Could a little piece of a magnetic rod embedded in either steel or Al disc (flywheel, pulley, etc.) do the trick?  I always thought magnetic pickups actually involved a magnet, or at least a dominant spike of metal passing the pickup.
        Magnetic reluctance sensors need a steel "tooth" on the moving disk to operate. The tooth (or teeth) must be a material that you can pick up with a magnet. Thus, aluminum won't do. You could put a steel "ring gear" on an aluminum pulley, I suppose.

        The sensor has a magnet inside. When the tooth comes near, it makes an easier path for the magnetic field from the sensor magnet. (Kind of like putting the keeper on a horseshoe magnet.) This increases the magnetic field in the sensor pick-up coil, generating an electric current. This voltage "pip" is sensed by the electronics.

>>  Homepage:  http://www.flyrotary.com/

>>  Archive:   http://lancaironline.net/lists/flyrotary/List.html