Make a force sensitive resistor (a pressure sensor) with spare parts instead of spending $5 - $20 each.

Step 1: Materials

Tools

Soldering iron
Hot glue gun
Knife/razor
Wire cutter

Components

Solder
Hot glue
One-sided copper PCB
Conductive foam
Wire

The foam

Conductive foam is what microcontrollers generally come packaged in. If you've received little ATmega microcontrollers or PICs, sometimes they'll be surrounded by conductive foam inside a little case or box. Not all conductive foam is created equal: some of it bounces back into shape faster than others. If you use PIC foam to make your FSR, it will respond quickly, but if you use ATmega foam will take a second to release. The fact that this FSR has a visible deformation is the primary difference from other FSRs.

Step 2: Sizing

Use the knife/razor to score your PCB into two plates that mirror each other. I went with approximately one-square-inch squares, but you could do any two shapes so long as there is copper in between.

Cut your foam into the same shape as the plate.

Solder one wire to each plate. You'll want to make sure the solder is going to hold the wire in place, so clean the copper beforehand if necessary and use plenty of solder.

Step 3: Connecting the Pieces

Glue the three pieces together. Only glue along the outline of the FSR, otherwise it will not conduct well. For mine, I just glued the top and bottom of both plates to the foam.

Step 4: Test It Out

Grab a multimeter and measure the resistance across your FSR. Your values will vary, but I got about 200 kiloohms at rest and 9 kiloohms when almost completely depressed. If your plates have a larger surface area, or the foam in between is thinner, these values will be smaller.

Step 5: Notes

Variations

Use it to Dim an LED (video + code)
Use it to Make some noise (video)
Try different kinds of foam (test resistance across the foam first to make sure it's conductive)
Cut unusual shapes
Test different foam configurations (e.g.: multi-layered foam)
Test different plate materials (e.g.: aluminum foil on cardboard/plastic/wood)
Make humongous FSR arrays

Links

SensorWiki FSR page explains FSR theory and use, with examples
Protolab explanation of FSR use in the context of other sensors

Thanks to Dane Kouttron and Zach Barth for introducing this technique to me, and leaving a few FSRs around the eclub.