A photocell is nothing but a switch, operated by a light source. So, during the evening, the lack of light closes the switch, enabling a lighting fixture to operate for example. During the day, light is sensed and the switch opens.
A photoresistor is commonly called a photocell . Its resistance changes with the exposure to light. the common one is the Cds type, or Cadmium sulphide type.The opto-electronic property of this semiconductor material makes it vary its resisitance when light falls on it.
This photocell drives a transistor whichin turn drives a relay which is connected to the load. A variable resistor adjusts the sensivity of the transistor to callibrate the amount of light needed to switch the relay.
