What is LDR? Basics of light dependent resistor

Light Dependent Resistor – it is a passive light transducer. It is also called as photo-conductive cell because its conductivity changes due to change in light intensity. image

Basic Principle – when light falls on it its resistance decreases and when it is dark its resistance is maximum. The change in resistance is directly proportional to intensity of light falling on it.

Construction – it is made up of photo sensitive material like cadmium sulphide (CdS), Selenium (Se), Cadmium Selenide (CdSe) or Lead Sulphide (PbS). It is deposited on insulating surface like ceramic substrate in the form of zigzag wire as shown in following figure. It is enclosed in round metallic or plastic case and two electrodes are taken out for external connections. The structure is covered with glass sheet to protect it from moisture and dust and allows only light to fall on it.

Applications –--

1- It is used in burglar alarm to give alarming sound when a burglar invades sensitive premises.
2- It is used in street light control to switch on the lights during dusk (evening) and switch off during dawn (morning) automatically.
3- It is used in Lux meter to measure intensity of light in Lux.
4-It is used in photo sensitive relay circuit.

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Constructional diagram of LDR