LDR Full Form, Symbol, Construction, Connection Diagram

LDR full form is Light Dependent Resistor. LDR is also commonly known as a Photoresistor because its working principle depends upon the absorption of photons. LDR is also known as a photocell, photoconductor, etc. 

Basically, LDR or Photoresistor is a semiconductor device and it is very different from a normal resistor with respect to construction and working principle. The working principle of LDR or photoresistor is very simple. Normally it has a very high resistance but when light falls on it, the resistance will decrease. Photoresistor or LDR has a wide range of applications in modern electronic circuits, cheap electronic devices, automation circuits, detector circuits, etc.

There are some other electronic components such as Photodiode, Phototransistor works on the almost same principle as a photoresistor. But photoresistors are widely used for their low cost, simple design, reliability, etc.

Photoresistor or LDR Symbol

Here you can see the symbol of LDR below. 

It is almost the same as the resistor symbol including two arrows shining on it that indicate the light source should fall on it to work.

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LDR Construction

LDR or photoresistor is a semiconductor device. There are two types of photoresistors available - 1. Intrinsic Photoresistor and 2. Extrinsic Photoresistor

Intrinsic photoresistors are made of pure silicon or germanium without doping and extrinsic photoresistors are made of silicon or germanium doped with some impurities.

If we see the construction of a conventional LDR, it is made of semiconductor materials. Its electrons are stored in a valance band and they are free to move. The photoresistor will conduct current when electrons are moved from the valance band to the conduction band. 

So when light falls on the photoresistor, it absorbs photons from the light source. These photons give energy to the electrons to break the barrier and move from the valance band to the conduction band. Once the electrons are moved from the valance band to the conduction band the photoresistor allows to flow the current through it.

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LDR Connection Diagram

Here you can see the connection of LDR or photoresistor below.

Here have made a very simple circuit of LDR to understand its working principle. The photoresistor or LDR has two terminals. They can be connected the same as a normal resistor.

Here we have made the darkness-sensing circuit using LDR. So when the darkness occurs the LED will glow and in the presence of brightness, the LED will be turned off.

Now let's know how this circuit works.

So when brightness is available near the circuit, the resistance of the LDR will be low and the electric current will flow through the LDR, not through the transistor. But when this circuit is put in a dark place, the resistance of the LDR will be high so the current will start to flow through the base terminal of the transistor, not through the LDR. So the transistor will start to conduct current flowing through it which will help to glow the LED.

There is a disadvantage of this circuit or we can say darkness circuit using LDR. When this circuit is put in a bright place, the LED will be turned off but the current will flow through the LDR. Although this is a very low amount of current, a continuous current will flow. So an LDR circuit always consumes current even when it does not do any useful work.

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