Abstract: A trackball in one embodiment includes a PCB including an aperture, a socket including upper and lower chambers communicated with each other, rollers disposed around bottom of the upper chamber, a ball rotatably supported by the rollers, a ring cover secured to the socket, a light guide plate mounted between the socket and the PCB and including an inclined light guide tube and an optical element; and an optical assembly mounted under the aperture and including a light source (e.g., LED) for emitting light through the light guide tube prior to impinging on the ball, and a photosensor. Rolling the ball will reflect the light from the ball through the optical element to be received by the photosensor such that the rotational movement of the ball can convert into a digital signal useable to move a cursor a corresponding distance and direction on a display.