Zero Crossing Detector circuit is basically an application of a comparator. In the article series Zero Crossing Detector with Op Amp is built using a comparator of an Op Amp IC741/351. The process of detection of this comparator is 0Volt input signal crossing point by making reference value at comparator 0Volt.
The output of the Zero Crossing Detector circuit with Op Amp is wave-shaped box that detection result of the crossroads of 0 volt input signal.