I use this circuit during my work as a lecturer in the high Technical Institute, to illustrate the power control of the thyristor experiment’
The circuit consist of ZCD ( Zero Crossing Detector ) to provide a synchronizing pulses during triggering the Thyristor .
The One shot circuit used to stretch the pulse ( by changing the RC circuit) from minimum to the maximum which is slightly less than the Time of the Half-Wave of the AC source.
I wish it will be useful for you
Please see the schematic as a PDF attached