R2 sets the circuit`s threshold. When the light intensity at PCI`s surface is decreased, the resistance of PCI a cadmium-sulfide photo-resistor is increased. This decreases the voltage at the inverting input of the 741. When the reference voltage at the 741`s noninverting input is properly adjusted via R2, the comparator will switch from low to high when PCI is darkened
This turns on Q1 which, in turn, pulls in relay Kl.