Magnetic shift tracks or magnetic strips, to which application of a rotating magnetic field or by rotation of the strip itself allows accurate determination of domain wall movement. An electric actuator coaxially coupled to the second pulley portion moves the second pulley portion with respect to the first pulley portion when the electric actuator moves the second pulley portion to change spacing between the first and second pulley portions. A primary portion of the emitted first wavelength light exits the LED from a top surface of the LED that has a minimum lateral dimension Wmin.