Working of Light Dependent Resistor (LDR)

LDR (Light Dependent Resistor) is one of the most cheapest light sensor and most commonly used in light sensor circuits in open area (e.g. auto switching street lamps). It is made of semiconductor materials and the semiconductor used in this systems are cadmium sulphide, lead sulphide, germanium, silicon and gallium arsenide.

The main role of LDR is, when the light increases resistance decreases and when the light decreases resistance increases and the operating temperature is  min = -60°c, max = 75°c and the circuit symbol of LDR is shown below :

ldr symbolSymbol of LDR

You may raise a question, when LDR detects light or dark, how long it takes the resistor to make a change?

  • Resistance Rise Time is 2.8 ms for 1000 LUX
  • Resistance Rise Time is 18 ms for 10 LUX
  • Resistance Fall Time is 48 ms for 1000 LUX
  • Resistance Fall Time is 120 ms for 10 LUX

At complete darkness the resistance value is 1MΩ.

According to the above description it is clear that, the resistance fall and rise time depends upon the intensity of light illumination and in the below table you can view the illumination of light for various source.

ldr various illumination sources

Sensitivity of LDR is the relationship between the light falling on the component (LDR) and the output of the resistance value (refer the below characteristic graph of LDR).

ldr graph

Solar Tracking Robot will be the best example to explore the working of LDR.

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Author: Raja Sundar

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