> >Picking up the ignition pulse is more exact.
>
> Most trucks are diesel and don't have one of these. Perhaps this is why he is
> looking at the alternator?
>
> My idea would be to have some form of sensor mounted to sense the teeth on the
> starter ring gear. This would probably give a more consistent pulse pickup then
> alternator frequency.

I have an alternator based tacho. The only real disadvantage is that
you have to calibrate it to your setup using an absolute method of
some sort. The W output is a sine wave output of moderately constant
amplitude and frequency proportional to belt speed.
For normal driving use, it is perfectly adequate. You probably only
read it to the nearest few hundred rpm anyway. If it's on a toothed
belt there isn't any slippage.

Steve.

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Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: steveb@tla.co.nz      fax +64 9 820-1929
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