A sensor may be formed with a transistor comprising a gate that has both n-type and p-type regions to increase the gate work function. This exchange-spring magnet uses the magnet alloy prepared by a liquid quenching method or a mechanical alloying method. Each of the cutter molds including a peripheral edge portion, a base portion and a curved, annular side wall defining a concave, generally bowl-shaped internal cavity, with the peripheral edge portion establishing a cutting edge for the cutter mold. A reversible motor is connected to drive the sprocket to urge the beads and, in turn, the pool cover fabric between open and closed positions.