A zero crossing detector or ZCD is a one type of voltage comparator, used to detect a sine waveform transition from positive and negative that coincides when the I/P crosses the zero voltage condition.
The zero crossing detector circuit is an important application of the op-amp comparator circuit. It can also be called as the sine to square wave converter. Anyone of the inverting or non-inverting comparators can be used as a zero-crossing detector. The only change to be brought in is the reference voltage with which the input voltage is to be compared, must be made zero (Vref = 0V). An input sine wave is given as Vin.