On 2014-05-02, Mike Spencer wrote:

Tim Wescott writes:

[ ... ]

Ah, tnx for that. Heard of "Hall effect", didn't know that commonplace
articles implemented it. Where would I look in old junk to find one?

Hmm ... near where moving magnets are sensed -- if you can find
"old junk" which is new enough. :-)

I just don't feel that doing this optically will be robust.

Someone gave me a DecWriter II in 1990, when it was at
archaeological-artifact stage of life, which I played with for a
while. The opto-mechanical unit for spacing eventually failed and I
was never able to get it working again. I suppose the similar little
wheels in a mouse would have the same potential for refractory
NFG-ness, which is why I thought of but didn't try that for my (real)
mouse tach.

The problem with the DecWriter-II is that the chopper was a thin
metal disc stuck to a hub with double-sided tape. When the temperature
gets too hot, it can shift off-center and stop being sensed properly.
(The head will bang into the stops at one end of travel or the other.)
(Ask Me How I Know This. :-)

So -- the problem there was not with the robustness of optical
sensing, but with the robustness of the mounting of the chopper wheel.
A very different matter. If you drill a number of holes through the
spinner, and put the sensor inside (assuming that you want this to be
sensing during flight, instead of just on a test bench) it should be
well enough protected, and the spinner will certainly stay put. It is
not mounted by glue, I hope. :-)

Enjoy,
DoN.

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