of duct-tape.
Homemade mat switches can be used for triggering an event when someone steps on the pad or other pressure is applied. They are used for automatic doors in supermarkets and you can use them to! I've used matswitches to sense when a person reaches a particular location to turn on a light, activate a tape-player, activate a pneumatic dummy for a Haunted House at Halloween, etc.
A matswitch works by putting a voltage potential across two pads and deforming a separating layer enough to let one contact touch the other and thus complete a circuit. In this case, the cardboard spacers compress and the top layer bends to allow the top foil layer to touch the bottom layer. I have made pads ranging from four inches by ten inches to 48 inches square and this technique has worked flawlessly for both.
Assembly is very simple. I start by layering two pieces of cardboard with foil. I typically use duct tape, although any insulating tape should work. Be sure to stretch the foil tight to increase the smoothness. You'll need to attach wires to the foil. Soldering the wires to the foil doesn't work (try it) but it is easy enough to spread the wire out along a long patch of foil, preferably on the back side of the pad, and tape it down. Remember, do not run high voltage through the pad, stay at 12 volts or under at low currents.
Next, it's time to cut the cardboard spacers. How wide should they be? How much space in between? It doesn't really matter, as long as the foil doesn't sag and bridge the gap when unloded. Also When the mat is assembled, it is easy to have problems with the edges permanently bending around the edge, so I try to put an edge around the perimeter of the entire pad to prevent shorting. Remember, you only need to put cardboard on one of the sides!
Now, put the foil and cardboard sandwich together with, what else, more tape. Check the continuity with a multimeter. If it never turns off, try stomping hard and repeatedly on the pad. Surprisingly, I've had some matswitches for years, and none have stopped working. I put one under the carpet in a stairway of my house and it still works after two years of abuse. Allen's Halloween Homepage has a great page describing how to make a similar pad with postarboard instead of cardboard, although it sounds as though he has encountered many more problems than I have with the cardboard. Again, make sure not to run high voltages/currents through the pad - it will heat up and catch fire from sparks/resistance. Use low voltage and a relay to trigger high voltage effects.