anyway, thank you so much for helping me with thisI mean what I say
The latch is done in Q2 and in your schematic, Q2 is connected to the bulb. But it could have been connected to other thing (e.g. another red LED).
So, in this case, the bulb will turn ON or OFF depending only on Q1, which will turn ON or OFF depending on the ac source.
really esp with that reference thing that didnt ever crossed my mind. thank you so much for the help 
