you see the street light turn on in evening and turn off in the morning automatically. how this all come true? the circuit described below gives the correct explanation of your question.the circuit uses the output from LM324 to drive the relay which can be further coupled to on/off bulb.
PROTEUS DESIGN
LDR used in the circuit provide input to LM324 comparator. positive side of LM324 is connected with variable resistor so adjusted the value at pin 3 of comparator will be 4.99V(V1).now when you bring torch away from LDR voltage V2 goes down that is below 4.99V(V2<V1),condition for darkness.that means the output of comparator at that time is high which cause relay to drive circuit which in turn make the bulb to glow. similarly when(V1>V2) put torch close to LDR bulb will turn on automatically.
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