One-Time Pulse Needed. Need Help!

[micro571]

Hi,

The end result that I need to obtain is get a pulse that will last for 30 seconds and then go off (and not go back on).

I figured that would be easy enough to do with a 555 timer in monostable mode ... but I've run into a problem.

There is no way to get the pulse to fire once on startup. If the trigger pin is held high on power-up, nothing happens, if it is held low, LED stays on forever because the circuit keeps re-triggering itself.

To trigger the 555, it requires a high-low-high pulse. So what I need is a high-low-high pulse on power-up to trigger the 555 once. Does anyone know how I could do that?

Are there any other solutions for getting a 30s pulse on power-up? I can't have push buttons in my circuit, so a traditional trigger won't work.

I've been pulling my hair out for almost a week on this thing. Any help would be greatly appreciated!

Micro571

 

[k7elp60]

To have a 555 monostable generate a pulse on power up all you need to do is put a resistor from pin 2 to Vcc, put a capacitor from pin 2 to ground.
With your requirement of a 30 second pulse I would use a 10k as the resistor and a 1 uf electrolytic with the postive on pin 2.
On power up pin 2 will be at ground, a short time later it will be a Vcc as the capacitor charges. Thus only generating a pulse on power up.

 

[micro571]

I've just tried that and it seems to be working! Thanks a lot!