This is the simple circuit of automatic emergency light. It is called dark dependent circuit. A 555 timer IC and LDR is used for this configuration. When light fall on the LDR, its resistance is decrease and in dark position its resistance is going high. Using these characteristics of LDR, automatic emergency light is made.
LDR is called Light Dependent Resistor. Its resistance is inversely proportional to the falling light on its surface. In dark, its resistance is approximately 7k to 10k. When light fall on its surface, its resistance is decrease to less than 1k. In the circuit, a voltage divider arrangement circuit is used here with LDR and 10K variable resistor.
When light fall on the surface of LDR, the resistance decrease to 1k. So the voltage of the timer pin-6 is greater than 2/3 of +Vcc. For this reason the output pin-3 goes low. The base voltage of transistor BC547 is less than 0.75 volt. The transistor is OFF state. So the light is OFF.
In dark position the LDR resistance increase is high so the voltage of the trigger pin-2 decrease to less than 1/3 of +Vcc. The output pin-3 goes high so the base voltage of the transistor BC547 is greater than 0.75 volt. The transistor is ON state. So the light is ON and it emits light.