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Diode-Resistor AND gate question

Discussion in 'Electronic Basics' started by Christopher Collins, Sep 20, 2004.

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  1. An AND gate can be made of two diodes and a resistor:

    A -|<|--+---- OUT
    B -|<|--+

    Here, R is a resistor and the |<| thingies are small diodes.

    As long as either of the inputs A and B is connected to ground
    (logical 0),
    the output will be a 0 as well (not exactly zero volts, since there's
    voltage drop over the diode). If both inputs are open, or connected to
    logical 1, the output will be a 1 as well. Hence, the output is the
    and of the inputs.

    My question: What's the point of the resistor here? It seems to me
    this could work without it.

  2. Brian

    Brian Guest

    Without the resistor, when both diodes go to a logical "1", there would be
    nothing to pull the output to a logical "1" (or +5 volts in this case).
  3. Hal Murray

    Hal Murray Guest

    Without the resistor, how much current would you get through
    one of the diodes when the inputis at 0V?
  4. Rich Grise

    Rich Grise Guest

    That's simple. All of it. ;-)
  5. The best way for you to understand the function of the resistor is to
    add a model of the load of the logical inputs that this and gate

    If they are cmos logic, they look something like capacitance ot the
    two supply rails. In that case, what is the source of charging
    current to pump these capacitors positive when the diodes are reverse

    If the load is TTL, each load gate contains some internal pull up
    resistance since the inputs are current operated (the input is a logic
    zero when you pull current out pf the input till the input voltage
    falls below about 1.2 volts). In that case, the diode pull up
    resistance may be redundant.

    So the answer is, it depends...
  6. We used to call this MML or Mickey Mouse Logic. Handy for CMOS (no
    loading) and slow signals like door open switches.
  7. Yeah. Besides, how didja come up with this "AND Gate" stuff? ;-)
  8. Didja hear that Michael Eisner is going to be leaving Disney?

    Because it's a Mickey Mouse Operation. ;-)
  9. Why did you give us this url ?

    To show a totally faulty way of doing it?

    Look at the first circuit on that web page.
    If both inputs are high there is no problem, the output is high, it is
    directly connected to the plus 6 Volt power supply.

    If input A goes low, what happens?

    Unless it can take out the power supply and kill it, nothing happens to
    the output. It is still directly connected to the plus 6 Volt power

    If the input to A is made up of a switch to ground it will burn the
    diode, or if a very sturdy diode is used, the power supply will burn.
  10. Clarence

    Clarence Guest

    The link shows an "OR" gate. There seems to be a disconnect in the definition.

    It is possible to have an "AND" gate or an "OR" gate using the same components.

    There is also the possibility that there may be some confusion in the
    definition of a 1 and 0. Let us assume a 1 = positive voltage above 1V. 0 =
    less than 1V.

    An "AND" for positive logic has a pull up to VCC and the cathodes of the
    diodes connected so that pulling the anode of either diode to a "LOW" (GND by
    definition) will give one diode drop (nominal 0.7V) at the output. GND +
    (nominal 0.7V)

    AN "OR" for positive logic (as shown in the link,) has the resistor pulling
    down, and the diodes connected with the cathodes common to the resistor.
    Pulling the Anode of either diode to VCC results in an output of VCC - (Nominal

    Is that any clearer?
  11. No, it doesn't. If anything it is an always gate, the output is always
    high because it is directly connected to the plus 6 Volt supply.

    How could it ever be anything but high, unless the power supply is turned
    off, or breaks down?
    No. And I can't believe that you are trying to defend that circuit as
    anything but a big mistake.

    Don't you see that the output is directly connected to plus 6 Volt?

    Compare how the connection to plus 6 Volt is drawn in the the following
    circuits. The connection is drawn exactly alike in the first circuit, so
    there can be no doubt that the output is connected to plus 6 Volt.
  12. Clarence

    Clarence Guest

    Ah-ha! I see what you mean. I thought it referred to the design maximum
    voltage. But IF it is actually a power supply (battery?) it is really wired
    wrong. But since it was an educational web site, perhaps the questions should
    be "WHAT is WRONG with this picture?"
    This web page is obviously not intended to show examples of faulty
    circuits, so it is simply a mistake.
  14. Clarence

    Clarence Guest

  15. Maybe just a bit too fast, from being wrong to being subtle.

    Remember that we are writing in an international newsgroup, where you
    cannot be too subtle without a big risk of being misunderstood. So we
    need to establish what is right and wrong clearly first, and then you can
    be subtle as much as you like.
  16. Clarence

    Clarence Guest

    I was not wrong, the web site was!
    Remember the diagram was NOT mine.
    I did not refer to it other than to say it was connected as an "OR" which is
    It has as you pointed out, a fatal flaw.
    What I wrote was entirely correct.

    Don't blame me if you didn't understand.
    English IS the common language here.
  17. Rich Grise

    Rich Grise Guest

    I heard that Mickey wants to divorce Minnie on the grounds of
    insanity because she's fucking Goofy.

  18. Owww, you get a howler for that one!! </Harry Potter>
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