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From what's been posted so far, here are what I think the answers are
to your questions:
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Shouldn't matter.
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Doesn't matter much.
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Doesn't matter much, since if it doesn't it's pretty much a no-brainer
to run it through a comparator and clamp it to the rails.
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He specified that he wants, as a trigger for the toggle, a pair of
knocks to occur within one second, so I'm interpreting that to mean
from the leading edge of the first clump of edges to the leading edge
of the second clump. That can be done by starting a non-retriggerable
one-shot with a period of 1s when the first edge is detected and
starting a retriggerable one-shot with a period of, say, 250ms at the
same time, Then using the output of the 1s one-shot to allow the
second high-going edge out of the 250ms one-shot (if it happens
within the 1s window) to toggle the eventual output. If the second
edge doesn't happen in time, then no toggle and everything goes back
to square one.
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Yes, since the likelihood of a pair of random occurrences meeting the
triggering criteria will be slight. But, if it's important, it's easy
to put in go no-go time windows to discriminate between too short and
too long knock envelopes.
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If the first two meet the triggering criteria the toggle will occur
and the third will be ignored, since he specified a two second inhibit
between knock pairs.
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After the learning curve, the acquisition of a programmer, and all the
rest of baggage that goes along with it, the simplest approach by far
would be to use a microcontroller. But, there _are_ all those snags,
so a couple of dual one-shots and a dual "D" flop is probably the
least painful way to get it done for a one-off.
I made a few errors in the circuit I posted earlier and I still need
to get the 2-second holdoff in there, so as soon as I get that done
(it won't be yesterday
I'll post it to abse and we'll see what
happens.