Instructables user DIYHacksAndHowTos put together this simple pressure plate from cardboard and aluminium foil for a haunted house, but we're intrigued by the home automation prospects. You could easily use it to turn lights or appliances on or off when you enter a room, or for anything else you want automated.
The video above does a great job of explaining in two minutes all of the different considerations involved -- how you can connect the plate to an Arduino to run a program, how you can connect it to a relay to work with DC appliances like lights or other devices, or how you can set it to activate a sound module.
The latter is more specific to the haunted house idea, but using a pressure plate to run an Arduino program could be useful for a number of things, like opening the blinds, for example. Alternatively, setting it to toggle something plugged in to a DC relay could be great for turning on the lights in the garage as soon as you step through the door, without having to install a motion sensor, for example. The sky's the limit, and the plate is remarkably easy to make.
Whether you're exploring home automation projects, or you really are building a haunted house, it looks like a good project to try.
Use a DIY Pressure Plate Switch to Automate Your Haunted House [Instructables via Make]