An adjusting apparatus includes a shaft, a rack, and a gear. The shaft includes a cam portion contacting a bracket and a securing portion. The cam portion includes a first outer edge and a second outer edge connected to the first outer edge. The cam portion defines a pivoting axis. The distance between the first outer edge and the pivoting axis is less than the distance between the second outer edge and the pivoting axis. The gear is toothed and secured to the shaft and the rack defines a plurality of slots engaged by the toothed gear. The rack is used to drive the gear and the shaft to be rotated.