A simple RC circuit should be enough to filter out the noise
of the coil signal. But you have to experiment. The only
disadvantage in counting the coil pulses is accuracy or resolution.
You would need to count the coil signal for exact one minute if
you want accuracy to be +/- 1 count.

If the counter of your DVM refreshes its count every second the
accuracy would be +/- 60 counts, caused by sampling time being
too short. An analog RPM meter may be equally accurate. Why not
just use an analog RPM meter?

I have an old Sears automotive tune-up tester. It has an analog
RPM meter on it. Don't know how accurate it is. It seems good enough
for me for setting idle.

David

--- tinker man <tinkerman007@GMAIL.COM> wrote:

> On 9/26/07, David Kao <dtkao0205@yahoo.com> wrote:
> > Why not just buy one and save all the time to build one?
>
> hehehe...you probably think I want to build everything...
>
> > Here is one that seems pretty good. I think there are lots
> > of it out there. Just google "digital tachometer".
>
> Pretty nice, but:
> 1. They don't ship worldwide.
> 2. I need a tach just for tuning (e.g. adjusting idle) and
> diagnostics, and for that, my external counter seems to be enough for
> my needs, just wanted it to better reflect the actual RPM.
> 3. For economy and legalities, I drive around 100Km/h (what
> economy?...). Good enuff for me. No need for a tach usually, and don't
> know where I'd stick a tach in my (desirably clean) dash?...
>
> cheers,
> TM
>

____________________________________________________________________________________
Boardwalk for $500? In 2007? Ha! Play Monopoly Here and Now (it's updated for today's economy) at Yahoo! Games.
http://get.games.yahoo.com/proddesc?gamekey=monopolyherenow