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Problems with 7400 series floating high

Discussion in 'Electronic Basics' started by [email protected], Feb 11, 2007.

  1. Guest

    Hi, I'm quite new to electronics, in the penultimate year at school,
    quite good with computers and wanting to teach myself electronics. I
    have made a binary adder circuit on Crocodile Clips which works. I'm
    now trying to put it together on a breadboard. Unfortunately the fact
    that the 7400 series chips float at a high voltage is causing me some
    problems.

    Here is a picture of what I have so far:
    http://heldtogether.co.uk/adder.png
    As you can see if both switches are off the 'carry' LED lights, which
    is clearly wrong, if one is on, the correct LED lights, and if both
    switches are on neither LED lights. It has crossed my mind to put in
    an inverter, but it seems awfully wasteful, and inelegant!

    Could someone help me to sort this out, and continue my studies?!?

    Thanks
    Josh
     
  2. The inputs of all TTL families of logic act as logic highs
    unless some external device sinks the internally supplied
    bias current.
    So the only thing I see wrong with your circuit is that you
    are assuming that an open grounding switch should produce a
    logic zero, but it actually produces a logic 1. With both
    switches off, you have two logic 1s going into the adder,
    and the result should be zero, carry the 1. If either
    switch is on (producing a logic 0 into the adder, the result
    is 1, with no carry. If both switches are on, you have a
    pair of 0s into the adder, and the result should be 0, with
    no carry.
     
  3. Guest

    Ah yes, of course, I was just looking at it in reverse, seeing an on
    switch instead of an off switch.
    Thank you!
     
  4. Mark Jerde

    Mark Jerde Guest

  5. Guest

    It's a screen shot of a java applet from the University of York,
    http://www.cs.york.ac.uk/netpro/bboard/. It simulates a breadboard
    with various chips, seems fairly well equipped with the basics. Gives
    you the ability to test things, really good actually, hope it comes in
    handy!

    Josh
     
  6. Mark Jerde

    Mark Jerde Guest

    Bookmarked. Thanks!

    -- Mark
     
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