> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of MCH
> Sent: 04 February 2010 17:02
> To: Microcontroller discussion list - Public.
> Subject: Re: [PIC] Fan speed check
> 
> Good info, but I won't be controlling the fan speed - I'm just looking
> for when it slows down or stops (due to age or what have you).
> 
> IOW, I need to make sure the fan is running and close to rated speed
for
> the (nearly constant) voltage provided. If it's normal RPM is 1300
RPM,
> I want to sound an alert when it drops below 1000 or 800 RPM.

In that case you don't really need to calculate RPM, just measure period
and compare against the period for 800RPM.  A bit of filtering may be a
good idea to eliminate any duff readings you may get back.

Regards

Mike

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