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:
<http://www.ko4bb.com/manuals/download.php?file=05_GPS_Timing/Simple_AC_Mains_Zero_Crossing_Detector.pdf>
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,
Charles
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