Photo cell schematic
This circuit is a classic application of LDRs (Light Dependent Resistors) or also called CdS cells, because they are made of Cadmium Sulphide. The LM358 Op Amp is used as a comparator. Its output is connected to a transistor used as a switch, to trigger a relay.
When light falls on the cell, the voltage on the non-inverting input of the Op Amp increases, when it reaches a voltage slightly higher than on the inverting input, the output will swing to 5V (approximately) and the transistor is switched ON, triggering the relay.
The Diodes D1 and D2 are flyback diodes (also called freewheel diodes). They protect the transistor from the back e.m.f. voltage spikes created by the coil of the relay when switched ON or OFF.
The Potentiometer R is used to adjust the sensitivity of the receiver, to adapt to different ambient light conditions.