A Zero Crossing Detector (ZCD) is one, which transforms the synchronizing signal into a square wave signal. By the very name it suggests that some thing should happen if it crosses 0V or if it crosses any preset voltage levels.

One of the following can be obtained.
(a) The output should be ON when the incoming signal is positive going and should and the output should be 0V, for the negative going signal. All signals commensurate w.r.t.DC.
(b) Alternately output should be 0V when the incoming signal is positive going and the output should be ON, for the negative going signal.

This can be accomplished by using diodes in the respective paths. One of the above refered signals are used to activate another output device. It could be a hardware, or a control circuit etc.

In this trainer the process of ZCD is experimented. In order to understand better, the experimenter has a variable reference signal to observe how the ZCD output behaves. The instrument is provided with necessary circuits to accomplish this task.