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Subject: Re: One-Shot Pulse (about 5 min length)

Date: 10/22/01 at 11:27 PM
Posted by: John Dunn - Consultant
E-mail: ambertec@ieee.org
Message Posted:

In Reply to: Re: One-Shot Pulse (about 5 min length) posted by N. Stevens on 10/21/01 at 8:00 PM:

Hi, Ned.

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
Ambertec, Inc.

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