The circuit is from India. None of their circuits work and I have pages of their circuits, showing the faults in their designs.
The circuit will not work. Not only is the NOR gate drawn incorrectly, but the BC557 will never turn off as Q bar will never rise to 12v as the chip is on a 5v supply. The circuit obviously has never been constructed
What is the purpose of D1?
T2 and T3 do not have to be power transistors. Why have a 5v regulator when the chips will accept up to 15v.
Finally, you don't design a sensitivity circuit as shown in the diagram. It's just a badly designed circuit by a person who is not electronics-capable.