In Reply to: Re: One-Shot Pulse (about 5 min length) posted by N. Stevens on 10/21/01 at 8:00 PM:
Although I have no doubt that the 555 approach has been done, is being done and will be done in the future, I do not recommend it. If the timing resistor is 100 Meg, then the leakage resistance of the timing capacitor must be in the thousands of Meg, a doable thing, but too easily corruptable.
Another problem is noise susceptibility as the timing interval nears its completion. The RC voltage ends up sneaking up on a threshold voltage which can be shifted ever so slightly by noise, resulting in a substantial timing uncertancy.
To my way of thinking, multiple minutes of timing accomplished by analog means is not a good design practice, not when digital counters can be so easily provided.
Still, Teddy, the choice is up to you. I imagine that it could be done as Ned suggests despite my misgivings.
Ned, thank you for your thoughts.
John Dunn - President