Auto Triggering of Timer

[abuhafss]

Originally Posted by Bernard

I would also add 1k resistors between pot and pins 7 to prevent accidental connecting Vcc to pins 7 via pot setting. Looks like all 555s should come up reset on power-up.

I am sorry but, what is POT?

By the way, one friend has advised me that "15 second period generated with a 555 is not likely to be very stable" so I should go for ICM7242 or for some PIC. What do you say?

[Bernard]

POT= potentiometer = variable resistor, or your 1M with arrow pointing at it. My one finger typing tends to make mistakes on long words, so ' tend to avoid them where possible. What are your timing accuracy requirements? Persional openion: 15 sec with 555 good for 1% or better. If far greater accuracy is needed, use a master clock, line frequency [ 50 or 60 Hz divided down to 5 sec] or crystal controlled clock driving a counter like 4017 & using gating to derive time periods. Or use 555 divided down, where a smaller cap with low temperature coeficient can be used.

[abuhafss]

Originally Posted by Bernard

POT= potentiometer = variable resistor, or your 1M with arrow pointing at it. My one finger typing tends to make mistakes on long words, so ' tend to avoid them where possible. What are your timing accuracy requirements? Persional openion: 15 sec with 555 good for 1% or better. If far greater accuracy is needed, use a master clock, line frequency [ 50 or 60 Hz divided down to 5 sec] or crystal controlled clock driving a counter like 4017 & using gating to derive time periods. Or use 555 divided down, where a smaller cap with low temperature coeficient can be used.

Thanks for the details. I got it.
The time accuracy is not so important here, 1-5% tolerance will be no problem.