What is Light Dependent Resistor/Photoresistor: All you Need to Know


Light-dependent resistors, also known as photoresistors, are the basis of devices for detecting the light level or lack of light. Therefore, manufacturers or repairers working on applications with light sensors have likely come across LDR. 

How these light-sensitive components work with light sources is essential in understanding how to implement them in our day-to-day electronic appliances. These resistors are becoming more common, and this article outlines how they interact with our luminous environment. 

Understanding a Light Dependent Resistor/LDR Photoresistor

A light-dependent resistor (LDR) is an electronic component applied to devices designed to detect light intensity. Equal parts like photodiodes or phototransistors are also applicable. However, light-dependent resistors provide a broader range of resistance for light-level changes. 

As the name suggests, LDR is a unique resistor whose function depends on the photoconductivity principle hence resistance varies under different levels of light. Its resistance is highest in the dark with sensor output of 1012 Ohm and decreases with light intensity. Automatically, the photosensitive components are prominent in light sensors, light meters, street lights, or other electronic applications used in areas of high light sensitivity.  

Light Dependent Resistor Symbol

The LDR symbol is simply the typical resistor circuit symbol with arrows showing light use. The basic structure is a rectangle which is the newer representation style. But, designers also use the older symbol with a zig-zag line.   

Lastly, the symbol also represents photodiodes and phototransistors as they have photoconductivity.  

LDR symbol with a rectangle

(LDR symbol with a rectangle)

Photocells Versus LDR

Unlike light-dependent resistors, photodiodes provide a faster response time; hence, engineers use them where quick light detection is needed, for instance, in optical communication and optoisolators. In particular, the function of a photodiode works by converting active light into electric energy. Nevertheless, the LDR working principle relies on light levels’ effect on resistance changes.

The LDR Structure

The photoresistor has a horizontal structure, and the format is such that the active layer is between a semi-insulating substrate and partially covering metal contacts on the top layer. As a result, this LDR structure is viral among small photoresistors or light-dependent resistors. 

The metals act as contacts for the photoresistor. Their surfaces are significant, especially in discrete light-dependent resistors, to ensure minimized resistance from the touch of light to the exposed active layer. The two metal contacts are separated to allow an area where the light can pass through the LDR active region. 

Numerous materials can be of use when making photoresistors. These materials include cadmium sulfide light, indium antimonide, InP, PbS, PbSe, Ge, Is, and GaAs. All these semiconductor elements have various properties, like the wavelengths of light sensitivity.

structure of a photoresistor

Types of Photoresistors

The classification of light-dependent resistors depends on the semiconductor devices used; hence fall into two categories:

Intrinsic Photoresistors

These photoresistors are made with accurate semiconductor devices like germanium and silicon without doping. Light falls on the LDR, exciting the electrons, and moving them from the valence band to the conduction band.

The charge carriers increase with ambient light, and the electrons conduct electricity. The higher the light level, the more electrons are liberated, and there is an increased level of conductivity. 

Extrinsic Photoresistor

These resistors used materials doped with impurities to create new energy bands above the valence bands. The bars filled with electrons decrease the bandgap. Hence, the required energy is less in moving them from the valence band. 

Extrinsic photoresistors are a common type of LDR used with long wavelengths of light. 

How Does the Light Dependent Resistor LDR Photoresistor Function?

Next, the function of the light-dependent resistor works because the material absorbs the component conductivity increases as light increases. As the incident light falls on the device, the electrons move to the conduction band from the valence bond in the semiconductor. However, photons in the incident light must have an energy gap more significant than the bandgap of the semiconductor material for the transition of electrons.

Therefore, more electrons jump to the conduction band when bright light strikes the device, creating many charge carriers. Consequently, the current starts flowing through the machine once you close the circuit, decreasing resistance.

Light Dependent Resistor Latency

Latency is a photoresistor property that highlights the time between changes in illumination and changes in resistance.  The rate at which resistance changes is the resistance recovery rate. Light-dependent resistors are efficient where light changes occur less frequently, for example, in audio compressors. The LDR responds with tens of milliseconds when light appears after darkness. The adjustment to darkness takes up to a second for resistance to reach its final level. 

It takes a substantial amount of time for changes in light level to affect a photoresistor making it a less appropriate electronic component where light conditions fluctuate rapidly. Mostly, in seconds. That is why you will find that the product details quoted on electronic applications with photoresistors are the dark resistance after a given time. There are two quoted values to indicate the resistor’s latency; one is for within a second, while the other is for five seconds. 

Applications of Light Dependent Resistor LDR Photoresistor

Light-dependent resistors identify the presence or absence of light in electronic components. Such applications are:

  • Night lights, street light
  • Outdoor Clock
  • Solar panels
  • Smoke alarms, Clock with automatic light
  • Ambulatory Surgery centers
  • Clock radios
  • OCB eye health services
  • Alarm circuits and alarm clocks 

Audio compressors use photoresistors because of their lack of immediate reaction to light. The delayed response is the required feature. Automatic light switching after sunsets also employs the excellent benefits of LDR as the switch’s design controls the light illuminated on the resistor sensor. 

image a grandfather clock that uses a photoresistor

 (image a grandfather clock that uses a photoresistor)


While photodiodes and phototransistors are commonly in electronic components, the LDR is also popular in devices that require a wide range of resistance. They allow minimal energy consumption through automatic on and off switching.

I hope you have found the article knowledgeable as far as photoresistors are concerned. If you need a partner in manufacturing or sourcing LDRs, feel free to reach us for assistance.

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