This is a volume pedal I built several years ago. The foot pedal part is a gas pedal from the auto parts store. It's attached to the base with a simple hinge. A slide pot is glued to the base, with a string going under two pulleys made from some old tape recorder tape guides. A spring connects one end of the string in order to provide tension. Some aluminum C stock is used to mount the pulleys and the input and output jacks, and a second pot sets the minimum volume level.