Missing pulse detector.

Discussion in 'General Electronics Chat' started by UMID Narziyev, Apr 15, 2017.

  1. UMID Narziyev

    Thread Starter New Member

    Mar 17, 2017
    Hi, everyone! I would like to ask the question about missing pulse detector. I understood that it is monostable mode and it detects the change in trigger pulse. But I don't know why the transistor Q1 is used. Could I apply the trigger pulse directly to trigger input without transistor? (The picture is from internet as a reference circuit diagram)
  2. bertus


    Apr 5, 2008

    The transistor is discharging the capacitor before the delay has ended.
    When a pulse is missing the capacitor will not be discarged and a pulse will show on the output of the 555.


    This image is taken from the texas 555 datasheet.

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  3. crutschow


    Mar 14, 2008
    Because of the way the 555 operates internally, you need an added transistor to keep the capacitor at pin 6 & 7 discharged when pulses are present so that the 555 only times out if a pulse isn't present.

    Pulses at the trigger pin do not do that.
    The 555 ignores trigger pulses after the first trigger (output goes high) which allows the capacitor to continue to charge until it times out and the output goes low.

    Thus the circuit won't work as a missing pulse detector without the added transistor.

    Edit: Below is the LTspice simulation of a variation on the circuit, using an NPN transistor that triggers on the leading edge instead of the trailing edge of the pulse.
    (Which may or may not be important to your application).

    Incidentally, Figure 16 in post #2 has an error.
    The capacitor is shown as starting to charge on the falling edge of the input pulse, but it actually starts to charge on the rising edge, since the transistor keeps the capacitor discharged until the input goes high.


    Edit 2: As another option, here's a missing pulse detector circuit using an LM339/393 comparator.
    It has a similar number of parts as the 555 circuit but uses one less active device package.
    Last edited: Apr 15, 2017
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  4. UMID Narziyev

    Thread Starter New Member

    Mar 17, 2017
    thanks for brief information