First, we will make the circuits. The whole system consists of two parts - a wall switch receiver circuit and chair leg transmitter circuit. The circuit for the chair leg is… power + pressure sensor + wireless transmitter. Depending on your situation, add some wires if necessary. For the wall switch circuit, you need power + wireless receiver + slide dimmer + servo (turn mode). Don’t forget that it is very critical to use same channel on both on the transmitter and the receiver in order for them to work together.
Using our template file, cut all the parts for the wall switch out of acrylic sheet. You can modify our design as much as you want to optimize it to your space.
Bend the large covering part with a heat gun. The etched lines indicate where you are supposed to bend.
Add 4 small wheels to an anchor-shaped part. These wheels help the gliding parts move smoothly and keep them in place. We used a Tamiya pulley set for the wheels (http://www.pololu.com/product/109), but you can laser cut them as well.
Pass the protruding center part of the anchor-like part through slit on the big covering. Then use acrylic glue to attach the anchor to the small plate with three rectangular holes.
Place the servo into the rectangular-shaped hole in the large case. Secure it down with M3 screws and nuts. Fix the servo arm (from the laser cut file) to the servo. Consider the position of the servo arm so that the anchor achieves a smooth sliding motion up and down when connected with the parts in step 7.
Connect the servo arm to the sliding plate, using the threaded shaft and joint. (http://www.pololu.com/product/733)
Place the acrylic covering over your light switch. We used gluedots to secure it down. Place the circuit for the wall switch on a mounting board, and secure the mounting board to the wall as well.
Set up the chair leg transmitter circuit on the underside of a chair or stool. Position the pressure sensing pad from the pressure sensor under one of the legs and hide all the other modules the best you can. We used adhesive shoes to easily attach and hide the circuit along the chair leg and underneath the seat. Stick some EVA foam under each leg of the stool to evenly distribute the pressure when someone sits down.
Everything is now rigged for a great prank!