On Fri, 4 Jul 2008, Bob Monsen wrote:
> This is a nice way to do it, which I learned here from a post by John
Fields
> a couple of years ago. (Yet another example of what a nice resource this
> group is). I'm not sure if it was his idea, or if he learned it from
somebody
> else. I haven't seen this idea in any of the data sheets I've looked at.
>
If you're talking about the resistor from the output to pins 6 & 2, that
was in "Electronics" over thirty years ago, albeit with a pullup resistor
on the output since it was the original 555 at the time. It was either
in "Designer's Notebook" or "Engineer's Casebook", I guess the former
is most likely.
It's included in Walter Jung's "IC Timer Cookbook" (which is a lot
easier to dig out than the original article) and credits it to the
June 21, 1973 issue of "Electronics". It does not bother with
the pull-up resistor, though it then suggests it as an improved
version of the simple circuit.
When I saw it, I thought it was great, no fussing with two timing
resistors, and no worry that the pulse width would get too narrow
over a wide range. So any time I need a non-specific clock, I use
the circuit, without the pullup reisstor. It works fine, it gives
far less variation in output pulse width than the standard circuit
with the two timing resistors, that require you to fuss with the
resistors to keep the pulse width reasonably constant.
For a lot of breadboarding, and even finished circuits, "close
enough" is good enough, so you don't need the pullup resistor
on the output, or a CMOS 555.
Michael
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