7 Segment Display Circuit - Pull Down Resistor

Discussion in 'General Electronics Chat' started by paulwoody, Jul 26, 2008.

  1. paulwoody

    Thread Starter New Member

    Sep 18, 2007
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    Volume 6 Experiments - 7 Segment Displays

    Hi Guys,

    I am slowly educating myself in the fine art of electronics and I would just like to check something concerning the circuit shown below.

    Would I be correct in thinking the four 10k Resistors used in the above circuit are Pull Down Resistors to prevent the signal lines from floating when not connected to the positive supply?

    Finally if I am correct and they are pull down resistors can you confirm that by removing them I will not damage the circuit - obviously I will not link to ground at all because I understand that will cause a short circuit.

    I appreciate I may get noise/stray voltage on the signal lines and maybe bounce from the switch but this is more to confirm that I am understanding the circuit as opposed to actually wanting to remove the connections.

    [​IMG]
     
    Last edited: Jul 26, 2008
  2. lightingman

    Senior Member

    Apr 19, 2007
    374
    22
    Hi,

    Yes you are correct. and if you are going to drive the inputs from anaother device you would not need the resistors.

    Daniel.
     
  3. paulwoody

    Thread Starter New Member

    Sep 18, 2007
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    0
    Many thanks for your assistance Daniel,

    I didn't realise I wouldn't require the Pull Up/Down's if connecting to another device.

    I am looking at controlling them through a PIC controller eventually to display a voltage once I get to grips with multiplexing etc.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The inputs to a CMOS device must never be left "floating"; if they are, they can oscillate unpredictably and cause difficult-to-diagnose problems.

    If your PIC's pins are set to OUTPUT, then they will either be near 5v or near 0v, whichever you've commanded them.

    If your PIC's pins are set to INPUT, they will be floating (high impedance). In that case, you would still need pull-up or pull-down resistors for an external CMOS device that was getting input from that pin of the PIC.
     
  5. paulwoody

    Thread Starter New Member

    Sep 18, 2007
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    Thanks for the advice SgtWookie. I will certainly bear that in mind.
     
  6. paulwoody

    Thread Starter New Member

    Sep 18, 2007
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    Well built the circuit this morning in about 20 minutes but must have spent nearly an hour trying to work out why I was getting erratic results.

    Firstly I had tied EL high holding the Latch on and then because I was being lazy I omitted the Pull Down Resistors.

    It was not obvious to me initially what was wrong, I though it was a bad ground connection. It was weird because when I put my hand near the circuit it seemed to work sometimes.

    Lesson Learnt! Don't skip the Pull Downs - I really had no idea how much of a headache it would cause to the circuit by removing them.

    Well learnt something new and its a lesson I wont forget in a hurry.

    Thanks to LightingMan and SgtWookie for your assistance.
     
  7. Wendy

    Moderator

    Mar 24, 2008
    20,764
    2,534
    Yep, they are what defines the logic 0. Without them the input is indeterminate, neither 1 nor 0.
     
  8. lightingman

    Senior Member

    Apr 19, 2007
    374
    22
    Just a note when using a PIC :-

    The PIC's I/O's will be set to the input state (hi Z) whilst it is booting up before you have set them to outputs in the software. In this case it is a good idea to leave the resistors in, but increase them to 47K.

    This is also the case if driving MOSFET's directly from a PIC.

    Daniel.
     
  9. paulwoody

    Thread Starter New Member

    Sep 18, 2007
    5
    0
    Cheers Guys I really appreciate the advice.
     
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