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4538 monostable input rise/fall restriction

Discussion in 'Electronic Basics' started by Pimpom, Jan 25, 2013.

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  1. Pimpom

    Pimpom Guest

    Does the 4538 monostable have any restriction on the
    rise/fall time of the triggering pulse? None of the
    datasheets I have mentions any such restriction. The block
    logic diagram shows the trigger input as an OR gate with an
    inverter at one of the OR inputs.

    Why I'd like to know: I want to trigger one of the two
    monostables in the package with a slight time delay after
    the other one. The first mono has to be still in the
    triggered state (Q output still high) when the second one is
    triggered. I can think of some ways to do this using other
    logic gates or another monostable. But I'd like to avoid
    adding another active device if possible. The delay is not
    critical - anything from 1 to several milliseconds will do.
    So I wondered if a simple RC filter between the output of
    the first mono and the input of the second will work.
     
  2. Pimpom

    Pimpom Guest

    "Jim Thompson"
    Oops! I had another look at the datasheets by TI and RCA
    (really the same) and they say that the inputs have Schmitt
    triggers. Don't know how I missed that. Thanks for pointing
    it out. NatSemi and Fairchild don't mention it.
    Question answered above^. Anyway, what I want to do is this:
    One monostable is triggered first to produce a pulse of ~50
    msec. The second mono is triggered a little later while the
    first one's Q output is still high, that is, before it has
    reverted to the idle state. It doesn't matter if the second
    mono is triggered by the first or they are triggered
    separately, as long as they are in the proper sequence. The
    two outputs go to different sections of the overall circuit.
     
  3. Pimpom

    Pimpom Guest

    Probably a manual switch - a momentary tactile type. The
    design is still in the early stages and many factors are not
    yet clearly defined.
     
  4. Pimpom

    Pimpom Guest

    Yes, that's exactly what I had in mind. What I didn't know
    when I made my opening post (because I missed seeing it in
    the datasheet) was whether the mono trigger inputs have a
    Schmitt trigger that accepts slow rise times.
     
  5. Pimpom

    Pimpom Guest

    The first monostable *was* intended to set up a circuit
    before and during application of the pulse from the second
    mono. But I've changed the design of the controlled circuit
    so that, although the mono pair will still be used, they now
    work independently and sequential triggering is no longer
    needed. What I didn't mention before was that the second
    mono was to have the option of either manual direct
    triggering or being triggered by the first mono.
     
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