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Light Dependent Resistor – special resistor based on internal photoelectric effect

Light dependent resistor (LDR) is usually used in electronic circuit design that needs to detect the presence or level of light, like streetlight. Have you ever wondered how some streetlights turn on automatically in the evening? Well, this magic behind this dependent resistors performed by LDR or light dependent resistor. An LDR has wide applications in our daily lives. Come let’s go into detail to know know more about the working of it.

What does a LDR do?

Light Dependent Resistor(LDR) is also called photoresistor  or photoconductor. A light dependent resistor is used to detect and sense light levels. In automatic security lights, for example, it detects lights and immediately turn on. It works in a way that with an increase in light intensity, their resistance decreases.

Generally, talking about the resistance, the resistance of an LDR is generally high at low light levels. Thus, little current can flow through the circuit. However, in bright light, the resistance is rather low in an LDR and more current can flow through the circuit.

What are the types of LDR?

Depending on the spectral characteristics of LDR, it is possible to classify them into three major categories. These three categories or types are visible light LDRs, infrared LDRs, and ultraviolet LDRs. A visible light LDR is used in different types of photoelectric control systems, street lights, and photoelectric tracking systems among others. Meanwhile, infrared LDRs are used in infrared spectroscopy, geography exploration, and missile guidance among others. In contrast, ultraviolet LDRs are more active or sensitive to ultraviolet light and are typically utilized for UV exploration.

What is LDR made of?

Typically, an LDR is made from different semiconductor materials. These materials allow them to possess light sensitive properties and it is possible that various materials can be used. However, cadmium sulfide is one of the most popular materials used for these photoresistors.

Recently, many cadmium-based materials have been restricted across Europe. Therefore, many producers have begun to use indium antimonide and lead sulfide to create semiconductor materials.

What are the possible advantages of LDR?

What-are-the-possible-advantages-of-LDRThere are several advantages of light dependent resistors. First of all, an LDR possesses high sensitivity. It is generally due to the large area that it can cover easily without any problem. Another major advantage of an LDR is its easy and simple employment. For instance, it can be installed in any device easily without experiencing major issues and problems. They are not sophisticated devices and that is why their installation is rather simple. Even an inexperienced person can install an LDR.

An important advantage of an LDR is its low cost. Since it is not that complex or sophisticated, it is easy to manufacture and develop LDR at low costs. Due to this low manufacturing cost, light dependent resistors are used widely in different devices and have wide applications. An LDR comes with a high light-dark resistance ratio, which makes it quite reliable even in uncertain conditions.

Where is LDR used?

Light dependent resistors are low-cost and simple devices and that is why they have wide applications. They are used whenever there is a need to identify and sense the absence and presence of light, LDR are being used. In addition to night light and illuminometer, audio compressor, another one example can be that hese resistors are utilized as light sensors and their applications include burglar alarm.

How do you make a light dependent resistor?

Making a light dependent resistor is not that simple. You can easily make an LDR by placing a light-sensitive material on an insulating substrate. This insulting substrate can be ceramic before placing the material in a zigzag shape. It will help get the required resistance and power rating. The area of zigzag play an important role in separating the metal-placed area into two different areas or regions.

After that, the ohmic contacts need to be made on the area’s either side. It is necessary to ensure that the resistances of the contacts are minimal. In this manner, a light dependent resistor can be created.

How difference can be highlighted between LDR and photodiode?

While an LDR is a circuit component that experiences a decrease in its resistance with an increase in light intensity, a photodiode is a semiconductor diode that is used for the conversion of light into electrical energy. An LDR is typically utilized in a forward bias condition while a photodiode is primarily used in a reverse bias condition.

Similarly, the materials used for the manufacturing of the two devices are also different. For instance, cadmium sulfide is used typically for the development of an LDR. On the other hand, silicon germanium is used widely to create and develop a photodiode.

FAQ about LDR


What is the resistance of light dependent resistor LDR?

It is important to note that the resistance of an LDR is quite high. Even though it is not fixed, it is still very high. For instance, it can even be over 1000,000 ohms. However, when they are illuminated or exposed to light, this resistance decreases significantly.

How does an LDR affect voltage?

Usually, when the amount of light decreases, an LDR’s resistance rises along with it. An increase in an LDR’s resistance results in a rise in the voltage applied across the LDR as well.

How LDR is used in street lights?

The resistance of an LDR is directly related to the intensity of light it is exposed to. When it is installed in street lights, it helps in automatically turn them on. In the dark, there is an increase in the resistance of an LDR in the control circuit. Therefore, the light is switched on. Meanwhile, during the day, the resistance is low and the lights remain off.

What is the range of LDR?

The range of an LDR can vary from approximately 10MΩ in darkness to around 100Ω in bright sunlight.


Overall, it can be said that light dependent resistors are quite common and they have been used in street lights and many other devices for years now. Even though they are simple and low-cost devices, they are quite important. Their applications are diverse and over the years, their usage has increased to a significant extent. Still, it does not mean that further research cannot be performed on it.



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