Zero-Crossing Detector for Microcontrollers

In lighting controllers and clock circuits that need the mains frequency as a parameter for evaluations carried out within a microcontroller, you will often find a transistor stage to convert the mains voltage (reduced by the transformer) into a 50-Hz squarewave signal that is suitable for input to the microcontroller. Generally speaking, this stage is unnecessary with modern microcontrollers if the port input is wired as a Schmitt trigger.