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Difference between Photodiode and Light Dependent Resistor (LDR)
In electrical and electronics engineering, the automation is day by day increasing which makes the use of various types of sensors. There are a lot of photo-sensors or light sensors which are used for making the electrical and electronic circuit automatically operating. The light sensors are sensitive to the intensity of light. The two most common light sensors are photodiode and light-dependent resistor (LDR).
In this post, we will take a look at all the differences between photodiode and LDR along with their description. A major difference between LDR and photodiode is that the LDR (Light Dependent Resistor) shows a bidirectional resistance, while the photodiode shows a unidirectional resistance.
What is a Photodiode?
A light-sensitive PIN junction semiconductor diode (a PN junction diode with a wide intrinsic semiconductor region between p-type and n-type semiconductor regions) which converts the light energy into electrical energy is called as photodiode. A photodiode absorbs the photons to produce the electric current.
The construction of a photodiode is such that it allows the light to reach the light sensitive part of the diode. Usually, a photodiode is designed to operate in reverse bias. A common practical example of photodiode is a solar cell.
The principle of operation of the photodiode is based on the photoelectric effect, i.e. when a photon of sufficient energy strikes the photodiode, an electron-hole pair is formed. If this absorption of photons occurs in the depletion region, then the charge carriers (electrons and holes) are swept from the junction by the electric field of the depletion region. The holes move towards the anode while the electrons move toward the cathode and hence results in the flow of photocurrent.
The applications of the photodiodes include: in the instruments used to measure light intensity, measurement of density of smoke, in the remote control systems, in optocouplers, etc.
What is a Light-Dependent Resistor?
A Light-Dependent Resistor (also called LDR or photoresistor or photocell or photo conductive cell) is a passive circuit element whose resistance changes with change in the intensity of light.
The resistance of an LDR decreases with the increase in the intensity of incident light. Therefore, the light dependent resistor exhibits photoconductivity. When no light is incident on the LDR, i.e. the LDR is in dark, then the resistance of the LDR is of the order of Mega-ohms whereas, when a light is incident on the LDR, its resistance decreases and becomes of the order of few hundred ohms.
The LDR is used in a number of applications such as nightlights, solar street lamps, laser-based security systems, dynamic compressors, etc.
Difference between Photodiode and LDR
Both LDR and photodiodes are two common types of light-sensitive devices used in a number of applications. However, there are several differences between LDR and photodiode which are given in the following table −
Basis of Difference | Photodiode | Light Dependent Resistor |
|---|---|---|
Definition | A photodiode is a PIN junction semiconductor diode which can convert the light energy into electrical energy. | A light dependent resistor is a passive circuit component whose resistance decreases with the increase in the intensity of incident light. |
Circuit symbol |  |  |
Alternate names | A photodiode is also called a photo-sensor or photo-detector or light detector. | An LDR is also called a photo-resistor or photocell or photoconductive cell. |
Biasing | A photodiode is mainly designed to operate in reverse bias condition. | LDRs are mostly used in forward bias condition. |
Device resistance | A photodiode acts as a unidirectional resistor. | An LDR acts as a bidirectional resistor. |
Material | The materials used for manufacturing a typical photodiode are Silicon, Germanium and Indium-Gallium-Arsenide. | Cadmium sulphide is mainly used for manufacturing an LDR. |
Response time | The response time of a photodiode is relatively less than that of the LDR, which makes it comparatively faster. | The response time of an LDR is more which makes it quite slow. |
Applications | Photodiodes are widely used in light meters, automatic shutter controls, smoke detectors, remote controls, optocouplers, solar cell panels, exact measurement of light intensity in scientific experiments and industries, etc. | An LDR is mainly used as a light sensor. Some other applications of LDR are in street lighting design, laser-based security systems, alarm clocks, solar street lamps, etc. |
Conclusion
Photodiodes and Light Dependent Resistors (LDR) are the most common types of photosensors. The major differences between photodiode and LDR are enlisted in the above table. The most significant difference between an LDR and a photodiode is that a photodiode is primarily designed to operate in reverse bias, while an LDR is designed to work in both directions (i.e. forward as well as reverse).
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