Photocell Light sensing is required by many outdoor lamps. Using Photocell eliminates the need to manually control the switch. A Photocell consists of a resistor connected to a photosensitive plate. As more light hits the plate, the resistance (the amount of current flowing through the resistor) changes, turning the light on and off. This technology is suitable for all types of outdoor locations. Therefore, light fixtures equipped with light sensors will automatically turn off during the day and turn on automatically at night.