Here are some alternative guides for building modern pedals:
Original Guide (Reference Only)
Adafruit carries a lovely foot switch which has a single pole-double throw switch inside. The switch can activate a number of items including an alarm or powerswitch tail to control your world.
A much-requested project by Adafruit customers is using the foot switch to interface with a PC via universal serial bus (USB). To get a switch activation to talk to the USB bus takes a controller - some smarts.
Adafruit's ATtiny85 based Trinket is low cost and contains enough smarts to perform USB communication (via some smart library software).
Only three parts are required. The foot switch, a USB cable, and a Trinket 5V. The entire project costs less than $20. And it takes less than an hour's work.
The wiring is shown above. The existing foot switch wires are cut short and two connected to Trinket pins #0 and GND (ground). When the foot switch is activated, the switch contact pair that creates a short connects Pin #0 with ground. When the Trinket detects the pin connected to ground, it outputs one or more characters as a PC keyboard.
The Trinket Keyboard tutorial states a Trinket 5V should be used due to the microcontroller clock rate the library uses.