An operator for a barrier, such as an upward acting door, includes a control unit having two optical limit switches providing output signals to a controller to effect shut-off of the operator motor when the door reaches open and closed limit positions. The limit switches preferably include LED emitter and phototransistor sensor elements and are preferably mounted on a circuit board in close proximity to a screw member which rotates in timed relation to the position of the door. Linearly movable nut members are mounted on the screw member and include or engage optical shield members which move into positions to provide respective output signals from the optical switches. The door closed optical limit switch may incorporate a pre-limit switch function to override a signal from a door bottom edge obstruction sensor.