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Power OFF 555 timer trigger?

Discussion in 'General Electronics Discussion' started by Oziam, Jun 9, 2012.

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  1. Oziam


    May 26, 2012

    I have a 12v DC circuit that I want to trigger a 555 or 556 when the power source is turned off!
    Is it possible without using a relay?

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Sure, it's easy. the 555 is triggered by a negative going pulse, so the main thing you need is a common ground and a small capacitor between the 12V rail and the 555's trigger pin.

    The power being turned off will generate the negative going pulse.

    Note that this assumes there are no capacitors in the 12V circuit that will maintain the voltage across the supply (or limit its rate of fall) after the switch is opened.

    This page may help.
  3. Oziam


    May 26, 2012
    Thanks Steve, yes of course, I was trying to re-invent the wheel as usual.

    cheers :)
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