Connect with us

LM3914 LED dot display

Discussion in 'LEDs and Optoelectronics' started by Old Grey, Mar 23, 2016.

Scroll to continue with content
  1. Old Grey

    Old Grey

    26
    2
    Feb 5, 2016
    I have this 3 - LM3914 LED display that works fine in Bar mode, that I want to change to Dot mode.
    [​IMG]

    The problem is that in Dot mode when the voltage passes the first 10 LEDs it goes blank, because I'm not using the middle 10 LEDs, before it continues to the last 10 LEDs, thereby not displaying whether it is reading a middle voltage, or off the ends of the display.

    How do I wire it so that all the middle 10 outputs from the LM3914 only lights up one LED.
    Basically the first 10 LM3914 outputs light-up the first 10 LEDs, all the middle 10 outputs light-up 1 middle LED, and the last 10 outputs light-up the end 10 LEDs.
     
  2. AnalogKid

    AnalogKid

    2,386
    663
    Jun 10, 2015
    With 10 diodes to a single LED.

    ak
     
  3. Old Grey

    Old Grey

    26
    2
    Feb 5, 2016
    Had time to think about it during that boring Batman v Superman: Dawn of Justice I saw today.
    It seams that the LEDs ground through the LM3914 when a stepped voltage threshold is reached by some sort of logic thingy, so can I get away with just joining the 10 pins together and attaching it to the neg led on the LED.
     
  4. AnalogKid

    AnalogKid

    2,386
    663
    Jun 10, 2015
    It might work in dot mode, but it will not work in bar mode. The outputs are not simple open collector transistors, they are controlled current sinks. That is why I recommend an isolation diode for each output being combined.

    ak
     
  5. Old Grey

    Old Grey

    26
    2
    Feb 5, 2016
    I only need Dot mode, so I will give it a crack.

    Thanks
     
  6. Old Grey

    Old Grey

    26
    2
    Feb 5, 2016
    It seams to work fine just joining all 10 outputs, but now I've got another problem.

    For some reason the 1st and last LED of each LM3914 says lit when the signal voltage is above their threshold.

    It doesn't matter if I do the 10 - 1- 10 configuration, or the full 30 LEDs' configuration, at full scale LED 1, 10, 11, 20, 21, and 30 stay lit.
     
  7. GPG

    GPG

    452
    66
    Sep 18, 2015
    show circuit
     
  8. Old Grey

    Old Grey

    26
    2
    Feb 5, 2016
    I think I worked it out, at least the LEDs' staying on part.
    I was using the Bar circuit when the Dot needs resistors to "+" - think they're called pull-ups -.
    The datasheet isn't entirely right as it only fixes the bright ones and leaves adjacent LEDs half on, but this one is better - the adjacent one is duller but still slightly on -.
    [​IMG]
    Because of the pull-ups I think I have to go the diode route, I just have to work that out - I'm having trouble thinking at the moment -.
    I found some old black diodes FR107, FR103 from an old ATX PSU, and some 1N4007.

    I think I'm eventually going to cut this down to 2 - LM3914s and link the middle 6 LEDs
     
    Last edited: Mar 28, 2016
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Similar Threads
Loading...
Electronics Point Logo
Continue to site
Quote of the day

-