We've had a foot pedal switch in the store for a while but some people have contacted us asking if there was a way to retrofit it to perform variable speed control, like a potentiometer. And here it is! This is a Foot Pedal Potentiometer (a.k.a Sewing Machine Speed Controller).
This is a really nice quality one, very sturdy, with a friendly 3.5mm 'audio' connector on the end, so it's incredibly easy to integrate into your project. You can use a 3.5mm terminal-block adapter, in-line wired jack, breadboard jack, or a panel mount jack. A lot of the cheap samples we looked at have a wall plug, are very low resistance (because they're meant to go in-line with the mains power) or are not very durable.
When no pressure is placed on the pedal, you'll read about 20 KΩ. When pressed down all the way, you'll read about 1 KΩ, and the resistance change is linear with the motion. Connect one side to ground, then the other side to a 10K resistor pull-up to your microcontroller's logic voltage. Then read the divided analog voltage, like you would with a photo-cell or other 'resistive' type sensor.