A combination of LED and photo detector is used to sense the distance traveled by the mouse. When mouse is moved across a flat surface, the ball protruding from the underside of the mouse and touching the surface starts to rotate in the direction of the movement.
Rotating ball touches and turns two rollers inside the mouse. These rollers are mounted at 90 degree angles to each other. One roller is used for vertical movement and the other roller is used for horizontal movement of cursor on the screen. Each roller is connected to a wheel, which rotates with movement of rollers.
As the wheel rotates, a pair of photo detector detects the number of openings on the wheel passing between them. Each opening on the wheel allows light from the LED to fall on the photo detector and generate electrical signal.
The direction in which the mouse is moving can be found out by finding the number of signals from vertical and horizontal rollers.
These signals are sent serially to the PC over the mouse cable. The mouse driver software converts these signals into distance direction and speed necessary to move the cursor on the screen.
Pressing a button on the mouse sends a signal to the PC. Based on which button is pressed, how many times it is pressed and the position of the cursor at the time of pressing the button, the mouse driver performs the task.
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