A first target wheel has an opening and rotates about an axis of rotation to move the opening along a generally circular path. A second target wheel adjacent the first target wheel has a second projection projecting into the opening of the first target wheel with the second projection being movable along the generally circular path. A single sensor is positioned adjacent the generally circular path and senses movement of the opening and the second projection past the sensor. An electronic controller may be used to process signals from the sensor to determine the relative angular difference between the first target wheel and the second target wheel.