Date: Tue, 25 Sep 2007 21:36:13 -0700
Reply-To: David Kao <dtkao0205@YAHOO.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Kao <dtkao0205@YAHOO.COM>
Subject: Re: Measuring engine RPM based on Hall sender/coil signals
In-Reply-To: <1b0df96d0709251856o3a4a8db7qcf933ff3dc55869c@mail.gmail.com>
Content-Type: text/plain; charset=iso-8859-1
Why not just buy one and save all the time to build one?
Here is one that seems pretty good. I think there are lots
of it out there. Just google "digital tachometer".
http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_524744_524744
David
--- tinker man <tinkerman007@GMAIL.COM> wrote:
> On 9/13/07, David Kao <dtkao0205@yahoo.com> wrote:
> >
> > --- tinker man <tinkerman007@GMAIL.COM> wrote:
> >
> > > Also, looking at the schematic, I see that the coil driver output goes
> > > in parallel to the dashbord to drive the tachometer,
>
> Yep, I've verified it and even got a slight electrical shock touching
> the wire on the relay socket by mistake...(coil primary kickback).
>
> > Before taking this signal to your counter you will need some RC signal
> > conditioning circuit to shape/filter the signal for the counter. Without
> > signal conditional it may be too noisy for the counter.
>
> OK, I know the theory, but lack the time to experiment and develop. I
> know this same coil signal goes also to the dynamic oil pressure alert
> system.
> Any suggestion for a conditioning circuit? (RC + diode limiter?).
> And hasn't anyone used this signal for a DIY tachometer?
>
> Anyway, I've connected my handheld counter/DVM to this pin and it
> reads 140-200Hz at idle, and increases with engine RPM (didn't check
> linearity). So maybe the noise is confusing the counter to read 150
> instead of 30Hz, but it's still OK as long as I know what to expect at
> idle. And more than that' it gives me a positive indication that the
> ignition system is working. One less reason to debug when motor won't
> start...
>
> > When the engine idles you will only see about 30 pulses per second of this
> > signal. But you will like to see 900 (RPM) displayed on the counter. But
> > you will see 870, 900 or 930 if your counter is set to count the pulse
> > at a rate of once per second. If you want to see 898, 899, 900, 901, 902...
> > (higher resolution) you may need to set your counter to count once per
> > minute. This will be a poor RPM display because it updates only once per
> > minute.
> >
> > The best way to design a digital RPM display is to measure the duration
> > of the pulse of the coil signal, instead of counting the pulses, which
> > has a limited resolution. By measuring the duration of the coil signal
> > you can make a fast updating precision RPM display. You can read 901.25
> > RPM at a refresh rate less than a second with a precision to +/- 0.01 RPM.
> > It is still a counter. But it counts an oscillator during the duration of
> > the coil signal. If the oscillator's frequency is high enough then you
> > get very high resolution.
>
> Very good idea David, and it's obvious your an experienced designer.
> Of course I could design it, but can you lower my burden and provide a
> simple implementation? Otherwise, I'm going to continue using my
> counter/DVM just for debugging and tuning when needed, don't really
> need a tach for changing gears, my ear and gut feeling suffice...:-)
>
> Cheers,
> TM
>
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