In this video I am adapting a sewing machine pedal to provide variable speed to my small Dremel drill. It’s not really suitable as the pedal is only rated to handle 80 Watts (0.42 Amps at 240 Volts) while the Dremel can take up to 125 Watts (0.52 Amps) of power, it should be alright for light work.
To control the power the footpedal has two stacks made of loose carbon disks, initially when the pedal is pressed power is passed through these and as more pressure is applied these compress to lower the resistance and allow more power through. When fully on, a metal disc engages across two contacts to allow all the current to pass. I think I would like to have a pedal that could handle more current, possibly something based around how a modern dimmer switch works.