What is the LDR Sensor?

A light dependent resistor allows for electrical current to be monitored or controlled as it varies depending on how much light is applied to it. This blog post will provide an overview of what LDR sensor is, its working principle, and various types that can be used for specific applications.

What is LDR?

Most home appliances, outdoor lighting, and streetlights are normally operated and maintained manually. This is not only dangerous, but it also wastes energy due to staff negligence or unforeseeable events when turning this electrical equipment ON and OFF. Therefore, by using a light sensor, we can automatically shut off the loads based on the intensity of the daylight.

Fig. 1. Typical LDR Sensor

By sensing the radiant energy present in a relatively small range of frequencies often referred to as "light," which runs in frequency from the "Infra-red" to "Visible" up to "Ultraviolet" light spectrum, a light sensor produces an output signal that indicates the intensity of light. The light sensor is a passive component that produces an electrical signal from this "light energy," whether it is in the visible or infrared portions of the spectrum. Because they transform light energy (photons) into electricity, light sensors are more generally referred to as "Photoelectric Devices" or "Photo Sensors" (electrons).

Fig. 2. Components of LDR Sensor


It can be divided into two primary categories: photovoltaics and photo-emissions and photo-resistors and photo-conductors, which modify their electrical properties.

LDR is also referred to as a photoresistor, photocell, or photoconductor. It is a specific kind of resistor, and the amount of light that strikes its surface affects how much resistance it exhibits. A light-dependent resistor or LDR is an example of an electrical component that responds to light. When light beams strike it, the resistance changes right away. An LDR's resistance levels can vary by several orders of magnitude. As the light level rises, the resistance value will decrease.

LDR resistance values range from many megaohms in complete darkness to only a few hundred ohms in strong light. As a result, these resistors are widely used in a variety of applications due to this variation in resistance. The wavelength of the incident light affects the LDR sensitivity as well.

Working Principle of LDR:

Fig. 3. Operation of photo conductivity

The photo conductivity theory underlies the operation of this resistor. It is nothing more than the fact that when light strikes its surface, the material's conductivity decreases and the electrons in the device's valence band are stimulated to the conduction band. These incident light photons must have energy larger than the semiconductor material's band gap. Consequently, the electrons quickly move from the valence band to the conduction band.

Variations in resistance to change in light intensity

The light-dependent resistor reacts to light very quickly. When the light is greater, which indicates that the resistance value for the LDR will be drastically reduced as the light intensity rises.

Fig. 4. Variations in resistance to change in light intensity

When an LDR is exposed to light, the resistance increases; this is known as dark resistance. Conversely, when the resistor is exposed to darkness, the resistance decreases. Any device that absorbs light will have significantly less resistance. The light intensity will increase, and the current flow will begin to increase if a stable voltage is applied. Therefore, the characteristics between resistance and illumination for a particular LDR are shown in the diagram below.
Since LDRs are not linear devices, the wavelength of the light that strikes them causes a change in their sensitivity. Because it depends on the material employed, some types of photocells are not at all sensitive to a particular range of wavelengths.

When light enters a photocell, the resistance changes within 8 milliseconds from 8 to 12 and takes a few extra seconds to return to its initial value after the light has been turned off. Therefore, this is referred to as a resistance recovery rate. This attribute applies to audio compressors.

Light Dependent Resistor Circuit:

The bridge rectifier circuit or a battery is used to provide the LDR circuit with the necessary DC voltage. This circuit converts the AC source to a DC source. To reduce the voltage from 230 volts to 12 volts, a step-down transformer is used in the bridge rectifier circuit. DC voltage is converted by connecting diodes in a bridge-like manner. The voltage regulator is utilised to transform the 12v DC to 6v DC, and the circuit is subsequently powered by this DC voltage. The bridge rectifier and load must both have a constant 230v AC supply for the light sensor circuit to operate constantly.

Fig. 5. Circuit diagram of the LDR


When the resistance is low around 100 ohms. As can be seen in the light sensor circuit above, the power source passes through the LDR and ground through the variable resistor and resistor. This is because the resistance provided by the light-dependent resistor is lower than the resistance of the remaining components of the sensor circuit during the day or when light is falling on the LDR. We are aware of the current principle, which states that current always flows in the direction of least resistance.

Types of LDR sensors:

There are two kinds of Light-dependent Resistor
1. Intrinsic Photo Resistors
2. Extrinsic Photo Resistors

1. Intrinsic Photo Resistors:

These resistors are pure semiconductors made of germanium or silicon. The number of charge carriers increases because of the electrons being stimulated from the valence band to the conduction band when light strikes the LDR.

2. Extrinsic Photo Resistors

The impurities that are added to these devices result in the creation of new energy bands above the valence band. There are electrons inside these bands. As a result, the band gap is reduced, and less energy is needed to move them. Long wavelengths are the principal application for these resistors.

Another kind of classification is done based on linearity,

  • Because of their linear performance and method of operation, liner-type LDRs are sometimes referred to as photoresistors instead of photodiodes.
  • The polarity via which they unite does not affect the behavior's of non-linear LDRs, even though they are frequently utilized.

Advantages:

  • Sensitivity is High
  • Simple & Small devices
  • Easily used
  • Inexpensive
  • There is no union potential.
  • The light-dark resistance ratio is high.
  • Its connection is simple

Disadvantages: 

  • The spectral response is limited.
  • The best materials have limited temperature stability due to the hysteresis effect.
  • Its chemical reaction in stable materials.
  • LDR Sensor is only used in situations when the light signal fluctuates dramatically.
  • It is not a particularly responsive tool.
  • As soon as the operating temperature changes, it gives the wrong results.

Applications:

  • The photoresistor is typically used to measure light intensity and presence.
  • Used in lights that turn on and off automatically based on light
  • Smoke Detector Alarm, Automatic Lighting Clock
  • Design of optical circuits
  • Proximity switch for photos
  • Security measures utilising lasers
  • Solar street lighting
  • Light metres for cameras
  • Radio clocks
  • Can be used in dynamic compressors; some compressors adjust the signal gain by connecting LDR and LED to the signal source.

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