phase controller
zero crossing detector is used to detect the start of input voltage ( +ve half or –ve half) , in some new designs uses a high value resister from supply zero crossing detector is used to detect the start of input voltage ( +ve half or –ve half) , in some new designs uses a high value resister from supply directly connects to any input pin to detect the polarity of the wave. If we know the start of the waveform then we can trigger the triac for the desired phase angle.