Computer Peripherals 433
during the early 1970s, a new input device emerged, which allowed some innovative usage of comput-
ers. We shall now discuss about this device: the mouse.
From a practical point of view, a mouse replaces the cursor control keys (scrolling keys, i.e., up, down, left
and right) and the Enter/Return key of the keyboard. However, its design makes the cursor control opera-
tion smoother and more effective for graphical design activities. Therefore, it is generally considered as an
inseparable part of a computer, especially by the PC-users. With the present day GUI, the user has to mostly
move and click the mouse to get the desired job done by the computer. Mouse may be of opto-mechanical
type or purely optical type. We shall discuss the basic working mechanism of both, in subsequent sections.
Figure 15.4 Opto-mechanical mouse (a) Speed sensing (b) Rotation sensing
Discs with slits
15.3.1 Opto-mechanical Mouse
By turning a mouse turtle, if you can observe a small spherical solid ball projected from a small circular
hole, you are observing an opto-mechanical mouse. When placed in its normal operational position and
moved over a plain surface, this spherical ball transfers its rotation to a pair of smaller at discs, located
orthogonally against each other [ Figure 15.4 (a)]. Because of their orthogonal placement, the movement
of the sphere is resolved into proportional movements (rotation in this case) of X and Y axes. As each of
the at discs is tted with optical encoder (LED, slotted disc and a light sensor), these proportional rota-
tions are transformed to electrical pulses [Figure 15.4 (b)]. Finally, these electrical pulses are digitized
and sent to the host along with the status of the mouse buttons.
One major disadvantage of these optical mice is the accumulation of dust particles over the spherical
ball and, thereafter, to the circular discs, generating unwanted errors in smooth operation. This is recti-
ed in optical mouse, as explained below.
15.3.2 Optical Mouse
All optical mice avoid the spherical ball and disc mechanism to sense the motion. Earlier models of
optical mouse used a LED and a photo detector to sense movements of the mouse over a special surface
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