There are two sensors in a slight angular shift. They are designed such
that there is overlap between there pulses. When you rotate the wheel in
one direction you get

A B
0 0
0 1
1 1
1 0
0 0

And when in the other direction you get this sequence reversed. A simple
finite state machine is used to extract from these sequences Increment
and Decrement pulses. The finite state machine is designed such that
even if the wheels changes direction arbitrarily, you don't accomulate
any error.

Tal

> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Jan-erik Soderholm (QAC)
> Sent: Tuesday, April 29, 2003 2:50 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [EE]: PC mouse optical detectors.
>
>
> Hi.
> I just scraped an old mouse (a Logitech wheel mouse)
> to see what optical components they used.
> I noticed that the wheel had one LED (IR?) and one
> 3 pin "receiver" mounted. Now, the wheel just have a row
> of spokes, but someway, the unit must not only detect
> spoke/nospoke, but also the direction the wheel is turning.
> Anyone know how these receivers work ?
>
> Maybe I'll de-solder a pair and setup a testbed...
>
> No marking on the components, of course...
>
> Jan-Erik.
>
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