LDR

                                                   LDR

        


              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 k Ohm 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!


LDR 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 required to achieve the desired effect, although any replacement must be at least 1K to protect the transistor from being damaged by excessive current.

By swapping the LDR over with the 10K and 10K variable resistors , the circuit will be activated instead by light. Whenever sufficient light falls on the LDR (manually fine-tuned using the 10K variable resistor), the LED will light up.