Digital Control LM3914

Discussion in 'General Electronics Chat' started by alec, Jan 3, 2009.

  1. alec

    Thread Starter Member

    Jan 3, 2009
    10
    0
    Hey,
    How can I control a 10 LED bar graph display digitally without using 10 outputs on the micro? There is always the option of using a 2x8bit shift registers or a LM3914 with a digital to analogue converter as the input to the LM..

    Any ideas....
    :confused:

    Ragards,
    Alec
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    You could use a r/2r resistor network on 4 outputs.( 16 values )
    This connected to the lm 3914 will give you your leds.
    If you use 5 outputs ( 32 values ), you could even have 3 LM 3914's.

    Greetings,
    Bertus
     
  3. alec

    Thread Starter Member

    Jan 3, 2009
    10
    0
    what i'd like to do is control the volume or gain of an amplifier with a digital potentiometer, as the gain increases i want the led's on the bar graph display to light up and vise versa if the gain decreases...

    i thought of writing a value to the bar graph display almost like with a 7-segment display and a 7447... but where would you find a similar decoder?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you're going to control the gain of an audio amplifier, you should use an audio taper pot; and in that case an LM3916 (VU meter LED dot/bar driver) would be your best bet.
     
    Last edited: Jan 3, 2009
  5. alec

    Thread Starter Member

    Jan 3, 2009
    10
    0
    Not sure about how to connect the LM and the pot... Do I connect the output from the pot to change the gain to the input of thre LM as well?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    No, the wiper voltage would go to pin 5 (sig in) of the LM3916 (I typo'ed the part number before; the LM3915 is a logarithmic LED dot/bar driver, you need the VU meter)
     
  7. alec

    Thread Starter Member

    Jan 3, 2009
    10
    0

    voltage to change the gain of the amplifier? or the voltage to increase/decrease the bars on the bar graph display? is the LM3916 a linear bar graph driver? why would that make such a big difference?
    :confused: sorry for all the questions, don't know much about the bar graph drivers and controlling gain of audio amps digitally..
     
    Last edited: Jan 4, 2009
  8. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    If you are going to use a digital pot you can connect directly the output of the pot to the input of the bar driver. If the pot has a linear output use the LM3914 or if the pot has a logarithmic output use the LM3915.
     
  9. alec

    Thread Starter Member

    Jan 3, 2009
    10
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    but will the same pot adjust the gain of the amplifier and move the bars on the bar graph display? iow, please see image attached.
     
  10. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Yes you can do it. If you are going to use a normal pot and want to minimize loading on it you can use a JFET input op amp wired as a voltage follower after the wiper pin of the pot. However, this is not necessary because both the op amp and the input of the bar driver you have on the schematic draw currents in the range of nA (negligible).
     
  11. alec

    Thread Starter Member

    Jan 3, 2009
    10
    0
    thanks a lot!!! i think this will actually work. so one digital pot wil be able to adjust the gain as well as the position on the bar graph display. i just want it to show the position on the poteniometer which will be proportional to the gain... i think...:rolleyes:
     
  12. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Yes and you don't need to use the op amp voltage follower I mentioned before with the digital pot. If you set the reference voltage of the bar driver equal as the maximum voltage of the pot it will work fine.
     
  13. italo

    New Member

    Nov 20, 2005
    205
    1
    On another note some pots are linear some are not. So make sure that you do not compound the the errors
     
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