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LED Indicator Range

Discussion in 'LEDs and Optoelectronics' started by heberjm8, Oct 29, 2010.

  1. heberjm8

    heberjm8

    13
    0
    Oct 11, 2010
    Hello. I've been working on a battery charge status light bar circuit. We purchased an LM3914 chip and have built the circuit but we've encountered a problem. The LM3914 only accepts reference voltages ranging from .125v to 1.25v. To elaborate, the LED connected through pin 1 turns on when the voltage reference is above 1.25v (up to it its max voltage). The LED connected through pin 18 (second LED in the sequence) turns on when the voltage reference is below 1.25v and equal to or above 1.125v. The LED connected through pin 17 (third LED in sequence) turns on when the voltage reference is below 1.125v and equal to or above 1v. The following 7 LEDs turn on for each increment falling within the range of its previous condition and - another 0.125. An equation illustrating this looks like the following:

    n=number of LED in sequence (pins 1, 18, 17, 16, 15, 14, 13, 12, 11, 10 - in that order)

    Minimum Voltage Reference=1.25-(n-1)*0.125

    The problem we are having is that the range we wish to use to illuminate our 10 LEDs must fall between around 10v to exactly 12.7v. The reason 12.7v is an essential value corresponding to an LED is because 12.7 represents our batteries charge status of 100%. Above this value is dangerous for the battery and below this value is less than fully charged. Ideally, we would use a range using only 1 LED below 11.89v and the remaining 9 LEDs between the range of 11.89 and 12.7v (to indicate the various charge levels of the battery). We don't need the percentages to be as neat as 0%,10%,20%,30%, etc... as long as we have at least 9 of our LEDs corresponding to percentages falling between the range of 11.89v and 12.7v.

    So we've been trying to figure out a way using voltage divider to construct a "level shifting" circuit to convert our raw reference voltages which will always range between 11.89v and 12.7v after steady state no load voltage has been reached to a range from 0.125v to 1.25v so that our LM3914 can be used. I believe we need to do this in a linear, 10 step fashion, but I'm not sure how to make it work without using additional chips to over complicate our light bar circuit. One idea was to use three quad comparator chips to use logic so that the voltage divider used will be dependent on the raw reference voltage, but this would require additional voltage supplies and I'd like to avoid that if possible. Simplicity is my aim. Is anyone able to help me find a solution to our little problem? As always, your help is highly appreciated. Thank you, gentlemen.

    Additional Potentially Helpful Information:

    LM3914 Data Sheet:
    LM3914.pdf

    Battery Charge Levels:
    0%=11.89v
    25%=12.06v
    50%=12.24v
    75%=12.45v
    100%=12.70v
     
    Last edited: Oct 29, 2010
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,178
    2,690
    Jan 21, 2010
    Have you read the datasheet?

    Did you get as far as page 11?
     
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