JAY EMRIE wrote in a message to ROY J. TELLASON:

RJT>JB>extra components would be needed to get a rate signal from a 2
RJT>JB>wire fan.

RJT>JB>it's easist to tap the monitioring terminsl on the fan PCB. (typically
RJT>JB>right next to the power terminals)

RJT>JE> HUH? None of the fans in my computers have EVER had a fan PCB. Am I 
RJT>JE> not understanding?

RJT>Ever take one apart?  :-)

RJT>There's likely much more than just a simple dc motor in there.

JE> That "likely" indicates that you "THINK" but do
not "KNOW" for sure 
JE> that there MAY be more than a simple DC motor in there.

I've not really felt like it was worth the effort,  either in terms of
doing something with the innards of one or in terms of getting any sort of
useful parts out of it.  I *have*,  however,  scrapped out a bunch of
floppy drives,  of various types.  The real early ones had pretty distinct
motors,  especially when you're talking about the real early belt-driven
models.  Later ones had a fair-sized magnet attached to the metal disc that
you often see underneath.  A few of those came in real handy to hold a tarp
on the back of my truck at one point.  I don't think that the wind would've
caused those to cut loose,  as they had to be *slid* off.  But anyway, 
under each of these was a set of coils,  and a specialized chip to drive
those coils,  along with feedback from some sort of a sensor that told it
how fast it was going.  I'm thinking that the stuff inside many small dc
fan motors is probably something fairly similar,  these days.

JE> IIRC, the subject was PC case fans - and simply looking at one will 
JE> show there is no circuit board in/on one of these.

I wouldn't bet on it.  Grabbed a couple of fans that were handy here,  but
unfortunately the first one runs off 115v,  and the other one is *glued* to
a Pentium chip,  so I can't see the other side of it.

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