I think you mean an 'encoder' -- and for 5 dollars, you'll probably need
to build one. The easiest route would probably be to print your own
encoder disk and use a Hamamatsu photodetector (around $2.00/unit). You
can mount the disk on the inside of a wheel or on the motor armature
shaft, if available.
There are other approaches available as well, including the use of
Hall-effect detectors (around $1.00 or so each) and magnets. This
approach works a bit better if the magnets are mounted on a disk
attached to the motor armature (again, if your motors have one). You can
mount magnets directly to the wheel, but you won't get great resolution.
I'm assuming you're using gearhead motors and only need speed (not speed
and direction or absolute position).
Hope that helps -- m
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