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Homework Help: 7 segments display difficult problem

  1. Apr 14, 2010 #1
    hi there ...

    I got a question about the 7 segments display .. I need help please because I am lost now ..

    the question is asking to design a combinational logic circuit using AND , OR and NOR gates for two outputs that will display the last two digits of my passport number . (note if the last two digits are same choose the next higher digits ,,

    this was the question ..

    I made the truth table because it's very easy ,, but then what should I do .. the problem is in the question they asked us to use the AND , OR and NOR gates only so how am I going to use it without using any decoder ,, also how to design the circuit so I can enter 4 digits number and display the last two digits ?

    seriously I am confused right now because I am in the 1st sem year 1 and I need any help in that ..

    thanks guys
    Last edited by a moderator: Apr 14, 2010
  2. jcsd
  3. Apr 14, 2010 #2


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    Staff: Mentor

    Welcome to the PF. Show us your truth table. Do you know how to draw a Karnaugh Maps from the truth table? That is the next step, and that will give you equations for your outputs that you can implement with the gates you mentioned.
  4. Apr 14, 2010 #3
    thanks berkeman

    I attached the truth table and karnugh map for the segment a .. but how to group it to find the equation ,,

    Attached Files:

  5. Apr 14, 2010 #4


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    Staff: Mentor

    I'm not sure which segment your K-map is for... which one is it? You need one for every segment that you are going to drive with your logic.

    Also, are your 7-segment displays common anode (CA) or common cathode (CC). It makes a difference in how you drive each segment -- with either a logic high or a logic low.

    After you draw your K-maps for each segment, you use grouping to come up with the simplest sum of products for each output term:


    Then you use AND-OR logic (with NOR gates used for any inversions of terms that you need as inputs) to make each sum of products output term to drive the segments.
  6. Apr 15, 2010 #5
    thanks a lot for ur information but I got two question ..

    when u want to simulate the circuit in multism how to get the common cathode or either the common anode ,, I mean how to calucalte the resistor and power supply ,.,

    as well what is the difference between common cathode and common anode when connect the circuit ?

    thanks again
  7. Apr 15, 2010 #6


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    Staff: Mentor

    You model them as LEDs, which they are. You calculate the series current limiting resistors' value by assuming about a 2V drop across the LED, and dropping the rest of the voltage across the resistor (assuming your drive gate or transistor has negligible voltage drop).

    This page: http://www.hobbyprojects.com/components/7_segment_displays.html
    shows some info about a CA 7-segment display, and discusses the CC option.
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