There are some very useful lights, especially for exteriors, which have an ambient light sensor that is a simple application of a switch controlled by LDR, very basic. This circuit detects ambient light and depending on the presence/absence of sufficient ambient light, causes a bulb to automatically turn on/off. automatically switches off when there is sufficient ambient light (for example, during the day when the sun is present no additional light is needed). These lights are very useful if you get home late after work or you are busy at night, you will not have to worry about your house is dark, with these bulbs it will always be alive at night. This type of light is also common to see in cars.