>         I'm relatively new to the PIC scene.  I want to use a PIC16F870 to
measure
> the rpm of a rotating shaft (0-15,000rpm).  There is a passive speed
sensor
> in the shaft that puts out a sine wave whose frequency is proportional to
> shaft rpm.  The amplitude of the signal increases with increasing
> speed.  The max. voltage of this signal is 70V peak-peak.

I've done this sort of thing a few times before.  First, you have to decide
what the minimum shaft speed you need to detect is.  That tells you the
worst case voltage and frequency of the sine wave you have to deal with.  It
also tells you how long you have to count pulses for to get to the desired
accuracy (5% in your case).  If this time is too long, then you need to
measure period and invert.  This is actually pretty easy with the CCP module
in capture mode.  All my tachometer-like applications have ended up doing it
this way.  You get quicker and more accurate updates which are also easy to
filter without requiring much memory.

Anyway, back to the pulse.  The sensor is probably a coil, which will have
an open circuit output voltage proportional to the frequency.  The worst
case is the low frequency end when the signal is smallest.  Design the
circuit to have enough gain to create a 0-5V square wave from the signal at
the smallest incoming amplitude.  There is always noise, so use hysterisis,
which gets you the square wave output for free.  I would set the hysterisis
to be about 25% to 50% of the minimum signal size, depending on how
confident you are that you really know what the minimum will be accross part
variations, temperature, etc.  Since your signal has an amplitude
proportional to f, you can apply a simple passive single pole filter with
1/f rolloff at or slightly above the minimum frequency.  This will help with
noise immunity at the higher frequencies because the hysterisis will stay a
roughly constant fraction of the signal amplitude.  You also want to avoid
overdriving the opamp or whatever.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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