Digital Tach and Newbie!

[Shane Moseley]

Santi Udomkesmalee wrote:

> >Since RPM seems to not need high accuracy you could do it with a LED array
> >of say
> >50 LED's for an accuracy of 120RPM and a range of 6000RPM...how does that
> >sound? I
> >guess it would start to get expensive WRT the drivers but leds are
> >cheapish.
>
> That sounds like a great solution, but I don't know where to start.  Where
> is a good place to pick up signal that I'll be dealing with.

<snip>

> So, if anybody is willing to help me with this tach, i'd really appreciate
> it!

Santi,

I'm willing to help ferret out the info required and maybe even do some
breadboarding/testing.  I once read an article about building an LED tach.  It
used the OBD2 diagnostic port on later model vehicles to retrieve this info from
the onboard computer.

As far as where to pick up the signal - I'd say via an inductive pickup on a
plug wire if interested in spark frequency at that point, in the distributor or
on the coil negative if interested in data from that point.  I like the idea of
what spark frequency the plug is seeing rather than what the computer is
delivering (or trying to deliver).  How about a device that has a feature of
multiple pickup points (other than via the diag port).

If pulling the data from the coil negative.  List - could someone describe the
voltage characteristics there.  I assume the negative side will go to zero volts
when collapsing and then what?  Float high?  What kind of spikes and current
shunts to expect?  Is opto-isolation absolutely necessary?  Thoughts on sample
rate?

Thanks!

Shane

ps - I can't remember if I mentioned it on this list but my newly installed
Edelbrock water injection system is warding off detonation (especially on hot
days) nicely but I'd have to say that my initial impression is not as good as
expected.  I was wanting to drop all the way down to 87 octane but not sure that
is gonna happen.  Mopars MP computer adds about 15 degrees more timing and I
installed my R/T camshaft at 4 degrees advanced.  Pulling 5000+lb out of the
hole on a hot summer day w/out the water is on the edge of detonating w/93
octane.  I'm now successfully running 89 octane w/very little detonation (still
need to tune the water setup).  The most noticeable difference is that WOT on a
hot day now feels like WOT on a cold day w/the water on.

--
'96 IndyRam-HisIndy-MPI TB Pulleys RTcam MPComp HVoilpump DynaGearDoubleRoller
WindageTray CompTAs
'96 IndyRam-HerIndy-numbered(#142)"Track Truck"
'74 Triple-Black Dodge Challenger Rallye 360 EFI R&D vehicle
'93 Dakota LE CC 318 - newest acquisition


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