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12vdc Pulse/trigger circuit

Discussion in 'General Electronics Discussion' started by omega1, Apr 6, 2012.

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  1. omega1


    Apr 6, 2012

    New here and looking for some ssistance in building a very basic circuit (but failing on my own, clearly!)

    I need a circuit that, when applied a voltage (12vdc) to the input will pulse an output for approx 1 second and then stop/discharge despite there still being a 12vdc voltage applied to the input.

    So, on 12vdc input (high), a 1 second 12vdc output (high) is present and then goes low, despite the input still being present. When input signal disappears, the circuit should go back to its normal state (low) and await another 12vdc input.

    I have been able to do this with a 555 but with my knowledge but cant get the output to go low while there is still an input present, so it doesn't work for me.

    I hope I have explained this OK.

    Thanks in advance for your help.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    I don't know what your circuit looks like, but you may be almost there. All that may be required is as RC differentiator at the trigger input. Thus the 555 will only see one trigger impulse and will not be affected by the voltage being present for longer than the monoflop's time.
    You can read up on this here:

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