Date: Wed, 26 Sep 2007 03:56:11 +0200
Reply-To: tinker man <tinkerman007@GMAIL.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: tinker man <tinkerman007@GMAIL.COM>
Subject: Re: Measuring engine RPM based on Hall sender/coil signals
In-Reply-To: <212545.68761.qm@web82714.mail.mud.yahoo.com>
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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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