Some of the circuits require zero cross reference mark of applied AC waveform as an input for their operation. The circuit will produce output based on these zero marks. The output is either a short positive or negative pulse when AC wave crosses zero line. For example in thyristor phase angle control circuit, zero cross reference mark of AC supply waveform is required to trigger thyristor at a specific angle. The phase difference measurement circuit will take zero cross mark inputs of two different AC waveforms and calculates phase difference between these two waveforms. Refer the waveforms given in following figure to understand above two examples better. 

Here in this application, I will show you how we can control the speed of single phase 230V AC motor wirelessly using IC555. Along with IC555 I am using zero cross detector (ZCD) circuit to trigger the monostable and an optocoupler to trigger thyristor. Let us first see the block diagram of the AC Motor Speed Control System.