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

 

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

Bob

 

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.

 

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

 

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.

 

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

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.

eT

 

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.