I have tried to use a comparator (LM358) to make a Light Sensor which gives output as high when a certain intensity level is crossed or vice versa but it has not given good output and I don't know the reason. So, I have to go for an alternative because the TASK is important at that time.So I have decided to go for a BJT (BC548) which is cheaper than the previous.This is the circuit which I have designed
Each and every component has its own physical significance. Let me explain briefly
The LED D1 always focus light on the LDR (light dependent resistor) whose resistance varies with light intensity. The potentiometer P1 is used as variable resistor in order to adjust the sensitivity of the LDR to the surroundings. When light falls on theLDR its resistance decreases and make the transistor(Q1) move into active region which may not sufficient to drive the load(L1) less than logic high voltage so I kept a transistor (Q2) for switching purpose operates in either cuttoff or saturation.
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