About computer mouse

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Many of you may have opened a computer mouse and have seen two slotted
wheels which rotates in one or another direction depending on how you
move the mouse.. (I am talking about an ordinary rolling ball mouse)
I was wondering how the bidirectional motion of these wheels are
detected by only one pair of sensors for each wheel.Or are there two
sensors embedded in one single module ? so that IR to one of the
sensors is cut first depending on which direction the wheel rotates ..

 

[Robert Baer]

Many of you may have opened a computer mouse and have seen two slotted
wheels which rotates in one or another direction depending on how you
move the mouse.. (I am talking about an ordinary rolling ball mouse)
I was wondering how the bidirectional motion of these wheels are
detected by only one pair of sensors for each wheel.Or are there two
sensors embedded in one single module ? so that IR to one of the
sensors is cut first depending on which direction the wheel rotates ..

One LED, shines thru slotted wheel to dual sensor; offset gives timing
difference to allow determination of direction.