AC line frequency is, depending on the country 50Hz or 60Hz. This frequency has small fluctuations in the short term, but is compensated daily by the power station resulting in a quite accurate frequency source for many timing applications. This circuit samples the line frequency from the secondary of a power transformer ( can be the one of the end application or a dedicated one ). This frequency is then divided by 5 or 6, and subsequently divided by 10 by 2 CD4017B ic´s, providing a clean 1Hz 50% duty cycle square wave.
Schematics:
SimpleTronic
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