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Why do you need multivibrators to create a pulse?

  1. Mar 28, 2013 #1
    A mono-stable multivibrator is used to create a pulse in response to an external trigger. Why do you need such a complicated circuit for that?

    Can't we have a simple circuit with resistances and voltage sources. Whenever there is an external trigger something completes the circuit(earlier the circuit was open) and hence current flows through thus creating voltage across the load. Now the circuit automatically opens after sometime(through some mechanism).
    So this way a pulse is created across the load resistance right?
  2. jcsd
  3. Mar 29, 2013 #2


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    Science Advisor

    A mono stable behaves in much the same way as you describe, except that the "something" you suggest is achieved with the mono stable circuit.

    You can get integrated circuits which contain mono stables and reduce the additional components to a minimum.

    A useful IC is the 74C221 or 74HC221. These contain two mono stables and require very few additional components to function.
  4. Mar 29, 2013 #3
    Can't we have a mechanical switch so that it closes when there is an external trigger and then opens after sometime. That's much easier to make rather than all the op-amps, capacitors etc. right?
  5. Mar 29, 2013 #4


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    Science Advisor

    Such arrangements are often used.
    Suppose you have a water tank. You could detect when the water level drops to a certain level then turn on the water supply for some exact time the top up the water level.

    However, using an integrated circuit is no big deal and often the best way to produce a long pulse from a short one or vice versa.
  6. Mar 29, 2013 #5
    I'm still not comfortable. Could you tell me why the integrated circuit is the better way?
  7. Mar 29, 2013 #6


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    Staff: Mentor

    Mechanical contrivances are way too slow, too noisy, and use too much power to provide a general solution for one-shot pulses in electronics. They can be built using a clock spring to furnish the timing element for durations of a few seconds to an hour or so. With the passage of time the spring will stretch a bit, causing the duration to change, and eventually the spring may break. Solid state electronics elements are usually far more reliable (though of course a poorly designed circuit may still fail), and extremely versatile: one good circuit can offer pulses with durations anywhere from microseconds to kiloseconds.
  8. Mar 29, 2013 #7
    Thank you very much. I guess that is what I was looking for.
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