Strange Problem with LM3914

Discussion in 'The Projects Forum' started by PFebby, Feb 19, 2018.

  1. PFebby

    Thread Starter New Member

    Feb 19, 2018
    3
    0
    Hello all,

    I am building a phase correlation meter using an LM3914 to drive an LED bar display. Everything is working correctly besides the LM3914. The LEDs light but they slowly cycle one at a time all the way up and all the way down the bar as if there’s a step time. I have no clue why this is happening or where the step time would be coming from considering the circuit was working completely fine a few days ago. I am using two equal sine waves as the inputs for the phase meter and as increase the frequency of the sine waves the step time of the LEDs seems to increase as well. Why is this happening and how can I fix it?
     
  2. LesJones

    Well-Known Member

    Jan 8, 2017
    2,204
    656
    Are you sure the "two equal sine waves" are realy EXACTLY the same frequency.

    Les.
     
    PFebby and ebeowulf17 like this.
  3. crutschow

    Expert

    Mar 14, 2008
    22,518
    6,598
    Feed the two inputs from the same sinewave.
     
    PFebby and ebeowulf17 like this.
  4. ebeowulf17

    AAC Fanatic!

    Aug 12, 2014
    2,897
    626
    Could you share a schematic of how you've got things right now? Otherwise anything we say will be uninformed guesses.

    You say everything else is working - how do you know? Are you measuring the voltage at the input to the LM3914? Are you sure the LM3914 isn't working properly?

    I see @LesJones posted while I was typing. He raises a good point - maybe the whole circuit is working properly and just showing you an unexpected result!
     
    PFebby likes this.
  5. PFebby

    Thread Starter New Member

    Feb 19, 2018
    3
    0
    Ahhhh interesting this result would actually make some sense if the two are slightly different frequencies. They are from two separate function generators so it's possible. I will look into it and report back.


    This is the design I'm using:
    http://donaupeter.de/audio/Korrelation/Korrelationsgradmesser_schematics.pdf

    However, I used the LM3914 design in the picture attached instead since i had a ten bar LED display and wasn't sure why the 1N4148's were required coming from the 9th LED. Screen Shot 2018-02-17 at 4.46.01 PM.png
     
    Last edited: Feb 19, 2018
  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    9,943
    2,406
    Because for some reason the circuit designer wanted both output 9 and output 10 to drive the 9th LED. The diodes perform an OR function.
     
  7. ebp

    Well-Known Member

    Feb 8, 2018
    2,332
    811
    The diodes just "OR" the top two outputs so the LED is ON if either of them is active. This makes sense only when the circuit is used in dot mode, not bar mode.
     
  8. ebp

    Well-Known Member

    Feb 8, 2018
    2,332
    811
    He probably had a display module with 9 LEDs. When using it in dot mode, all LEDs would be off at full-scale without the ORing diodes.
     
  9. ElectricSpidey

    Active Member

    Dec 2, 2017
    627
    152
    I don't think you even need the diodes, considering the outputs are just open collector.

    I admit I have never tied the outputs of the same 3914 together, but I have tied the outputs of 2 different 3914s together to create a crisscross display.
     
  10. ebp

    Well-Known Member

    Feb 8, 2018
    2,332
    811
    Spidey is quite correct. The outputs are open collector NPNs with emitter resistors to make crude but adequate constant-current sinks.

    I haven't used an LM3914 in many many years. I remember it from when it was a new product. I'm a bit surprised they are still in production. I see TI offers it in SM in a PLCC, which ain't exactly a modern package. That typical circuit drawing sure looks like a copy from a National Semi data sheet from auld lang syne.

    Unless two signal generators are DDS types with decent crystals, the probability that they will deliver exactly the same frequency is pretty much zero. 0.1% difference at 1 kHz is 1 Hz, or 2π radians per second.
     
  11. PFebby

    Thread Starter New Member

    Feb 19, 2018
    3
    0
    I have an update as well as another small problem!

    It turns out LesJones was correct! I monitored the sine waves and they weren't in phase. I adjusted this and the circuit works perfectly! The only problem now is that my indicator is reading the exact opposite of what it should. When the sine waves are completely in phase I have no indication and when the waves are out of phase I have full indication. I thought this might have been occurring earlier on in testing and now I realize that the design has the LEDs placed in the opposite order (my 3914 pin 10 is green and my pin 1 is red. I have a bar LED so i can't simply change the order of LEDs and wiring the opposite way would get messy. How can I invert the signal coming out of the CD4070? Would an XNOR instead of an XOR chip have the result I'm looking for? I guess I don't want to INVERT the signal out of the 4070 but just switch the results of the logic to be opposite. Is this correct?
     
  12. LesJones

    Well-Known Member

    Jan 8, 2017
    2,204
    656
    Try swapping over the inverting and non inverting inputs on ONE of the two comparitors to produce a 180 degree phase shift in one channel.

    Les.
     
    PFebby likes this.
Loading...