How LDR Sensor Works

What is an LDR?

An LDR or Light Dependent Resistor is also known as a photoresistor, photocell, or photoconductor. As the name states it is a special type of resistor that works on the principle of photoconductivity. This means that the resistance changes according to the intensity of light. Its resistance decreases with an increase in the intensity of light.

In the dark, it has a very high resistance, sometimes up to 1 MΩ, but when the LDR sensor is exposed to light, the resistance drops drastically, to as low as a few ohms, depending on the light intensity. The sensitivity of LDRs varies with the wavelength of the light applied and are nonlinear device. 

How does an LDR work?

This resistor works on the principle of photoconductivity. It states that when the light falls on the surface of an LDR, the conductivity of the material reduces. This is because the photons in the incident light have energy greater than the band gap of the semiconductor material used in the LDR. Making the electrons jump from the valence band to conduction.

LDR

These devices depend on the light, when light falls on the LDR then the resistance decreases, and the resistance increases in the dark. From the previous explanations, it can be deduced that when an LDR is kept in a dark place, its resistance is high and, when the LDR is kept in the light its resistance will decrease

The impact of light on an LDR

Variation of LDR Resistance with Variation in Light Intensity:

If a constant “V’ is applied to the LDR, the intensity of the light increases, and the current increases. The figure below shows the curve between resistance Vs illumination curve for a particular light-dependent resistor. As per the property of LDRs, the amount of light entering the LDR is inversely proportional to the resistance of the sensor, and the graph is hyperbolic.

Resistance vs Light Intensity graph

What is LDR used for?

Light Dependent Resistors are often used as light sensors. They are usually utilized when it is required to detect the presence and absence of light or to measure the light intensity. Examples are night lights and photography light meters. An interesting hobbyist application for light-dependent resistors is the line following robot, which uses a light source and two or more LDRs to determine the needed change of course. Sometimes, they are used in other sensing applications, for example in audio compressors, because their reaction to light is not instantaneous, and so the function of the LDR is to introduce a delayed response.

What are the types of LDR?

The classification of LDR can be done based on linearity or based on the photosensitive materials used in the LDR. The most frequently used materials while designing LDR mainly include cadmium sulfide, thallium sulfide, cadmium selenide & lead sulfide.

The classification based on the material are:

1. Intrinsic photoresistor:

These types of photoresistors are made with pure semiconductors without any doping. Pure semiconductor materials like germanium and silicon are used. When the incident light having an adequate amount of energy falls on this then electrons gain that energy and get excited, which leads to a few of them going to the conduction band.

2. Extrinsic Photoresistor:

These types of photoresistors use doped semiconductors. This means some impurities are mixed with the semiconductor such as phosphorus to make this photoresistor.

Extrinsic light-dependent resistors are generally designed for longer wavelengths of light, with a tendency towards infrared (IR).

The classification based on the linearity are:

1. Liner type LDRs:

They are called photodiodes however in some application areas, it is used as photoresistors because of the linear performance they present & their operation.

2. Non-linear LDRs:

They are commonly used but their behavior does not depend on the polarity through which it unites.

LDR Sensor Module or Photoresistor sensor Pin Diagram

LDR Sensor Module or Photoresistor sensor Pin Diagram

LDR sensor module is a low-cost digital sensor as well as an analog sensor module, which is capable of measuring and detecting light intensity. This sensor also is known as the Photoresistor sensor. This sensor has an onboard LDR (Light Dependent Resistor), which helps it to detect light. This sensor module comes with 4 terminals. 

Where the “DO” pin indicates the digital output pin and the “AO” pin indicates the analog output pin. The output of the module goes high during the absence of light and it becomes low in the presence of light. The sensitivity of the sensor can be adjusted using the onboard potentiometer.

The VCC pin indicates the +5 v power supply Input Pin and the GND indicates the ground (-) power supply Input Pin.

LDR Sensor Module or Photoresistor Sensor Hardware Overview

The LDR sensor module mainly consists of the LDR, LM393 Comparators, Variable Resistor (Trim pot), Power LED, and output LED.

1. LDR or Light Dependent Resistor

LDR or Light Dependent Resistor is one type of variable resistor. The features and functions have been previously discussed in detail. 

2. Variable Resistor (Trim pot)

The LDR sensor module has an onboard variable resistor or potentiometer, this variable resistor has a 10k preset. It is used to set the sensitivity of the LDR module. When the preset knob is rotated the sensitivity of the light intensity detection is varied.

If the preset knob is rotated in the clockwise direction, the sensitivity of the light intensity detection will increase.  If the preset knob is rotated in the counterclockwise direction, the sensitivity of the light intensity detection will decrease.

 3. Power LED

This onboard LED indicates if the power supply is ON or OFF. When we turn on the sensor power supply this Green LED is also turned on. 

4. Output LED

When the LDR sensor detects the light, the green LED is activated. When the LDR sensor detects darkness, the green LED is turned off.

How to make an LDR Sensor Module

The Components Required to build an LDR sensor module are IC: LM358 or LM393 (You can choose any one of these ICs), LDR or Light Dependent Resistor (R2), two 1K Resistor (R1, R4), 10K Resistor (R3), 10K Potentiometer (VR1), two 0.1 uF Ceramic Capacitor (C1, C2) and two Green LED (D1, D2).

Connect the components as shown in the circuit diagram to build an LDR sensor module:

1. +5 V power supply is taken 
2. 1K resistor is connected in series with the LED
3. Capasitor C1 is connected parallel to the above.
4. PIN 2 is connected to a 10K Potentiometer VR1.
5. The LDR which is parallel to capacitor C2 is connected to PIN 3.
6. The output DO is taken from PIN 1 of the IC.
7. The output AO is taken from PIN 3 of the IC.
8. Detection LED in series with a resistor R4  is also connected to PIN 1.

Circuit Diagram of an LDR sensor module

How LDR Sensor Module Works

The LDR sensor module has to be connected to a 5 V power supply. Then the threshold voltage needs to be set at the Non-Inverting input (3) of the IC according to the present light intensity by rotating the preset knob for setting the sensor sensitivity.

When light intensity increases on the surface of the LDR, the resistance of the LDR decreases. Which pimples that the maximum amount of voltage will be allocated across the resistor(R3). Therefore a Low amount of voltage from the LDR is given to the Inverting input (2) of the IC. Then the Comparator IC compares this voltage with the threshold voltage. In this case, the input voltage is less than the threshold voltage, so the sensor output goes LOW (0).

In contrast, when light intensity decreases (low/dark) on the surface of the LDR then the resistance of the LDR increases. Then the maximum amount of voltage will be allocated across the LDR (R2). Therefore, a High amount of voltage from the LDR is given to the Inverting input (2) of the IC. Then the Comparator IC compares this voltage with the threshold voltage. In this condition, this input voltage is greater than the threshold voltage, so the sensor output goes High (1).

Applications of an LDR Sensor:

LDR sensor modules are used where there is a need to sense the presence and absence of light is necessary. These resistors are used as light sensors and the applications of LDR mainly include alarm clocks, street lights, light intensity meters, and burglar alarm circuits.

Nowadays, the lightening-up of highways is done through HID or LED lamps but, the energy consumption of these lights is high as well as there is no particular mechanism to turn on/off the lights from sunrise to sunset other than it being done manually. To overcome this problem, an alternative method can be the power conserving of intensity-controlled street lights using LDR.

Conclusion

In this blog post, we have learnt that Light Dependent Resistors (LDRs) are essential components in various electronic applications and have wide-ranging use in industries such as lighting, photography, and security systems. The working principle of LDRs is simple yet effective, making it possible to sense light levels and trigger events based on ambient light conditions. With the knowledge of different types of LDRs, one can choose the right one for their specific project needs.

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