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A Light Dependent Resistor ( LDR, photoconductor, or photocell) is a device which has a resistance which varies according to the amount of light falling on its surface. They will be having a resistance of 1 MOhm in total darkness, and a resistance of a 1 to 10 of kOhm in bright light. A photoelectric device can be either intrinsic or extrinsic. Applications: An LDR can even be used in a simple remote control circuit using the backlight of a mobile phone to turn on a device - call the mobile from anywhere in the world, it lights up the LDR, and lighting can be turned on remotely! Light Dependent Resistor Circuits: There are two basic circuits using light dependent resistors - the first is activated by darkness, the second is activated by light. In the circuit diagram on the left , the led lights up whenever the LDR is in darkness . The 10K variable resistor is used to fine-tune the level of darkness required before the LED lights up. The 10K standard resistor can be changed as require