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Tri-state with transistor?

Discussion in 'Electronic Basics' started by FyberOptic, Sep 6, 2006.

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  1. FyberOptic

    FyberOptic Guest

    Hiya folks, I'm not up to speed as well as I probably ought to be on
    analog electronics, but I have a particular case where such a thing
    might be simpler to implement than adding in a whole 'nother IC for one
    logic connection, so I thought I'd ask the smart folks to verify
    something for me.

    Basically, I want to be able to toggle between tri-state and ground for
    a single signal. Here's my impression of how this should work, using
    an NPN: I connect my signaling line which will do the toggling to the
    transistor base, then tie the signal I want to be able to go into
    tri-state to the collector, and then tie the emitter to ground. When
    the base goes high, this should make the other signal go low, and when
    the base is low, the other signal would theoretically be tri-state.

    Something I thought that might be an issue, if my above method is even
    correct, is that an IC might not be able to source enough power to
    operate a transistor. Could this be the case?

    The only alternative here is to stick in a tri-state buffer or
    something of that sort, just for this one line I need to toggle. I
    guess if it's absolutely necessary I can, but there's got to be a way
    to tri-state something using more analog methods; I mean, that's what's
    happening at some level inside the IC's, after all!
  2. Randy Day

    Randy Day Guest

    You're using 'tri-state' a bit differently than the industry norm.
    What you're referring to is called 'open-collector'.

    True tri-state devices have the following 3 possible output

    Active-low (logic 0, output pulled to ground - base-high in
    your example)
    Open ( output is open circuit, base-low in your example)

    Active-high(logic 1, output pulled to +V by an upper transistor
    in a 'totem pole' arrangement)
    Replace 'tri-state' with 'open collector' and you're
    absolutely right.
    Not usually. Most IC outputs can provide several mA
    of current; proper selection of your transistor and
    its base resistor will keep you within spec...

    Check the datasheet for the IC you have in mind.
    Yep. There are also 'open'collector' versions of the
    common logic gates available.

  3. FyberOptic

    FyberOptic Guest

    So is this to say that "open-collector" is the same as the
    high-impedance state something like a buffer or latch goes into when
    its output is disabled? From what I gathered from your post, this is
    possibly the case, but I'm wanting to make absolute sure. For the
    signal I have in mind, I'd want to be able to take it from being forced
    'low' (aka tied to ground) to being the equivalent of being
    disconnected (or floating, whatever you want to call it).

    Hmm, would something like an open-collector inverter be able to perform
    the same task I'm referring to? I'd put the toggler line on the input,
    and the togglee on the output, and this would supposedly pull the
    togglee low when the input was high, and let the togglee float when the
    input was low?
  4. jasen

    jasen Guest

    Yup that'd work , you've just "invented" the open-collector output.

    Another trick is to put a diode in series with the ic's output.
    That depends on the IC and the transistor.
    if it's an issue use two (or more) transistors.

    As long as your using small transistors and only switching typical logic
    level signals it will not be an issue.

  5. LOgic families usually can supply and source a voltage or current.
    That means they switch between a high level and a low level.
    "Low" is not an absence of voltage, it is an active positioning of the
    output to ground or close to it.

    You can't connect anything in parallel, because one may try to be high while
    the other gate output is trying to be low (bad for the ICs, and unless
    it's really controlled, you don't want the results).

    But sometimes you want to be able to select between two outputs connected
    to the same point, and devices with tri-state outputs are one solution.
    The in effect disconnect the output of the IC, the point that goes
    between high and low, from the output pin, so the circuitry connected
    to that pin will never know something is connected there. It looks
    like the IC is never actually connected to the circuit. Then you
    can run the other device connected to that point like nothing else
    is there.

    This is the high impedance state, the "tri-state" that is neither high
    nor low.

    An obvious example is a computer data bus, when many devices need
    to output to that bus feeding the CPU. Using tri-state devices
    ensures that only one device is driving the data bus at a time.

    Open collector devices can only provide a low, ie ground or
    close to it. But they require some external voltage source in
    order to operate, without that voltage there is nothing to pull "low",
    even if the transistor could operate without any voltage on the collector.
    ICs that provide open collector devices are usually used to drive
    external circuitry, where levels will not be at the logic level of
    the device. Thus you can provide the needed pull up resistor to
    the +supply, the resistor supplying the "high" to the following circuitry.
    Or, the open collector device is feeding something that doesn't need
    a "high" output, but merely the low. These are things that have
    their other side connectoed to the +supply, such as relays and Nixie
    tubes, and speakers.

  6. FyberOptic

    FyberOptic Guest

    I seem to have a knack for "inventing" things. Maybe next I'll invent
    the telephone.
    Hey that does sound like an interesting trick indeed. I don't think I
    actually have any around, though, but I do have some transistors. I'll
    have to check and see if they're the right kind for the job.

    Something I was wondering about is whether I need to put a resistor
    between the emitter and ground. Or between any of the other transistor
    connections, for that matter. I've never connected an IC's output to
    ground before, and while I think there might be resistance in the
    transistor to prevent a short, I'm not going to go wiring it up and
    take a chance on burning everything out just yet!
  7. FyberOptic

    FyberOptic Guest

    At risk of sounding like an even bigger beginner, I'm afraid I can't
    make heads or tales enough out of the datasheets for the transistor
    type I have and for the IC I want to activate it with.

    My toggler line to the transistor base is coming from a 74LS374 (octal
    flip-flop), and the transistors I have are PN2222A. First, I found the

    ...but as mentioned, I don't understand how to read the charts well
    enough to figure out what volts/amps are output from the one and
    required for input on the other. I was looking in the "On
    Characteristics" of the transistor datasheet, but it says stuff like
    50ma at the base and 500 at the collector, then 2v to the right of
    that. I don't get what that means, exactly, if that's even what I'm
    supposed to be looking at. I looked at the electrical characteristics
    for the '374 as well, but I can't find an output current.

    Something I did notice about the '374 is that if I'm understanding the
    sheet, its active signals are actually around 3.4v, instead of the 5v I
    assumed. I've never really looked at the electrical characteristics
    for IC's before, and while I know 3.4v is enough to be TTL 'high',
    maybe this is just common voltage output for most of'em.

    I also want to say thanks to everyone who's replied so far, it's nice
    to know there's folks who know their stuff out there willing to help!
  8. Randy Day

    Randy Day Guest

    No. A tri-state device can put the output to +V, ground
    or open.

    Our open-collector (transistor) idea has only two
    possible outputs: ground or open.

    Here's a drawing of the output stage of a logic gate,
    buffer or latch; the 'totem-pole' I mentioned:

    +---- output
    (created by AACircuit v1.28 beta 10/06/04

    If the top transistor is turned on, the output is
    connected to Vcc (logic 1). If the bottom transistor
    is turned on, the output connects to ground (logic 0).
    Think of the open-collector circuit as the bottom half
    of a tri-state circuit.

    If *both* transistors are off, you get the 'open'

    If both transistors are ON, you get the 'smoke'
    state. ;)

    See also:

    Here's an interesting application for an open-collector
    gate: As long as the max. voltage and current are low
    enough, a 5v logic gate can control a higher (or lower)
    voltage load.

    Be sure you read the datasheet on the gate before you
    try this, though!

    5v +-+
    | | | load
    |\ +-+
    -| >O--|

    (created by AACircuit v1.28 beta 10/06/04
  9. Winfield

    Winfield Guest

    Below is basically what you need. The 2.7K (to 10K) resistor limits the
    current fed into the base of the transistor to an acceptable level. e.g.
    When the transistor is forward biased, (turned on), the base will be at
    about 0.6V. If the chip's output is 3.4V, then the voltage across the
    resistor will be 3.4-0.6V=2.8V. 2.8V into a 2.7K resistance will be about
    1mA. Anything from about 2.7K to 10K should do the job fine.

    .... Fred

    | +------ 0V / Hi-Z
    +------+ |
    | | |/
    | '374 |---[2.7K to 10K]---| PN2222A
    | | |>
    +------+ |
    | |
    === ===
    = =
  10. Jerry R

    Jerry R Guest

    Be sure to use a resistor in series with the base of your open collector

    Rb = [ V(high) - 0.6v ] / ( Ic/beta )

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