Evolution of the Apple Mouse

Tony Duell ard at p850ug1.demon.co.uk
Tue Sep 28 15:15:19 CDT 2010


> >    Not really.  The replacement of one expensive, failure-prone 
> > mechanical part with a cheap-and-getting-cheaper silicon part is
> > almost always a good thing.
> 
> I won't quibble wth the "cheaper" part, but exactly how "failure-
> prone" are microswitches?  I can remember only one failing and that 
> was on an automatic coffeemaker (handled lots of current in a wet 
> environment; arcing eventually destroyed the contacts).

Didn';t somebody say here last week that they had ordered some 
replacement switches for their mouse? So presumbly they do fail.

I can think of 2 microswitches that I've had problems with in computer 
devices farily recently. The devices themsevles were over 25 years old, 
so I can't moan.

One was a track 0 switch in a Tandon TM100 drive in an HP9836. It 
intermeittanylu failed to make contact properly. Since it was debounced 
by an SR flip-flop this caused some interesting faults. Since the drive 
will step towards the spidle when the track0 siganl is active, but won't 
step out again once the stepper ahs got to the right phase, this caused 
the itneresting effec that the drive would step in ith no problems but 
would only step out by 3 or 4 cylinders. This switch was a low-force one, 
but amazinygly I foudn the exact replacement (size and operating force) 
in a current catalogue. So I replaced it.

The other was the paper-out swtich in an HP 2631 printer. I think I 
mentiond this on the lis a few months back. The contacts were shorted, 
even though it still clicked properly. This led me a merry dance trying 
to work out why the printer appeared totally dead.

I guess switches are less reliable than ICs, but equally they are easier 
to test and repair/replace. So I am not at all convinced that replacing 
the former wit hthe latter is a good thing.

-tony



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