ACTIVE LOW TRIGGER Circuit
This circuit sits LOW and the
output goes HIGH when the push-button is pressed. When the
normally-closed push button is pressed, it opens and the
uncharged 1u will be pulled to nearly 0v rail via the 10k and this will
take pin 2 LOW to make output pin 3 HIGH for the duration determined
by the 22u and 100k. If the push-switch stays open, the 1u will
charge via the 100k and eventually the output of the 555 will go
low.
But normally the switch must be pressed for a short period of time
so that the timing components (100k and 22u) make the output go
HIGH for a short period of time. This circuit is called an ACTIVE
LOW TRIGGER
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