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Led and Photodiode

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Submitted By SurajSahni
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Light emitting Diode Light-emitting diodes are elements for light signalization in electronics. The basic principle behind the working of LED is electroluminescence. The Light emitting diode should be forward biased to get the light. In Light emitting diodes, electrons are injected from low work function cathode to the conduction band of the n-type semiconducting material and holes are injected from high work function anode to the valence band ot the p-type semiconducting material. When the electron in the conduction band combines with the hole in the valence band, energy is released. In case of indirect band gap semicondutors, phonon will be released to conserve of both energy and momentum. But in case of direct band gap semiconductor, light will be emitted whose wavelength depends on the band gap of the semiconductor.

Different parts of a Light emitting diode

Radiative recombination in direct and indirect bandgap semiconductor

Cartoon showing radiative recombination in a direct band-gap semiconductor Schematic diagram of working of an LED Light emitting Diode and its circuit symbol The main advantage of Light emitting diode over other light sources is its increased efficiency. LEDs are available in red, orange, amber, yellow, green, blue and white. Blue and white LEDs are much more expensive than the other colours. We have employed low cost Red LED in our electronic circuit. Light Dependent resistor A light dependent resisitor is a resisitor whose resistance changes with the intensity of incident light. The working principle of light dependent resistor is photoelectric effect. A light dependent resisitor is made of a high resistance semiconductor. If the energy of the incident light is greater than the band gap of the semiconductor, electron -hole pairs are generated. The photogenerated electron-hole pair transits the device giving rise to photoconductivity. The essential elements of a photoconductive cell are the ceramic substrate, a layer of photoconductive material, metallic electrodes to connect the device into a circuit and a moisture resistant enclosure. Light sensitive material is arranged in the form of a long strip, zig-zagged across a disc shaped base with protective sides. For additional protection, a glass or plastic cover may be included. The two ends of the strip are brought out to connecting pins below the base as shown below.
Side view
Side view
Top view
Top view Top view and side view of Light Dependent Resisitor The commercial photoconductive materials include cadmium sulphide (CdS), cadmium selenide (CdSe), Lead sulfide (PbS) and Indium antimonide (InSb) etc., There is large change in the resistance of a cadmium selenide cell with changes in ambient temperature, but the resistance of cadmium sulphide remains relatively stable. Moreover, the spectral response of a cadmium sulphide cell closely matches to that of a human eye. Hence, LDR is used in applications where human vision is a factor such as street light control or automatic iris control for cameras. The above mentioned features drive us to opt for CdS based LDR in our electronic circuit for Automatic street light controller. Light Dependent Resistor and its circuit symbol

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