[time-nuts] Simple AC mains zero-cross detector

Charles Steinmetz csteinmetz at yandex.com
Tue Dec 16 11:38:11 EST 2014

Every so often, the subject of logging the zero-crossings of the AC 
mains comes up.  There are any number of ways to couple the AC mains 
to logic circuitry (coupling with very high value resistors, 
capacitor coupling, and optical isolation have been mentioned).  A 
simple AC mains ZCD that is transformer isolated and gives excellent 
results, is posted at ko4bb.com:


The ZCD uses a small, dual-primary power transformer, two 
transistors, and a few diodes, resistors, and capacitors.  It 
produces a ~100uS logic-level pulse at every positive zero-cross, the 
leading edge of which is predictably and stably related to the AC 
mains zero-cross.

Best regards,


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