A motorized operator for moving a barrier, such as a garage door, between open and closed positions, includes a magnetic sensor for determining position and/or velocity of the barrier. The operator motor is drivingly connected to a speed change transmission which is connected to the barrier. The magnetic sensor includes a housing supporting a speed change mechanism including an output shaft supporting a magnet. The magnet is mounted adjacent a Hall effect sensor circuit which measures the change in the magnetic field generated by the magnet to determine position of the barrier, as well as speed. Control circuitry enables accurate determination of the position of the barrier for setting open and closed limit positions, for example.