Photo cells and timers are necessary to control the landscape lighting transformer, which in turn controls the lighting system. Different situations may call for a different device or combination of devices. A commercial installation looking for enhanced security may desire the lighting system to be active continuously throughout the entire night. A homeowner may want the house lit up only for specific hours, or may want the house lit up any time it is dark (such as during a storm).
Timer and Photo Cell: This configuration allows for the daytime advantages of a photo cell while still turning off the lighting system at a desired time during the night. For example, the timer can be set to be ON at 6AM and OFF at midnight (timer ON for 18 hours). In this configuration, the photo cell will keep the lights off during the day unless it becomes sufficiently dark because of a storm, and the lights will turn on. When the sun comes out again, the photo cell will turn the lights off until night falls, when the lights will turn on as normal. Once midnight arrives, the timer will override the photo cell and the lights will go off and stay off until 6AM the next day, when lighting conditions will again determine if the system should be on or off.