The device includes a body having a cap with a number of push buttons locked to its upper side. The body accommodates a circuit board (13) and has a hole in a bottom side for accommodating a ball (15). The body further has one or more encoders (16), each of which includes a wheel (17) with a spindle (18) fixedly provided with a roller. A securing seat (20) is disposed below the wheel, and has a number of plate-like mountings (24-26) for supporting the spindle of the wheel. The mountings further support an idle wheel at a position lower than that of the roller. The idle wheel is disposed intermediate the roller and the ball and contacts with both of the roller and the ball for transmitting rolling actions of the ball to the roller. A light source (31) and a sensor (32) are respectively disposed on both sides of the wheel and secured in a couple of positioning slots formed in the securing seat.
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