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Need to multiply rev counter signal ?

Discussion in 'Electronic Design' started by George Gosbee, Jan 2, 2006.

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  1. Hello all,

    I'm after advice for a simple frequency multiplier, I need to increase the
    rev counter signal on my car by 1.5 i.e. if revs are 2000 I need a signal of
    3000. So it needs to be 12 vdc supply 0 - 7000 rpm input 0 - 11000 output.
    Capable of easy build on a bit of Vero board.

    Its to give the correct rev signal to a LPG system I have fitted but for
    some reason it only reads 2/3 of the rev's. It may be something to do with
    the fact that its a V6 working on a lost spark system. I.e. the plugs fire
    every rev not just on the compression stroke.


  2. John Fields

    John Fields Guest

  3. Thanks for the quick reply,

    Its digital, and possibly a very spiky pulse. It is a wide ranging input, in
    that it can get its signal from the HT coil primary or a wire wrapped around
    one HT lead or the TDC sensor or the feed to the taco gauge.

  4. the Jackal

    the Jackal Guest

    I'd say you have at least 3 alternatives to solving your problem :
    1) Tougher way which though directly answers your question:
    Transform a typical Class C single BJT/MOSFET amplifier circuit with
    a parallel LC circuit at the collector/source.
    Choose the L and C components such that LC tank resonates at 1.5f
    where f is the frequency of your input signal.

    2)Easier way.
    There is probably a microcontroller that is counting the number of
    revs from the engine shaft. You'll have to check for a DAC chip in
    your tach that changes those engine pulses to an analog voltage. They
    normally have an output pin where you can connect a pot to adjust the
    output voltages. If the pot is already there everything is simpler, so
    you'll avoid all the hassle of building up a seperate circuit to get
    the job done, which would in any case be a simple circuit. You might
    need the DACs datasheet to get that done.

    3) Even more easier: Maybe next time....

  5. John Fields

    John Fields Guest

    I'm confused. If you have so many input types available it seems
    like there must be at least _one_ available which would fill the
    bill. Does the possibility exist that you could change from the
    signal source you're now using to another one which might work right
    off the bat?

    For example, the output from the TDC sensor gives you just one pulse
    per revolution, which the interface might like a lot.

    If you can't do it by selecting the signal source, then I can think
    of two methods that might work, the first being to use your current
    stream of pulses to fire a non-retriggerable one-shot with a small,
    fixed period, then integrate those pulses and use the resultant DC
    to drive a VCO which would, in turn, drive the interface.

    The other way would be to use the input spikes to drive a
    non-retriggerable one shot with a period of about half the time
    between two input spikes at the highest speed at which the system is
    supposed to work. Then use an EXOR with a delay on one of its
    inputs to double the output frequency of the one shot, and a circuit
    to remove every fourth EXOR output pulse. That way, for every 2
    spikes in you'll get 3 EXOR output pulses out, which is what you
    want, and if the interface is just counting pulses and time and
    isn't worried about symmetry, then it'll probably work.

    If you can't make it work by using another input source, and you
    need a schematic, let me know, here, and I'll put something together
    for you.
  6. Ken Smith

    Ken Smith Guest

    This sounds like a job for a CD4046.

    You divide the frequency by 2 to feed one input of the phase detector-2
    and the VCO by 3 to feed the other. Since an engine can't change RPM too
    fast, it would be ok, I think.
  7. Ken Smith

    Ken Smith Guest

    Better yet, times three then divide by 2.
  8. Sounds good, will this chip need many discreet components? as you can guess
    electronic design is not my profession.

    I'm a HVAC controls man.

  9. Ken Smith

    Ken Smith Guest

    Go find at least a hobbiest electronics guy and suggest this to him:

    ! !
    4046 part 4046 part ! ----- !
    ------- ----- -!D Q! !
    in ---------! PD II !---/\/\/---+----! VCO !-------!> ! !
    ------- ! ----- ! Q/!-+--+-- Out
    ! --- R1 R2 C1 ----- !
    ! --- !
    ! ! C2 !
    ! GND !
    ! !
    ! ---------- !
    --------------! Div by 3 !----------------
  10. John Fields

    John Fields Guest

    Don't you mean:

    ! !
    4046 part 4046 part ! ----- !
    ------- ----- -!D Q! !
    in ---------! PD II !---/\/\/---+----! VCO !----+--!> ! !
    ------- ! ----- | ! Q/!----+--Out
    ! --- R1 R2 C1 | -----
    ! --- |
    ! ! C2 |
    ! GND |
    ! |
    ! ---------- |
    --------------! Div by 3 !-----+
  11. Ken Smith

    Ken Smith Guest

    Oooops yes!
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