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Working of light dependent resistor
This circuit explains the principle of operation of LDR (light dependence resistor). The circuit has various applications like shadow alarm, automatic night/morning lamp.
LDR, an acronym for light dependent resistor is a resistor whose resistance is dependent on light. The resistance of LDR is of the order of Mega Ohms in the absence of light and reduces to a few ohms in presence of light. In this circuit when the light falls on LDR, the resistance of LDR becomes low and the entire voltage drop takes place across the variable resistance VR1 (10K). As a result the base of transistor (T1) gets high input and it gets biased thereby glowing the LED. When no light falls on LDR, the resistance of LDR becomes high so almost entire voltage drop takes place across it and the base of transistor is at low potential. So transistor does not gets biased nor it becomes conducting, hence switching off the LED. The sensitivity of the circuit can be adjusted by varying the preset VR1.
LDR, an acronym for light dependent resistor is a resistor whose resistance is dependent on light. The resistance of LDR is of the order of Mega Ohms in the absence of light and reduces to a few ohms in presence of light. In this circuit when the light falls on LDR, the resistance of LDR becomes low and the entire voltage drop takes place across the variable resistance VR1 (10K). As a result the base of transistor (T1) gets high input and it gets biased thereby glowing the LED. When no light falls on LDR, the resistance of LDR becomes high so almost entire voltage drop takes place across it and the base of transistor is at low potential. So transistor does not gets biased nor it becomes conducting, hence switching off the LED. The sensitivity of the circuit can be adjusted by varying the preset VR1.
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