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Thread: The grey wire.

  1. #21
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    The grey wire.

    Hey Tom,

    How did you get on in the end with your RPM gauge?

    I got mine set up on the bench and it worked beautifully with a sine wave generated by my function generator. Good linear response that was exactly proportional to the frequency of the wave generated. I took it down the airfield last night and it wasn't so pretty when hooked it up to the tacho signal on the Rotax. It worked up to about 3000RPM (although reading a bit high) and then somewhere between 3000-3500 jumped off the scale. Same thing was repeatable time and time again.

    As the LM2907 detects zero crossing I suspect that electrical noise is creeping in and it's not a clean sine wave. Hard to say for sure without a scope but the airfield is off-grid so a scope is not an easy proposition.

    I think I will need to implement some input smoothing on pin 1 to clean it up a bit. Just wondered if you'd had more luck?

    EDIT: Looking at this in some depth I think I'm going to implement a low pass filter using a MAX292 - They do an 8th order Bessel filter on an 8-pin DIP for about 5. It's clock tunable for 0.1Hz to 25kHz cut-off so I'm thinking a 1kHz cut should provide a decent frequency range for the tacho and hopefully reduce any noise coming down the line.

  2. #22
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    The grey wire.

    Interesting... I tried a "cheapo Universal digital car rev counter from China with similar results on the 503. (DCDI) went bonkers above 3500 rpm.
    This rev counter could be set to trigger on the "negative going" output from an alternator output.... or trigger directly from the feed from the Cap Discharge feed
    Actually worked ok when used on one of the 503 CD trigger feeds and also ok on a Dual ignition MZ 2 stroke from the alternator output.. but grey wire... don't want to play!

  3. #23

    The grey wire.

    Hi Tim,
    "EDIT: Looking at this in some depth I think I'm going to implement a low pass filter using a MAX292 - They do an 8th order Bessel filter on an 8-pin DIP for about 5. It's clock tunable for 0.1Hz to 25kHz cut-off so I'm thinking a 1kHz cut should provide a decent frequency range for the tacho and hopefully reduce any noise coming down the line."
    I'd have thought a resistor and capacitor might just suffice ;-)
    Dave

  4. #24
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    The grey wire.

    Dave Smith wrote: Hi Tim,
    I'd have thought a resistor and capacitor might just suffice ;-)
    Dave
    You're probably right - bit frustrating as my workbench with my scope and test tools is 30-40 mins from my engine. If I could sit down with both I suspect I could rig something up that would do the trick with resistors and capacitors. I am no electronics expert - my background is software so I am very much learning on the project. The MAX292 seemed like a nice option as I could fairly easily 'field tune' it by adjusting the cut-off until I get the desired result.

    Wally Hayward wrote: Actually worked ok when used on one of the 503 CD trigger feeds and also ok on a Dual ignition MZ 2 stroke from the alternator output.. but grey wire... don't want to play!
    That is interesting. May be worth having a play around with different inputs. The benefit of the LM2907 circuit (rather than a direct input to the micro) is that it has built in resilience to over-voltage or spike conditions so should tolerate any AC input within reason.

  5. #25
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    The grey wire.

    Yes, I'm getting the same problem but have been dealing with other issues this week as the ignition box had died over the winter. A new one finally got the engine started but the primer was leaking and is now rebuilt. The ' scope is in France right now so it may need to wait until I get back. I'm guessing that there is some frequency doubling going on as even with dodgy zero crossing, it will cross. Those who use Rotax tachos on the grey wire soon stop doing so, it would seem. a little signal conditioning may be required but I'm using a good screened wire, earthed to the block all the way to the chip and wouldn't expect this. I expect that it is the fault of the Rotax stator and wonder if grabbing the alternator signal might be worth doing, bearing in mind that it would need to be potted down. Isolating the signal through a small transformer might soak up some spikes.
    The maximum frequency is 680 Hertz so rolling it off at about 800 wouldn't need anything sophisticated, fragile or dear.

    Low pass filter: 390 ohms each from earth and grey wire to input of tacho with 330 nanoFarads across the tacho input.

  6. #26
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    The grey wire.

    Mike Stratman of California Power Systems has some relevant info which may be of use in para 2 of this article: http://www.cps-parts.com/cps/pdf/Part49.pdf
    In another article (Part 13) which I can't find on the web now, he indicates that the ignition circuit is a massive source of RF and we all know what that does to high impedance loads! He does not mention the grey wire at all in that but says "The generating coils and flywheel magnets are naturally shielded and are not considered to be a source of RF waves.
    (The external ignition) coils are a great source for static interference. The polarized windings are designed to amplify spark voltage to the 20,000 plus volt range. These coils radiate a massive amount of RF waves in a highly scattered pattern". It is these coils he shields but the current feeds go back to the magneto and thus into the grey wire.
    I think that simple LPF's may not give the results you'd like and suggest a multi-pole LPF with each pole shielded from each of the others.

  7. #27
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    The grey wire.

    In which case, it ain't worth the bother. Grab a signal off the plug lead, bang a 555 with it and integrate the output through a Norton diode. Shove that into the adc.
    Which would be fine, but the grey wire is designed for the purpose and the input of the chip is for a magnetic input so it ought to be low impedance.

  8. #28
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    The grey wire.

    Does anyone happen to know what's the peak voltage the grey wire produces?

  9. #29
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    The grey wire.

    No, not me sorry.

  10. #30

    The grey wire.

    Try the bloody resistor and capacitor...
    KISS

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