Need circuit to multiply tach signal for use with other tach

Discussion in 'The Projects Forum' started by Dr Evil, Aug 24, 2011.

  1. Dr Evil

    Thread Starter New Member

    Aug 24, 2011
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    I tried searching and found some similar issues, but nothing that I found answered my questions completely. Please forgive if this is a common subject and I missed it on search.

    What I am trying to do:

    I have a Moto Guzzi that I am customizing. The Guzzi uses two separate ignition circuits with two separate 6v coils, points, etc. I would like to use an aftermarket tach on my project, but all of the ones I see that I like are car related and thus allow for customization to 4, 6, 8, 10 cyl. As such, I believe that I need to multiply the single fire signal x4 to utilize these gauges. Correct? There are costly products out there that do similar to this, but still not exactly what I need with out some serious massaging. As such, I would rather build what I image would be an easy circuit to solve this issue. I have built mega jolts, sparks, and squirts, but the programming was always a PITA as I was self teaching.

    Dose anyone have any guidance?

    A little background on me:
    3 years of electronics in HS (back in early 90s), 4 years as an avionics tech in the USCG (12 years ago). I can read a schematic, order parts, understand basics, but I am a little rusty. Solid state stuff is my weakest point so I may ask some simple questions.

    Thanks for your help :)
     
  2. jaclement

    Active Member

    Apr 15, 2009
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    Don Lancaster had the solution to your problem in his book " CMOS Cookbook".You use a phase-locked loop with the proper division to give the number of pulses so that your readout is correct.
     
  3. Dr Evil

    Thread Starter New Member

    Aug 24, 2011
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    Interesting book. I may have to add it to my library. Thanks for the tip and the direction to search, PLL. Looking into it and it looks like the deal. I will see if I can figure out how to program one/hook one up.
     
  4. wayneh

    Expert

    Sep 9, 2010
    14,369
    4,980
    Not sure it would work, but you might consider just rectifying the pulses. Kinda depends what signal the tach is looking for - it may not work if it needs a zero-crossing.
     
  5. Dr Evil

    Thread Starter New Member

    Aug 24, 2011
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    Any info on where to buy this PLL and hook it up? Or is it an IC that I need to source and buy based on inputs and desired output? Is it a circuit I have to build with a CMOS?
     
  6. Dr Evil

    Thread Starter New Member

    Aug 24, 2011
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    Some answer here. http://ecelab.com/circuit-pll-osc.htm

    Now I think the cook book will help me figure out how to time the thing to put out 4:1 signal. Unless anyone wants to take a stab at it.
     
  7. Dr Evil

    Thread Starter New Member

    Aug 24, 2011
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  8. Dr Evil

    Thread Starter New Member

    Aug 24, 2011
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    What about a buffer amplifier?
    http://www.tpub.com/neets/book9/35h.htm

    Will this only quadruple a base frequency that is stable, or will it multiply all them in the RPM band of the engine?

    (Never mind, I see that the tank will only be set to the harmonic of a specific reference freq and thus is static. So, this wont work.)
     
  9. Dr Evil

    Thread Starter New Member

    Aug 24, 2011
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    Being that I am playing around in the 8.33Hz-166.67Hz range, I am finding it difficult to find a circuit that will operate in this band. I may be misunderstanding.
     
    Last edited: Aug 26, 2011
  10. Dr Evil

    Thread Starter New Member

    Aug 24, 2011
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  11. Phobos77

    New Member

    May 10, 2018
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    I know this is an old thread and Dr. Evil hasn't visited this thread for a while. Hoping my post will trigger an email to him.

    Dr Evil. Did you manage to find a solution. I have a similar "problem". I also have a Guzzi (850GT, 1973) but with a single coil. I want to double my tach signal to a tacho meter which is suited for 4-6-8 cylinder engine (4 stroke).
     
  12. crutschow

    Expert

    Mar 14, 2008
    18,608
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    For that, a simple digital frequency doubler circuit using a CD4070 XOR gate, as shown below, may work for you.
    You may have to tweak the values of R and C to get it to work properly over the full RPM range (6000 RPM shown).

    Where are you going to get the tach signal?
    If it's from the coil primary, there will need to be some circuitry added to the input (shown from V1) to protect the circuit from the high voltage spikes.

    upload_2018-5-10_11-15-11.png
     
  13. Phobos77

    New Member

    May 10, 2018
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    Hello Crutshow,

    Thanks for the quick reply.
    I will have a look at this.
    I am no electrical engineer I am a technical engineer so where do I start with tweaking the R and C values?
    If I know the max RPM can I some how simply calculate what these R and C values are?
    If not with how much do I increment or decrement the values?

    With regards,

    Peter
     
  14. crutschow

    Expert

    Mar 14, 2008
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    The values shown should work for a maximum of 6000 RPM for a 2-cylinder 4-stroke with a single coil (one spark per revolution).
    If different from that, you can reduce the value of R1 in proportion to any increase.
    Thus for 7000 RPM maximum you would change it's value to 6000/7000 *20k = 17kΩ.
    The exact value is not critical.
     
  15. Phobos77

    New Member

    May 10, 2018
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    Clear.

    The circuit on the left (Vdd) is to power U1. Resistor R2 has a value of 10. To be sure, this is 10 Ohm, not 10K Ohm, right?

    I had a look at the setup and yes I will get the signal from the primary coil. Can you propose a circuitry to protect it from high voltage spikes?
    But why would there be voltage spikes?
    The primary coil is powered by 12 Volts en the secondary coil is the high voltage coil for the spark plugs. So I am thinking the input signal can not be higher then 12 volts. What am I overlooking?
     
  16. Phobos77

    New Member

    May 10, 2018
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    I think I understand why I need a protective circuit.
    The capacitor parallel to the contact breaker gets charged with a higher voltage then the battery before it collapses into the primary coil.
    I always thought the capacitor acted only as a snubber preventing spark erosion at the contact point and that you could even have a working system without the capacitor only that it would wear your contact points faster.
     
  17. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    1,810
    Coils have inductance. Inductance is a strange animal which objects to changes of current through it. Current takes time to build up in it, but when you interrupt the current the inductance creates a high voltage which tries to keep the current going. Thus your 12V can easily become several hundred Volts across the primary coil. The capacitor tames this somewhat, but forms an oscillatory circuit with the inductance and causes 'ringing' until the coil energy is dissipated in the coil's inherent winding resistance.
     
    Phobos77 likes this.
  18. Phobos77

    New Member

    May 10, 2018
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    Hello Crutschow, Could you please advise me some protective circuitry? With regards, Peter
     
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