Basic logic gate troubleshooting

Discussion in 'General Electronics Chat' started by chiman777, Feb 7, 2013.

  1. chiman777

    Thread Starter New Member

    Jan 26, 2013
    9
    0
    Hi,
    I'm working on the basic logic gate troubleshooting worksheet, and I'm just having some trouble understanding the pull-down resistors in questions 18-21. I understand how they work regularly, but I'm not understanding what happens when they have an open. Correct me if I'm wrong, but usually with opens you'll get the voltage across that open but not the current, so I'm just not sure whether or not in these cases whether or not the logic would be a 1 or a 0 going towards the logic gates. So yeah, I'm just a little bit confused.
     
  2. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    This is the normal application of a pull-up/down resistor-to force the logic to one state while not being driven. In the case of #18, the pull-down resistor ensures the input to the NAND gate is pulled low(input = 0). When the switch is connected, the input becomes a logic 1, as it is connected directly to Vdd...

    ...also, include a link if you are referencing something on the internet.. I don't want to have to search for the thing you have open in another tab....
     
  3. chiman777

    Thread Starter New Member

    Jan 26, 2013
    9
    0
    Ok, I actually understand R5 in question 18, its R2 and R4 that I'm a little bit confused about. But I suppose they wouldn't really be considered pull-down resistors would they? Since R1 and R3 are resistors themselves. So in that case say R4 was open, yeah I'm looking at the circuit and I'm still a little confused because I would have said in that case that the logic coming out of the comparator would be a logic 0 but apparently it's a logic 1...Yeah, haven't dealt with many circuits in a while so I kind of need a refresher lol.
    Sorry about the link too... I'll know for next time.
     
  4. tshuck

    Well-Known Member

    Oct 18, 2012
    3,531
    675
    Those are just to create a voltage divider... without them, the comparator inputs are connected to ground. As the resistance of the sensors change, the current in that branch changes, changing the voltage dropped across those bottom resistors.

    ...and no, those aren't pull-down resistors...probably could call them bias resistors...
     
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