Never observe how street lamps will turn on automatically at night and off by itself in the morning? No doubt you also already know that no one is in charge to turn on and off the lights? Behind it there is a series of electronics that can do this job correctly. This circuit uses light that is used as an indicator. So
when nighttime no light then the electric current will be channeled by a
series of leading lights so that the lamp will be lit, otherwise when
the evening light then there is no electric current is not channeled by a
series towards the lights so that the lamp is off.
A device can be made dark or light is activated by slightly changing the circuit configuration. This
idea found a variety of applications such as, Automatic watering of the
garden in the evening, night lights, siren activated automatic dark and
so on.
This is very convenient for turning on the lights at a time when we are not there at home. Automatic night light circuit can be used to light the stairs, Office and others.
SCR and LDR together played the role of automation in this circuit. In the picture, the resistance of the LDR is low during the daytime and high at night. So
the required trigger pulses was developed around the SCR when be
darkened and it causes the SCR trigger and the current flows.
Good Luck ...
Caution This series relate to high voltage!