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Constructing ab' with a NAND

Discussion in 'Electronic Basics' started by Rikard Bosnjakovic, Oct 26, 2005.

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  1. I'm designing a small alarm-circuit for a veroboard and I've run into a
    problem that I hope anyone can help me with.

    The truth table for triggering my alarm is this:

    a b trig alarm
    0 0 0
    0 1 0
    1 0 1 <--
    1 1 0

    That is, ab'.

    In the circuit I'm using a 74LS03 (quad 2-NAND O/C) and up to this moment
    three of the four gates are used. Because of the limitations of the
    project size I cannot add another IC to the board, but there is room for a
    couple of diodes and transistors.

    What I need to do is manually constructing the ab' output. Since I'm
    having a free NAND gate, I can invert either a or b to get 0/0 or 1/1
    respectively, but I'm unable to get any further since I haven't been able
    to figure out what to do next.
  2. Method 1:
    Invert b with a transistor stage to give b', then NAND(a,b') = 0, then
    invert that with another transistor stage to get Out = 1

    Method 2:
    Invert b with NAND gate to give b', then AND(a,b') = 1 = Out with a
    couple of diodes, like this:

    | |
    | | 2k2
    o-------o OUT
    IN a --|<------o
    IN b'--|<------o

    (created by AACircuit v1.28.6 beta 04/19/05
  3. Tim Williams

    Tim Williams Guest

    Use a single transistor as inverter, or use the NAND as an inverter for DTL?

  4. What's DTL?
  5. John Fields

    John Fields Guest

  6. Somehow I cannot follow these links. Doesn't work in either Mozilla's
    news-reader, Outlook express, Gnus or pine.

    Can you name the subject and date of the article?
  7. John Fields

    John Fields Guest

  8. Thank you, that method worked fine.

    I do however have some questions. Using the schematic you posted, if I
    connect a LED between OUT and GND, the LED will light when a and b are
    high (this is expected, since the circuit is an AND).

    Since this AND-gate is controlling a buzzer (alarm), I connected it to a
    transistor and the buzzer. If I dont use the transistor, it won't work
    because of too low current (I think).

    So, I grabbed a BC547 NPN from my box, connected OUT to the base, buzzer
    red wire to +5 and buzzer black wire to the collector. The emitter I sent
    to ground. However, this circuit makes the buzzer buzz all the time, just
    as if there is always a current to the base of the transistor, completely
    regardless of the a and b inputs.

    I'm by all means an utter novice to transistors, but the fact that the LED
    was off when either the a and b inputs to the AND was low made me think
    that there will be absolutely no current to OUT, and therefore the
    transistor should not let current flow through its collector and emitter,
    and therefore the buzzer should not buzz either.

    Obviously, I'm thinking wrong somewhere but I can't figure it out.
  9. Having confirmed last night that my main HD is at risk of failure, my
    priorities have suddenly changed! So I'll have to be brief, and anyway
    I expect you just want to get it working. Wire the buzzer between the
    EMITTER and ground, with collector to +5V.
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