help with 7447 decoder

Discussion in 'The Projects Forum' started by idmond, Dec 18, 2010.

  1. idmond

    Thread Starter Member

    Oct 5, 2010
    18
    1
    i was asked to design a circuit to count from 0 to 9 and display the number on 7Seg.
    so i used 74ls93 counter with decoder 7447 to drive the 7Seg. common anode.

    but here's the problem:
    when i checked the output for the 7seg. decoder, i noticed that it outputs
    0.06v as LOW and 0.14v as High.
    my question is how these voltage levels can drive the 7Seg. CA???
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    They can't. The IC is meant to operate with a common anode display. With such a display in circuit and the proper current limiting resistors in the segment lines, the 7447 will work just fine by allowing the proper current through the LED's.
     
    idmond likes this.
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    To further emphasize; you must have a current limiting resistor for EACH of the seven segments.

    The outputs of the 7447 are open-collector. This means that they can sink current, but cannot source current. For testing purposes, you could connect resistors (say, 1k Ohms) from the a through g outputs to +5v; and then measure the voltage on the output pins. Wired in this way, when a segment is supposed to be lit, you'll measure <0.2v on the output pin(s), when it's supposed to be off, you'll measure neary 5v.
     
  4. idmond

    Thread Starter Member

    Oct 5, 2010
    18
    1
    Thanks alot!! ,SgtWookie. you realy made everything clear now.
     
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