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You could use the device that you descirbed but there would't be a sharp transistion between on and off. You would probably see some "bounce" from the brush that would give a false reading.
With a transmitter/ receiver pair you would drill or mill a series of holes in your wheel, each a equal distance apart. Now looking at your Digikey catalog you want a infrared transmitter as your light source mounted on one side of your wheel. Using infrared will eliminate false signals from other light sources. Now mount your infrared receiver directly opposite the transmitter and the slots will block then pass the light through the wheel. Measure how long it takes to see the number of slots you have and you have the speed for one revolution.
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