[chbot] Mouse Scroll Wheel Question

[Andrew Errington]

On Sat, February 16, 2008 23:37, Richard Jones wrote:
> A friend of mine wishes to use a mouse scroll wheel circuit as a low cost
>  USB input for a software defined radio, removing the scroll wheel and
> providing his own pulses from a rather nice 'knob'. Has anyone actually had
> a mouse apart apart and figured it out? The initial assumption was that
> the wheel would have an optical quadrature detector but there seems to be
> only one optical detector, and I suspect that the inputs are multiplexed
> from buttons etc. Any ideas welcome.
>
>
> Richard Jones

Hi Richard,

In the mice I have dismantled I have found several variations in design
(some quite elegant), but generally they fall into the two expected types-
opto-mechanical and electro-mechanical.  I think that electro-mechanical
is most common as the requirement is for only a dozen or more clicks per
revolution, whereas the mouse ball itself needed to generate hundreds
(before the purely optical sensor was introduced).

The one that you describe is probably optical if the optical sensor is
next to the wheel.  Even though you see only a single optical package
there, look for 3 pins connected to it.  Often there are two optical
sensors (offset, of course) moulded into one package.  The three leads are
one for each sensor (A & B) and a common connection.  I doubt that the
buttons are multiplexed with the wheel quadrature, but there will be a
button attached to the wheel as well somehow which might be confusing.  It
might be cleverly integrated mechanically with the wheel assembly, but it
will have its own I/O line.

If it's electro-mechanical then look out for some weird bent spring wires
inside the wheel itself.  Often there is a quadrature pattern of PCB
traces etched onto a circular piece of board inside the wheel.  Spring
contacts will pick up A B and Common traces.  Another design has a little
spring that flicks one way for 'up' and the other way for 'down'.  You
can't use quadrature techniques for this design.

By the way, if you're looking for a neat input switch take a look at
Dickies P7531 Rotary Encoder.  It's hard to get now as it's been on
clearance for a while, but it's nicely made (and currently cheap).

HTH,

Andrew