What is LDR ?

     An LDR shorted from Light Dependent Resistor or photoresistor or Photo Sensor. Which is a passive electronic component, basically,  LDR is made of a high resistance semiconductor that absorbs photons that are based on the quantity and frequency of absorbed photons.


How LDR works:
  The LDR is made of high resistance, the resistance is very high in the darkness it's almost 1M ohm. But light falls on LDR, probably the resistance is falling down rapidly to the 5-10k ohm.

 The LDR comes with the vivid symbols of LDR given below,
LDR Above diagram showing the symbol for the LDR. Next, we discuss characteristics of LDR.  

Characteristics of LDR:


 LDRs are a passive element. Light-dependent resistor, name showing meaning itself like the resistance depends on the light falls on the LDR.
 The resistance is very high in the darkness, this also called for a dark resistance like 10M ohms, if the device is allowed to absorb light its resistance will be decreased drastically. If a constant voltage is applied to it and intensity of light is increased the current starts increasing. Below diagram shows clearly resistance vs illumination curve.


LDR characteristics

Fig 1. Characteristics of LDR




Working of LDR:
   
   A light dependent resistor works on the principle of photoconductivity. Photoconductivity is an optical phenomenon in which the conductivity of the material is increased when the light is absorbed by the material. 

When light falls i.e. when the photons fall on the device, the electrons in the valence band of the semiconductor material are excited to the conduction band. These photons in the incident light should have energy greater than the band gap of the semiconductor material to make the electrons jump from the valence band to the conduction band. Hence when light having enough energy strikes on the device, more and more electrons are excited to the conduction band which results in a large number of charge carriers. The result of this process is more and more current starts flowing through the device when the circuit is closed and hence it is said that the resistance of the device has been decreased. This is the most common working principle of LDR.

Based on the material used, classified as two types, those are 

1. Intrinsic Photo Resistor: 

   These are made up pure semiconductor materials such as silicon and germanium, these are an undoped semiconductor. Electronics get excited from valence bond to conduction band when photons enough of energy and number of charge carriers.

2. Extrinsic Photo Resistor: 

   These are doped elements and it is also called as dopants. These are also semiconductor materials. These dopants create new energy above the valence band which are filled with the electrons. It needs less energy to excite them and reduces the band gap.

   

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