You have security lights outside your house.
How do you turn them on in the evening? You probably flip a switch.

Then as the day begins to break, you turn the security light off to save on electricity.

These are all well and good if you remember to flip the lights on and off daily.

However, no matter how diligent you are, you're going to forget sometimes. That would make your place either less secure, or it would cost you more in electricity charges.

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Automation

What if there was a way to automate the process of turning the lights on from dusk to dawn?

That is where Photocell comes in. It is a light sensor.
When it gets dark outside, the sensor completes a circuit to turn on the light.
Conversely, as day breaks, the circuit opens to turn off the light.

With a photocell installed, you'd no longer need to worry about turning lights on or off manually.

A single photocell can be used for one, or multiple lights bulbs.
So you don't need to have a photocell for each light bulb.

Also, photocells are inexpensive, and easy to install.

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Photocell wiring

A photocell is easy to install. It comes with three wires, colored red, white, and black.

Take a look at the following images for how to install it.

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Applications

We've looked at the use of photocell in sunset to sunrise lighting that automatically turns on whenever intensity of light is low.
Photocells are also used in other applications like intruder alarms, automatic doors, gates, elevators, etc.

Photocells are used extensively for street lights.