Light Dependent Resistor (LDR)

Light Dependent Resistor(LDR) is a photosensitive device which changes its resistance on the basis of intensity of light hitting its sensitive surface. These resistors have a variety of functions and resistance. A typical light dependent resistor has a resistance of 1MOhm in the darkness , and a resistance of a couple of KOhm in the brightness. Resistance of an LDR is inversely proportional to the intensity of light that falls on its surface. They are also called as photo conductors, photoconductive cells or simply photocells. They are made up of semiconductor materials having high resistance.

Fig: Light Dependent Resistor

Generally the following two symbols are used to indicate LDR, where the arrows indicate an incident of electromagnetic radiation.

Fig: Symbols of LDR

WORKING OF LDR

A light dependent resistor works on the principle of photoconductivity. Photoconductivity is a phenomenon in which the material increases its conductivity when light is absorbed by the material.

The semiconductor material of LDR is of high resistance because there are only few electrons available for conduction in the conduction band. When light/photons of suitable energy fall on LDR, some electrons jump from valence band to conduction band. This happens because electrons in LDR absorb the energy of photons. Now since some extra electrons are in the conduction band more current flows through LDR. So we can conclude that resistance of LDR decreases with increase in light intensity and vice versa.

If we plot a graph between intensity of light incident on LDR and resistance, a curve as shown in figure below is obtained which also shows that resistance of LDR decreases with increase in light intensity.

Fig: A graph showing inverse relation between light intensity and resistance

USES OF LDR

Light-dependent resistors are simple and low-cost devices. These devices are used where there is a need to sense the presence and absence of light.

• These resistors are used as light sensors and the applications of LDR mainly include security alarm clocks, smoke detectors, light intensity meters, street light control circuits, light activated control circuits, etc.
• It is also used for automatic contrast and brightness control in television receivers.

Here we'll discuss how it works on automatic street lights.

LDR in Automatic Street Light

Street lights are designed to switch on automatically when it's dark and switch off when there is light to save energy. A simple electrical circuit used in automatic street light is shown below:

Fig: Circuit Diagram

We know LDR offers very high resistance at night time and its resistance drastically falls during day time. In the circuit shown above, the LDR and bulb are connected in parallel. Here a transistor is used as a two-way switch. During day time, LDR gives low resistance as a result the current through led decreases which turns LED off. During night time, LDR offers high resistance as a result the current flowing through the led increases so the LED glows.

ADVANTAGES OF LDR

• LDR are cheap and readily availble in many sizes and shapes.
• There is no union potential.
• The light-dark resistance ratio is high.

DISADVANTAGES OF LDR

• Spectral response is narrow.
• Temperature stability is low for the best materials.
• In stable materials, it responses very slowly.
• The use of LDR is limited where the light signal changes very quickly.