LM3914 Range Settings... Help

Discussion in 'The Projects Forum' started by WonHungLow, Feb 18, 2014.

  1. WonHungLow

    Thread Starter New Member

    Jun 29, 2009
    9
    0
    [​IMG]
    im currently using the above schematic...
    my question is how can i set the upper limit to light up all LED's @ ~8.4V
    and set the lower limit to cut off all LED's @ ~6.0V
    i think i will be ok with the 10k pot for the upper limit but the lower limit is confusing to me. now it has to reach 0V to turn off all led (pin 4 to ground)
    am i correct that i would need a voltage divider and the VOUT goes to pin4 set at ~6.0V
    [​IMG]
    since input is 12v i could use 2x 2.2kohm resistors (or any 2 of the same value)
    is this correct or am i missing something else.
    thanks guys
     
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    2,498
    507
    Look at the attached schem. It shows how the internal resistive divider is floating and you can reference the top and bottom of the string to any voltage you want. You just set up resistive dividers to create the voltage that corresponds to whatever level you want the first and last LED to be.
     
  3. bertus

    Administrator

    Apr 5, 2008
    15,650
    2,348
    Hello,

    Have a look at the internal diagram of the LM3914:

    [​IMG]

    You can see that the total resistance between pin 4 and 6 is about 10 K.
    You can set the lower limit by putting a potentiometer on pin 4 to ground.
    To have about 6 Volts min. a value of about 25 K is needed.

    Bertus
     
  4. WonHungLow

    Thread Starter New Member

    Jun 29, 2009
    9
    0
    ok here is what i did. i added a 100k Ω pot (pin1=Vcc; pin2=pin4 on LM3914; pin3=GND)

    i then set my bench power supply to 6v and adjusted the pot till all LED's turned off

    then i increased the V on the bench power supply to my upper limit of 8.4V (2x 18650 Li-Ion batteries in series)
    then set the other 10k Ω pot so that all LED's light up.

    tested with adjusting the bench power supply and it works like a charm..

    thanks alot. much appreciated...
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,650
    2,348
    Hello,

    The way you connected the potmeter on pin 4, will make the range not linear, as the voltage on pin 4 will change with the battery voltage.
    Better put pin 1 and 2 of the potmeter together on pin 4 of the LM3914 and pin 3 of the potmeter to ground.

    Bertus
     
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