Digital Tach and Newbie!

[David Hunt]

At the coil the voltage goes to zero when the points (or replacement) goes
near zero volts.  When the current is interrupted the voltage goes quite far
below zero.  Then the voltage goes quite far above 12 Volts.  You can used
an RC in front of a zener to get control of this signal and provide it to
the further signal.

dh




----- Original Message -----
From: "Shane Moseley" <smoseley at ix.netcom.com>
To: <diy_efi at diy-efi.org>
Sent: Wednesday, September 05, 2001 6:53 PM
Subject: Re: Digital Tach and Newbie!


> 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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