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LDR 1480405 6 7 HDR Enhancer 1. A photoelectric device can be either intrinsic or extrinsic. An intrinsic semiconductor has its own charge carriers and is not an efficient semiconductor, for example, silicon. Large CdS Photocell from a street light. A photoresistor is less light-sensitive than a photodiode or a phototransistor.

The latter two components are true semiconductor devices, while a photoresistor is a passive component that does not have a PN-junction. Photoresistors also exhibit a certain degree of latency between exposure to light and the subsequent decrease in resistance, usually around 10 milliseconds. The lag time when going from lit to dark environments is even greater, often as long as one second. This property makes them unsuitable for sensing rapidly flashing lights, but is sometimes used to smooth the response of audio signal compression. The internal components of a photoelectric control for a typical American streetlight. The photoresistor is facing rightwards and controls whether current flows through the heater which opens the main power contacts.

At night, the heater cools, closing the power contacts, energizing the street light. Photoresistors can be placed in streetlights to control when the light is on. Ambient light falling on the photoresistor causes the streetlight to turn off. Thus energy is saved by ensuring the light is only on during hours of darkness. They are also used in some dynamic compressors together with a small incandescent or neon lamp, or light-emitting diode to control gain reduction. Majority of street lights, outdoor lights, and a number of indoor home appliances are typically operated and maintained manually in many occasions. What is a Light Dependent Resistor?

An LDR or light dependent resistor is also known as photo resistor, photocell, photoconductor. It is a one type of resistor whose resistance varies depending on the amount of light falling on its surface. When the light falls on the resistor, then the resistance changes. Working Principle of  LDR This resistor works on the principle of photo conductivity.

It is nothing but, when the light falls on its surface, then the material conductivity reduces and also the electrons in the valence band of the device are excited to the conduction band. These photons in the incident light must have energy greater than the band gap of the semiconductor material. These devices depend on the light, when light falls on the LDR then the resistance decreases, and increases in the dark. When a LDR is kept in the dark place, its resistance is high and, when the LDR is kept in the light its resistance will decrease. V’ is applied to the LDR, the intensity of the light increased and current increases.

The figure below shows the curve between resistance Vs illumination curve for a particular light dependent resistor. Types of light Dependent Resistors Light dependent resistors are classified based on the materials used. Intrinsic Photo Resistors These resistors are pure semiconductor devices like silicon or germanium. When the light falls on the LDR, then the electrons get excited from the valence band to the conduction band and number of charge carriers increases. Extrinsic Photo Resistors These devices are doped with impurities and these impurities creates a new energy bands above the valence band.

These bands are filled with electrons. Hence this decrease the band gap and small amount of energy is required in moving them. These resistors are mainly used for long wavelengths. Circuit Diagram of a Light Dependent Resistor The circuit diagram of a LDR is shown below. When the light intensity is low, then the resistance of the LDR is high. This stops the current flow to the base terminal of the transistor.