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.
--
rgds
LAurence
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