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crutschow

Missing Pulse Detector Circuits Using LM339/393 Comparator or CD4049 Gate

Watchdog timer circuits, alternate to 555 designs, will timeout on either high or low hang

  1. crutschow
    A common missing pulse detector circuit design uses a 555 and a transistor to perform this function.
    As an alternative, here are similar circuits using only a single active device package.
    The total number of parts is about the same but with one less active device package.
    These circuits will time out when the pulses stop, independent of the pulse DC voltage at that point.

    Here is the LTspice simulation of a missing pulse circuit using the LM339/393 comparator IC.

    upload_2017-4-16_11-55-18.png

    U1 discharges C1 at the positive edge of each clock pulse, keeping it below the Ref voltage of U2, so its Out signal remains low.
    When the input pulses stops, C1 will then charge and exceed the Ref voltage after approximately one R1C1 time-constant, at which point Out goes high.
    R1 and C2 can be changed to give the desired delay for Out to go high after the pulses stop.
    Typically this is made somewhat longer than one pulse period.
    The delay for output turn-on is largely insensitive to supply voltage, staying close to one R1C1 time-constant.
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    Below it the LTspice simulation of a missing pulse circuit using a CD4093 CMOS Schmitt trigger NAND gate package.

    upload_2017-4-17_0-42-36.png

    It's operation is similar to the LM339/393 circuit.
    U1 charges C1 through D1 at the falling edge of each clock pulse, keeping its voltage above the logic threshold of U4 and thus the Out voltage low.
    When the input pulses stops, U1's output will go low due to the bias from R2 and R3, causing C1 to discharge and go below U4's threshold voltage after approximately one R1C1 time-constant from the last input pulse trailing edge, at which point Out goes high.
    R1 and C2 can be changed to give the desired delay for Out to go high after the pulses stop.
    Typically this is made somewhat longer than one pulse period.
    The delay for output turn-on is fairly insensitive to supply voltage, staying close to one R1C1 time-constant.
    Its delay is not quite a stable as the LM339/393 circuit from temperature and voltage changes but should be adequate for most applications.

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