Photo Resistors Or Light Dependent Resistor ( LDR ) Science Tutorial

Presented By IngenuityDias

A photo-resistor or light-dependent resistor (LDR) or photocell is a light-controlled variable resistor. The resistance of a photo-resistor decreases with increasing incident light intensity; in other words, it exhibits photo-conductivity. A photo-resistor can be applied in light-sensitive detector circuits, and light- and dark-activated switching circuits.LDR (Light dependent resistor), as its name suggests, offers resistance in response to the ambient light. The resistance decreases as the intensity of incident light increases, and vice versa. In the absence of light, LDR exhibits a resistance of the order of mega-ohms which decreases to few hundred ohms in the presence of light. It can act as a sensor, since a varying voltage drop can be obtained in accordance with the varying light.

LDR Light Sensor as Lamp Switch Using LED-

Material Used for Making LDR
An LDR or photo-resistor is made of any semiconductor material with a high resistance.It is made up of cadmium sulphide (CdS).An LDR has a zigzag cadmium sulphide track. It is a bilateral device, i.e., conducts in both directions in same fashion.Lead sulphide (PbS) and indium antimonide (InSb) LDRs (light-dependent resistors) are used for the mid-infrared spectral region. Ge:Cu photoconductors are among the best far-infrared detectors available, and are used for infrared astronomy and infrared spectroscopy.

LDR Light Dependent Resistor using Photo Diode-

Suggested Further Readings for Students:-

LDR – Priciple Of Working -How an LDR works? -Explain Working Of LDR with example.

A photo-resistor is made of a high resistance semiconductor. In the dark, a photo-resistor can have a resistance as high as several megohms (MΩ), while in the light, a photo-resistor can have a resistance as low as a few hundred ohms.

It is first necessary to understand that an electrical current consists of the movement of electrons within a material. Good conductors have a large number of free electrons that can drift in a given direction under the action of a potential difference. Insulators with a high resistance have very few free electrons, and therefore it is hard to make the them move and hence a current to flow.

LDR-Resistance variation In Presence Of Light-

LDR has a high resistance because there are very few electrons that are free and able to move – the vast majority of the electrons are locked into the crystal lattice and unable to move.



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