LED Tachometer LM3914

Discussion in 'The Projects Forum' started by LoCkEd31, Apr 27, 2010.

  1. LoCkEd31

    Thread Starter New Member

    Sep 22, 2009
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    I am trying to make an LED tachometer similar to the one on this thread:

    http://forum.allaboutcircuits.com/showthread.php?t=16291

    Mine is on page 13 of the national semiconductor datasheet for the LM3914. I want to use an input voltage from 0-5V, but cannot figure out what resistor values I need to use to do this. Any help is greatly appreciated. Thanks!

    here is the datasheet,
    http://www.national.com/ds/LM/LM3914.pdf

    the Bar Display with Alarm Flasher is the schematic I'm using.
     
    Last edited: Apr 27, 2010
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Someone else had a very similar request about a month ago.

    Here's the modification to the schematic you'll need to make:

    [​IMG]

    But in the original request, the person wanted 4.5v for the high level.

    You'll need to increase the 3.3k resistor to 3.9k; that's a "ballpark" number. If the range isn't high enough, increase the 3.9k resistor slightly until it is. You'll get about 11mA current through the LEDs.

    [eta]
    I still had this circuit on a breadboard, so I slapped in a 3.9k resistor and gave it a spin.
    The actual values for the resistors I used measured 1.43k for the 1.4k, and 3.85k for the 3.9k.
    The 10th LED illuminated and began the flashing display when the SIG input (pin 5) was raised to 4.98v.
     
    Last edited: Apr 27, 2010
  3. LoCkEd31

    Thread Starter New Member

    Sep 22, 2009
    2
    0
    Thanks a lot sgtwookie for your help!! I had to drop the 1.4k to a 1k to get the range but it worked great!

    Thanks again!
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Hmm, interesting you had to change it that much.

    You'll reduce the power dissipation in the LM3914 a good bit by adding 100 Ohm resistors to each LED.
     
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