Need more details on the fan. Are you suggesting this is of the type
usually found in computers? Brushless DC?
Many (most?) of those have a tachometer output on the third wire....

If that's not the case, I'd probably look for some way to mount a magnet on
the moving part and a hall sensor on the stationary part.
Non-contact, dust immune, bug immune, etc.


-Denny
--=20
http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.