Laurence Taylor spake thus:

Dave Plowman (News) wrote:

The trouble with this approach is that it is best for short time delays.
Ten minutes is a relatively long time, so circuit leakage may cause the
delay to be longer than what you would calculate based on the R-C time
constant.

If you use the CMOS version - usually 7555 - the low current allows times
of much more than 10 minutes.

The problem is that you end up needing absurd component values - ten
minutes delay would need 1.2Mohms and 470mf, for example, and the latter
is likely to be inaccurate and probably variable.

You can use two of them (or a 556), or there used to be a chip that
would work up to many hours delay, the number of which I cannot recall;
I think it was a TDA- something.

Why would you want to mess with a 555 at all? Not precise, and more
complicated. I already posted a possible solution using an oscillator
(1MHz) and two 74HCT4040 chips, which will give you an *exact* time and
uses fewer components. (Like no resistors or capacitors, except for
maybe one 0.01 uf bypass cap.)


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