If you guessed that the part was from a sewing machine, you were right. Before these modern electronic sewing machines, the motor in them was controlled directly with an adjustable resistor that was located inside the foot pedal. This controlled the amount of voltage that was fed to the motor thus resulting the higher or lower speeds.
There are many places where you could get your hands on one. I visited a local sewing machine repair shop and asked for used foot pedals but you could find these from recycling centers, flea markets etc. I ended up paying 5 euros, that is around 7 dollars for this particular pedal. Just look for one that is not meant for electronically controlled sewing machine.
Motor on my Ferm rotary tool is rated for 135 Watts. As you can see from this plate that was located under the foor pedal, this pedal is rated to handle maximum load of 90 watts. This is not an ideal situation as the rotary tool can take more current that the resistor inside the pedal is able handle. Try to look for a pedal that has a higher power rating that the motor you are planning on controlling. You may see only Amperes printed on the label but you can convert the given values to watts quite easily. Just multiply the ampere value with the mains voltage and you have the power rating of the unit in watts. For example 0.85 amperes * 110 volts or like here in Finland 0.4 amperes * 230 volts = 92 Watts.
What we can find inside?
Shop owner gave me a short tour on different pedals. There are several models out there so you may or may not be able to get the same model I'm using. You may need to get the pedal part off to get inside but this model had a removable bottom plate. Under the plate we have the actual resistor that is controlled with the top part of the pedal.
I'm certain that there are different models of these resistors too but they should work more or less the same way. Top part of the resistor in this image houses the mechanism that is pushed deeper inside the resistor when you press the pedal. This lowers the resistance of the resistor this giving more "juice" to the device you are powering via it. Capacitor on the left is there to smooth out the electicity but you don't need to touch it at all.
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