LIGHT DEPENDENT RESISTOR(LDR)
OR
PHOTO RESISTOR
Introduction:
• A light dependant resistor(LDR) or a photo resistor is a light controlled variable resistor. Its resistance changes with the light intensity that falls on it.
• The resistance of a photoresistor decreases with increasing incident light intensity. In other words, it exhibits photoconductivity.
• The resistance range and sensitivity of a photoresistor can substantially differ among dissimilar devices.
Typical leaded light dependent resistor
• They are made up of semiconductor materials having high resistance.
• Photocells or LDRs are non linear devices. There sensitivity varies with the wavelength of light incident on them.
Some photocells might not at all response to a certain range of wavelengths.
Working Principle of Photoresistor (LDR)
So how exactly does a photoresistor (i.e. a light dependent resistor or LDR) work? Photoresistors work based off of the principle of photoconductivity. Photoconductivity is an optical phenomenon in which the material’s conductivity is increased when light is absorbed by the material.
When light falls i.e. when the photons fall on the device, the electrons in the valence band of the semiconductor material are excited to the conduction band. These photons in the incident light should have energy greater than the bandgap of the semiconductor material to make the electrons jump from the valence band to the conduction band.
Hence when light having enough energy strikes on the device, more and more electrons are excited to the conduction band which results in a large number of charge carriers. The result of this process is more and more current starts flowing through the device when the circuit is closed and hence it is said that the resistance of the device has been decreased.
Types of Light Dependent Resistors (LDRs or Photoresistors)
Light dependent resistors, LDRs or photoresistors fall into one of two types or categories:
Intrinsic photoresistors: Intrinsic photoresistors use un-doped semiconductor materials including silicon or germanium. Photons fall on the LDR excite electrons moving them from the valence band to the conduction band. As a result, these electrons are free to conduct electricity. The more light that falls on the device, the more electrons are liberated and the greater the level of conductivity, and this results in a lower level of resistance.
Extrinsic photoresistors: Extrinsic photoresistors are manufactured from semiconductor of materials doped with impurities. These impurities or dopants create a new energy band above the existing valence band. As a result, electrons need less energy to transfer to the conduction band because of the smaller energy gap.
Regardless of the type of light dependent resistor or photoresistor, both types exhibit an increase in conductivity or fall in resistance with increasing levels of incident light.
Construction of a Photocell
The structure of a light-dependent resistor consists of a light-sensitive material which is deposited on an insulating substrate such as ceramic. The material is deposited in a zigzag pattern in order to obtain the desired resistance and power rating. This zigzag area separates the metal deposited areas into two regions.
Then the ohmic contacts are made on either sides of the area. The resistances of these contracts should be as less as possible to make sure that the resistance mainly changes due to the effect of light only. Materials normally used are cadmium sulfide, cadmium selenide, indium antimonide, and cadmium sulfide. The use of lead and cadmium is avoided as they are harmful to the environment.
Characteristics of Photoresistor (LDR)
Photoresistor LDR’s are light-dependent devices whose resistance is decreased when light falls on them and that is increased in the dark. When a light dependent resistor is kept in dark, its resistance is very high. This resistance is called as dark resistance. It can be as high as 1012 Ω and if the device is allowed to absorb light its resistance will be decreased drastically. If a constant voltage is applied to it and the intensity of light is increased the current starts increasing. The figure below shows the resistance vs. illumination curve for a particular LDR.
When light is incident on a photocell it usually takes about 8 to 12 ms for the change in resistance to take place, while it takes one or more seconds for the resistance to rise back again to its initial value after removal of light. This phenomenon is called a resistance recovery rate. This property is used in audio compressors.
Also, LDR’s are less sensitive than photodiodes and phototransistors. (A photo diode and a photocell (LDR) are not the same, a photo-diode is a pn junction semiconductor device that converts light to electricity, whereas a photocell is a passive device, there is no pn junction in this nor it “converts” light to electricity)
Applications of Photoresistors (LDRs)
Photoresistors (LDRs) have low cost and simple structure and are often used as light sensors. Other applications of photoresistors include:
· Detect absences or presences of light like in a camera light meter.
· Used in street lighting design (can be combined with a good Arduino starter kit to act as a street light controller)
· Alarm clocks
· Burglar alarm circuits
· Light intensity meters
· Used as part of a SCADA system to perform functions such as counting the number of packages on a moving conveyor belt
Advantages
LDR’s are cheap and are readily available in many sizes and shapes. Practical LDRs are available in a variety of sizes and package styles, the most popular size having a face diameter of roughly 10 mm. They need very small power and voltage for its operation.
Disadvantages
Highly inaccurate with a response time of about tens or hundreds of milliseconds.
Interference of Arduino with PHOTORESISTOR ON TINKERCAD
Schematic:
Code:
int sensorValue = 0;
void setup()
{
pinMode(A0, INPUT);
Serial.begin(9600);
pinMode(9, OUTPUT);
}
void loop()
{
//read value from the sensor
sensorValue = analogRead(A0);
// print the sensor reading so as you know its rangeSerial.println(sensorValue);
// map the sensor reading to a range for the led
analogWrite(9 , map(sensorValue, 0, 1023, 0, 255));
delay(100);// wait for 100 millisecond(s)
}Photodiode
Definition: A special type of PN junction device that generates current when exposed to light is known as Photodiode. It is also known as photodetector or photosensor. It operates in reverse biased mode and converts light energy into electrical energy.
The figure below shows the symbolic representation of a photodiode:
Principle of Photodiode
It works on the principle of Photoelectric effect.
The operating principle of the photodiode is such that when the junction of this two-terminal semiconductor device is illuminated then the electric current starts flowing through it. Only minority current flows through the device when the certain reverse potential is applied to it.
Difference between diode and photo diode:
Construction of Photodiode
The figure below shows the constructional detail of a photodiode:
The PN junction of the device placed inside a glass material. This is done to order to allow the light energy to pass through it. As only the junction is exposed to radiation, thus, the other portion of the glass material is painted black or is metallised.
The overall unit is of very small dimension nearly about 2.5 mm.
It is noteworthy that the current flowing through the device is in micro-ampere and is measured through an ammeter.
Operational Modes of Photodiode
Photodiode basically operates in two modes:
· Photovoltaic mode: It is also known as zero-bias mode because no external reverse potential is provided to the device. However, the flow of minority carrier will take place when the device is exposed to light.
· Photoconductive mode: When a certain reverse potential is applied to the device then it behaves as a photoconductive device. Here, an increase in depletion width is seen with the corresponding change in reverse voltage.
Let us now understand the detailed circuit arrangement and working of the photodiode.
Working of Photodiode
In the photodiode, a very small reverse current flows through the device that is termed as dark current. It is called so because this current is totally the result of the flow of minority carriers and is thus flows when the device is not exposed to radiation.
The electrons present in the p side and holes present in n side are the minority carriers. When a certain reverse-biased voltage is applied then minority carrier, holes from n-side experiences repulsive force from the positive potential of the battery.
Similarly, the electrons present in the p side experience repulsion from the negative potential of the battery. Due to this movement electron and hole recombine at the junction resultantly generating depletion region at the junction.
Due to this movement, a very small reverse current flows through the device known as dark current.
The combination of electron and hole at the junction generates neutral atom at the depletion. Due to which any further flow of current is restricted.
Now, the junction of the device is illuminated with light. As the light falls on the surface of the junction, then the temperature of the junction gets increased. This causes the electron and hole to get separated from each other.
At the two gets separated then electrons from n side gets attracted towards the positive potential of the battery. Similarly, holes present in the p side get attracted to the negative potential of the battery.
This movement then generates high reverse current through the device.
With the rise in the light intensity, more charge carriers are generated and flow through the device. Thereby, producing a large electric current through the device.
This current is then used to drive other circuits of the system.
So, we can say the intensity of light energy is directly proportional to the current through the device.
Only positive biased potential can put the device in no current condition in case of the photodiode.
Characteristics of Photodiode
The figure below shows the VI characteristic curve of a photodiode:
Here, the vertical line represents the reverse current flowing through the device and the horizontal line represents the reverse-biased potential.
The first curve represents the dark current that generates due to minority carriers in the absence of light.
As we can see in the above figure that all the curve shows almost equal spacing in between them. This is so because current proportionally increases with the luminous flux.
The figure below shows the curve for current versus illumination:
It is noteworthy here that, the reverse current does not show a significant increase with the increase in the reverse potential.
Advantages of Photodiode:
· It shows a quick response when exposed to light.
· Photodiode offers high operational speed.
· It provides a linear response.
· It is a low-cost device.
Disadvantages of Photodiode:
· It is a temperature-dependent device. And shows poor temperature stability.
· When low illumination is provided, then amplification is necessary.
Applications of Photodiode
1. Photodiodes majorly find its use in counters and switching circuits.
2. Photodiodes are extensively used in an optical communication system.
3. Logic circuits and encoders also make use of photodiode.
4. It is widely used in burglar alarm systems. In such alarm systems, until exposure to radiation is not interrupted, the current flows. As the light energy fails to fall on the device, it sounds the alarm.
In case of a typical photodiode, the normal reverse current is in tens of microampere range.
Interference of Arduino with Photodiode on Tinkercad
Schematic:
Code:
int ir;
void setup( )
{
Serial.begin(9600);
pinMode(6,OUTPUT);
pinMode(A0,INPUT);
}
void loop( )
ir=analogRead(A0);
if(ir>120)
{
digitalWrite(6,HIGH);
}
else{
digitalWrite(6,LOW);
}
}
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