Zero crossing threshold detector

A comparator is an electronic component that compares two input voltages. Comparators are closely related to operational amplifiers, but a comparator is designed to operate with positive feedback and with its output saturated at one power rail or the other. An op-amp can be pressed into service as a poorly performing comparator if necessary, but its slew rate will be impaired.

Bistable output that indicates which of the two inputs has a higher voltage. That is,

where ${\displaystyle V_{{\text{S}}+}}$ and ${\displaystyle V_{{\text{S}}-}}$ are nominally the positive and negative supply voltages (which are not shown in the diagram).

The threshold detector with hysteresis consists of an operational amplifier and a series of resistors that provide hysteresis. Like other detectors, this device functions as a voltage switch, but with an important difference. The state of the detector output is not directly affected by input voltage, but instead by the voltage drop across its input terminals (here, referred to as Va). From Kirchhoff's Current Law, this value depends both on Vin and the output voltage of the threshold detector itself, both multiplied by a resistor ratio.

...
Wikipedia