Changing The 7447 Decoded Value To Display Another Number???

Discussion in 'Homework Help' started by samrat, Apr 12, 2008.

  1. samrat

    Thread Starter Active Member

    Apr 2, 2008
    30
    0
    Good day guys.. The 7 segment decoder(74LS47) gets the converts binary numbers to display their decimal value on the 7 segment display(common anode in my case). My question is, when the values for 3 is decoded(a=1,b=1,c=1,d=1,e=0,f=0,g=1), HOW CAN I CONVERT THIS VALUES to read as a 5(a=1,b=0,c=1,d=1,e=0,f=1,g=1) on the 7 seg display? I have tried to implement some logic gates between the decoder and the 7 seg display but to no success. I am dealing with numbers from 1 to 7 only. All I need is for 5 to be shown on the 7 seg everytime the binary value fo 3(011) is inputed into the 7447...I am stuck and this is the last bit of my mini-project that is due tomorrow.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You can't change the internal logic of the LS47. All you can do is to reroute the LS47's output to the display. Hint: which segment is ON for 3 and OFF for 5, which is OFF for 3 and ON for 5?
     
  3. samrat

    Thread Starter Active Member

    Apr 2, 2008
    30
    0
    Thats exactly what I am trying to do Beenthere. Rerouting the output of the 7447. But I have to be sure that it does not affect the other numbers that are to be displayed(0,1,2,4,5,6 and 7).My problem is that after a whole night of trying to input logic gates between the 7447 and the 7 seg display, I am still stuck and I need to know how it is done. This is all part of a syncronous counter design mini proect. I tried putting a 3 input NAND gate throught the input connected to segment b with the inputs of the NAND gate been the inputs to segment a,c and g. This was to turn the LED at segment b off when the number 3 was to be displayed but it didn`t work because segment b remained off even when when other numbers were displayed.. All I wanted to do was turn segment b off and segment f on so that 5 is displayed when the count comes to 3 from the syncronous counter. I do need to be shown the exact way of going about doing this Beenthere.. thanx .
     
  4. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
    2
    When your BCD input is LLHH you need to set BI to float the outputs on the chip and turn the right segments on with some other circuit. I think when BI is set to turn off the outputs that means they are floating so you can control them with some other device.
     
  5. samrat

    Thread Starter Active Member

    Apr 2, 2008
    30
    0
    thanks mrmeval,that opened a little door. A step in the right direction but I am trying to implement a circuit to cater for all the numbers but to no avail. Any hints as to what gates should I connect at the wires going to the 7 seg display on segments b and e???My knowledge is quiet fresh.
     
  6. WayneTho

    Member

    May 29, 2009
    12
    0
    To accomplish this I connected a 2-input AND gate to bcd lines a & b (1 and 2), and a 2-input NOR gate to lines c & d (4 & 8), then sent those outputs to a 2-input AND gate to get a "3=True" signal. I then took the b and c lines and ran each one through its own 2-input XOR gate with the 3=True signal as the second input on both XOR gates. I sent the outputs of these gates to their respective inputs on the 7447 chip. Now the BCD input to the 7447 will have a "0101" (5) anytime the actual BCD number is "0011" (3).

    The additional logic tapped bits a & b and were inserted between the BCD source and 7447 on bits b & c so only the results of the circuit go into the decoder.

    I tried this in a logic trainer program and it works. By the way, I am a novice at Logic circuits and haven't taken any classes on the subject. I wonder if there was an easier way to accomplish this, as this solution involved three extra ICs.
     
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