An auxiliary input device for menu control on a portable computing device is provided. The auxiliary input device includes a lighting unit, an optical lens, and an optical sensor. The lighting unit is for projecting light to a surface upon which the portable computing device is moved upon. The optical lens is for receiving reflected light from the surface and focusing the reflected light on the optical sensor. The optical sensor is for periodically capturing images of the reflected light and transmitting information of the images captured to the portable computing device. The images captured map a movement of the portable computing device. Accordingly, the portable computing device scrolls through a menu displayed thereon according to the images captured. A related method is also provided.