problem with logic gates

Discussion in 'The Projects Forum' started by whitearrow, Jan 17, 2009.

  1. whitearrow

    Thread Starter Member

    Jan 6, 2009
    13
    0
    i have to build a circuit which has the truth table shown in the attachment. i have built the circuit (shown in the attachement as fig1) and in theory it gave me the wanted results. but when the inputs where given with a comparator circuit(shown as fig2), the values where not aquired. can any one help?
     
    Last edited: Jan 17, 2009
  2. Wendy

    Moderator

    Mar 24, 2008
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    2,534
    What is the goal here? My first suspect is the comparators, and when they are triggering.

    I could open your word document once, but no more. There are lots of versions of word out there, and not all of them are compatible.
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Comparators typically have open collector outputs, so you must use pull-up resistors in order to obtain an output.

    A good rule of thumb for the pull-up resistor value is roughly 3.5ma current across your V+/V- supply, or roughly 250-300 Ohms per volt. Therefore, if you're using +5v and -5v, then it's 10v/0.0035a = 2857 Ohms.
    2.7k and 3k are standard values. Use the closest. I can't see what you're using for V+ and V- supply voltages.
    [eta]
    Attached images of the contents of the Word document.

    OK, now I see you're trying to use a TL074 quad opamp as a comparator. This is a bad idea, as the outputs will be in constant saturation. Also, the common-mode input range does not include the negative rail; more like 3v above it to V+.

    Use an LM339 quad comparator instead of the TL074, and use 1.5k pull-up resistors to +5v. Their common-mode input range includes the negative rail; you can use ground for the negative rail.

    Your V+ should be 4.75v to 5.25v inclusive, as that's the range for TTL logic.
     
    Last edited: Jan 17, 2009
  4. whitearrow

    Thread Starter Member

    Jan 6, 2009
    13
    0
    thanks for that useful information. can you tell me between which pins i have to connect these pull up resistors?

    thanks again
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Where is the circuit? Do you drive the gate inputs with a comparator? What logic family is it (TTL or CMOS)?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    On an LM339, the output pins are 2, 1, 14, 13 for A, B, C, and D respectively.
     
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