triggering 555 in a different way

Discussion in 'The Projects Forum' started by lowrise4, Jun 20, 2014.

  1. lowrise4

    Thread Starter Active Member

    Sep 6, 2010
    I have a project where I would like to have a 555 emit a single, short pulse - but not by the usual way of connecting the trigger pin to ground via a momentary switch. I would like the 555 to emit a single pulse when it is powered up (voltage applied to pins 1 and 8). I have a 2.2K pullup resistor on the trigger pin, as usual.

    Simply keeping the trigger pin always connected to ground doesn't work...the output stays on constantly.

    I thought I could connect the trigger pin to ground via a small value capacitor. I thought this would provide a temporary low voltage to the trigger when power is applied. It seems to sometimes work, sometimes not. Even when it does sometimes work, if I cut power to the 555 and re-apply power again, it doesn't work until some time has passed, if at all.

    P.S. the reason I'm trying to accomplish this is because I'm doing a project in Make: Electronics and it suggests a modification to a completed circuit and I think this is the way to accomplish this
  2. BobTPH

    Senior Member

    Jun 5, 2013
    Add a pullup resistor and it will work. Calculate the time constant (RC) to be well below the time of the desired output pulse.

  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    use a 1meg pull up and a 1uF cap on pin 2 to ground, this will give you a 1 second pulse on pin 2 to trigger the ic,if you need less trigger time reduce the pull up resistor to 100 or 10k.
  4. Alec_t


    Sep 17, 2013
    Adding a reverse-biased diode across the pull-up resistor will ensure the cap discharges at power-down, ready for the next power-up.
  5. Bernard


    Aug 7, 2008
    Not tried, but after you have a workable trigger that may be longer than desired output pulse, add a diode, anode to pin 3, cathode to small R, say 1 k to pin 2. On powerup as soon as pin 3 goes high, the hold down C is rapidly charged, preventing interuption of normal output pulse.
  6. eetech00

    Senior Member

    Jun 8, 2013
    Hi :)

    See attached...

    I've used this circuit for a lamp POST. It provides an output for 2 seconds after power "on". You'll need to modify the RC values for the pulse duration you need.

  7. lowrise4

    Thread Starter Active Member

    Sep 6, 2010
    Hi everyone,
    Thanks for all the help and sorry for taking so long to reply...I just got around to trying the suggestions.

    eetech, I tried out the values you used in your circuit and it works perfectly. I'm also surprised that it works again immediately after power is re-applied. I thought I might need a reversed diode as was suggested earlier, to discharge the cap.