Look at the schematic
That is the classic 555 astable multivibrator circuit, with the addition of another external trigger. That is NOT what should be there. He would want it as a retriggerable monostable multivibrator. Stays on while light is blocked then times out when light is resumed.
That circuit will not work properly. The better idea is to use the transistor to short the timing cap (threshold pin) to ground and ignore the pin 2 trigger input entirely.
The author of the article, Pankaj Khatri, has no clue as to what he's doing. If he did manage to get it to work (as planned) it isn't because it was designed to work as planned.