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Help request 30 led bargraph

Discussion in 'Hobby Electronics' started by RexT, Jan 25, 2005.

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  1. RexT

    RexT Guest

    I am trying to make a 30 led bargraph to display the output from a MAP
    sensor.
    Found this circuit - http://cp_www.tripod.com/rotary/pg27.htm#30ledaf
    My question is - How do I incorporate pots to adjust zero and full scale ?
    Any help gratefully received.
    RexT.
     
  2. Ken Taylor

    Ken Taylor Guest

    That's the purpose of the pot's in the circuit, as described in the note on
    it. Look here:
    http://www.national.com/pf/LM/LM3914.html
    for a more detailed description on how to use this IC ("Application Hints"
    section is useful).

    Ken
     
  3. RexT

    RexT Guest

    Thanks Ken,
    Have read the spec sheet but with my limited experience I have trouble
    joining the dots.
    If the only way of zero adjust and span is to use the multiple trim pots
    then my original idea is out of the window.
    I am building two of these to tune a twin cylinder motor and was going to
    use a ganged pot to control zero and another for span.
    This would give me the ability to tweak the displays on the bike (as it is
    also destined to tune other machines for friends)
    I was going to include a left/right switch to prove that both sides are
    equal.
    I normally use a mercury in glass pair of manometers on my own bike, but it
    is not portable and friends tend to want their machines tuned at their own
    locations (then I get to drink their tea/coffee).
    I was hoping that someone would come up with a mod for the circuit - just
    pop a ..... in next to part A and that would give me the external adjustment
    I need.
    Thanks for your reply - it was a big ask to hope for someone to jump in and
    do some design work for me.
    Best regards,
    RexT.
     
  4. Ken Taylor

    Ken Taylor Guest

    Well, you can get twin pot's which are independently variable, if that's
    what you need. The fact that the schematic description says 'trimpot' and
    you want to use pot's doesn't change the fact that they are both variable
    resistors - they'll do the same job, just the pot's being larger (and more
    unstable, AOTBE). Try it out, you may be pleasantly surprised.

    Cheers.

    Ken
     
  5. RexT

    RexT Guest

    Where do I put a Zero adjust pot in that circuit ?
    How do I amend it to run off a MAP sensor instead of a Lambda sensor ?
    Is there anyone that I can chat to who can design these things ?
     
  6. Richard

    Richard Guest

    You need to give a little more information.

    Like, what is the expected output voltage range from the MAP sensor for the
    range of pressures expected.

    Then using the design information in the LM3914 data sheet you can set up
    the required reference and offset voltages with (trim)pots and resistors.

    Richard.
     
  7. RexT

    RexT Guest

    Expected possible output will be from 5v to zero, but under operating range
    will be more in range of 3v to 1v.
    This makes the Zero and FSD adjustments so important to keep the actual in
    the 30 led range.
    Reference and Offset - I take it that reference will be the zero point will
    it ?
    Then the offset will produce full scale deflection adjustment ?
    Thanks for your interest - life is tough when you are missing the basics
    (and 57 years old does make entering a whole new field a little difficult).
    Best regards,
    RexT.
     
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