common anode LED display problem

Discussion in 'General Electronics Chat' started by kuxz2008, Nov 21, 2009.

  1. kuxz2008

    Thread Starter Member

    Nov 16, 2009
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    i would like to know why the LED did not have a display like zero?
    is there any mistake in the connection?
     
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  2. Wendy

    Moderator

    Mar 24, 2008
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    The 7447 has a zero suppression feature. I don't have time to study this schematic in depth, but I suspect you have that pin enabled.
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    The problem appears to be pin 4 pulled high. That is an output pin. It may have toasted the IC. Try leaving pin 4 open, and pull pin 3 high instead - that is the lamp test input.

    As with all TTL logic, strapping input directly to Vcc is not a good idea. Using a resistor of about 10 - 20K will hold the level up, and protect the power source if the chip fails.
     
  4. BMorse

    Senior Member

    Sep 26, 2009
    2,675
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    according to the data sheet :

    So change RBI to be tied low and RB0 should be kept tied high for 0 to display....

    My .02
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    I beg to differ. LT\, RBI\, and BI\/RBO\ are all connected correctly - but I suggest that it would be a good idea to use a 330 Ohm to 510 Ohm resistor between them and Vcc. Were they CMOS instead of TTL, I'd suggest 10k to 20k for pull-up resistors.

    There is no path for V1, the 5v supply to connect across J1. You will need to add some temporary jumper wires so that the circuit will be powered. This is the reason there is nothing showing on the display.

    I don't see a Vcc connector on the schematic. Some simulators require the presence of a Vcc to power TTL IC's; the power pins aren't shown.

    Your J2 key won't have much function here, unless the 193 counter happens to power up in a non-zero state.
     
  6. kuxz2008

    Thread Starter Member

    Nov 16, 2009
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    i have connected the BI/RBO to low
    the problem still occur ....
     
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  7. jgessling

    Active Member

    Jul 31, 2009
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    On my breadboard the 7447 has pins 345 disconnected and it counts all the digits. Are you sure your counter is sending zero? I've had some success of diagnosing things like this by jumpering in some LED's to show the logic signals. Like on the ABCD inputs to the 7447. Fun to watch binary counting too.

    Just noticed, the 74ls193 is a binary counter which means it goes 0-15. I think you want a decade counter (0-9). Like a 7490 or 74390 (dual).
     
  8. BMorse

    Senior Member

    Sep 26, 2009
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    I just went and checked my circuit for a 2 digit 7 segment display I made using the 7447, but I used a 74LS90 for the decade counter..... and in my circuit I left pins # 3,4,5 on the 7447 unconnected in my circuit and it displays the leading and trailing zero's....


    Are you just running your circuit in an emulator?? Or do you actually have the circuit built??

    It seems as if you would have to have the data pins on the 74LS193 connected to something in order for it to "display" something, try pulling all four data lines low.....

    My .02

    My .02
     
    Last edited: Nov 22, 2009
  9. lihle

    Active Member

    Apr 12, 2009
    83
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    hey man try to test the driver with the pins A,B,C,D all connected leaving theother pins not connected. Or you can change the driver and use the 7448 its similar but make sure you leave the other pins not connected.

    if this is not working i will show you my design.
     
  10. kuxz2008

    Thread Starter Member

    Nov 16, 2009
    84
    0
    do i need to change the on current and forward voltage drop for the 7 segment LED common anode display?
     
  11. SgtWookie

    Expert

    Jul 17, 2007
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    The defaults should be OK.

    What are they set to currently?
     
  12. kuxz2008

    Thread Starter Member

    Nov 16, 2009
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    0
    on current: 5mA
    forward voltage drop: 1.6v
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    That sounds fine.

    You need to figure out why the 7447 isn't sinking current.

    Try wiring one or more of the segments to ground; see if they light up.
     
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