increase signal input on LM3914

Discussion in 'The Projects Forum' started by laurich, Jan 30, 2011.

  1. laurich

    Thread Starter New Member

    Jan 22, 2011
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    I got my LM3914 bargraph to work finally. I really want to thank SgtWookie and AudioGuru for all their help. I have another request hopefully it won't be too involved. In the attached circuit R1 is 680 ohms, R2 is removed and bypassed to ground, the supply voltage is about 9 vdc. My problem is the signal input only allows 4 to 5 LED's to light. Is there any "simple" way to amplify this signal to have all ten LED's lit. The signal come from a small generator my daughter made and I really need all 10 LED's to light. Thanks in advance for your help.
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    See Page 6 of the Datasheet

    It explains how to change the resistors between REF OUT and REF ADJ to get the range you need, there's a small formula in the datasheet for getting the exact values.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Ahh, is this the thread from where you were trying to sort stuff out?
    Link: http://forum.allaboutcircuits.com/showthread.php?t=48799

    You know, we really do need an accurate schematic of your entire circuit.
    Showing us something from the datasheet and then saying "It's just like this, except..." just makes it that much harder for us to help you, as it usually generates more questions than provides answers. It is very easy to misinterpret what people write, or intended to write.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    But he needs less voltage, not more. He already has it set at minimum.

    You appear to be getting only about 0.6V from the generator. The simplest fix is probably to connect a 4.7k resistor in series with a 10k pot, wired as a rheostat, between REF OUT (pin 7) and RHI (pin 6).

    EDIT: I should make it clear that this does not increase the input signal; it reduces the voltage spacing between comparator thresholds.
     
    Last edited: Jan 30, 2011
  5. Audioguru

    New Member

    Dec 20, 2007
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    "R2 is removed and bypassed to ground"???
    Then pin 8 has a capacitor to ground instead of being connected to ground?

    If pin 8 is connected to ground then the 10th LED lights when the input is at +1.25V.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I thought he used "bypassed" to mean "shorted". You and I know what bypassed means, in electronic parlance. The OP probably does not.
     
  7. laurich

    Thread Starter New Member

    Jan 22, 2011
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    Ron H, thank you that did the trick. I was able to adjust my voltage for 0.65 as max and it works great. Guys I,m a 55 yo nurse who works alot in a busy ER and I haven't been able to find the time to acquire and learn one of those schematic programs. I did work in the electronic field while in the USAF but that was in 1979 and most of our equipment used vacuum tubes. Much has changed since then. My 6th grader is doing a science project with magnets and we thought it would be nice to use this LED display to show the results. I realize the work you do here is free of charge and I really do appreciate it.
    Thanks
     
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Excellent! I think it's great that you went to so much effort to help your daughter with her science project.
     
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