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Originally Posted by sanddrag
I'm wondering if there is anything from an automotive location that would do it. I believe the motors are actually windshield wiper motors, or they could possibly be window motors. How does a car's windshield wiper system get the different speeds? Does it have all different resistors to get the different speeds?
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Many, many, many moons ago I did a science project in high school where I needed to dim a car head light like you would a light in your house. I went to the local auto parts store and asked if I could look a some car dimmer switches. This was a long time ago but back a then a lot of GM cars had a single knob that controlled all of the lights in your car. You pulled the knob out half way and your park lights came on. You pulled it out all the way and your headlights came on also. You turned the knob to the left and your interior dome light came on. Now the important part: You turned it to the right and it dimmed the dash lights. If you wanted to hit your high beams you either pulled the turn indicator or mashed a switch on your floor board under your floor mat. This was long before anything like daytime running lights. So anyway, I roamed up and down some of the shelves behind the counter at the auto parts store until I found this type switch. I put it in a little aluminum box and wired the headlight to the interior dimmer terminals and a 12V battery and it worked great. The little aluminum box got a little warm but I had a 12VDC infinitely variable dimmer. I don't see why this wouldn't work just as well for a small 12VDC motor. Now a dash dimmer switch for a 20+ year old GM car may or may not be a stock item at your local AutoZone or Pep Boys I don't know. If not I bet a GM dealer will know what you need or head to an auto salvage yard. The dimmer switch shouldn't be expensive. Remember K.I.S.S. Sometimes you need to think "Old School".
Edit: The technical term for the dimmer switch is a rheostat and it looks like
this.