Generally, the LDR is a photocell and it works on the principle of photoconductivity. A resistor is a passive component whose resistance is decreased when the intensity of light decreases. In the light varying sensor circuit, the optoelectronic devices are used mostly. The LDR is a device which is made up of high resistance semiconductor material. Some of the applications of LDR are street lights, clock radios, light alarm, reflective smoke alarm, outdoor clocks, and camera light meters. In this article, we will discuss the working & construction of light dependent resistor.
What is a Light Dependent Resistor?
The light dependent resistor (LDR) is designed with the highest resistance semiconductor metal. If the light falls on the device, then the photons produce the electron energy and it makes them bound into the conductive band. Hence it conducts electricity. The LDR circuit is also known as photo resistors. Here we will see the symbol of the LDR.
Working of Light Dependent Resistor
Here we are discussing the most common & simple working principle of the LDR. The working principle of the LDR is based on the photoconductivity and it is an optical phenomenon. The material conductivity is increased when the light is riveted by the material. If the light falls (photons) on the device, the electrons which are in the valence band of the semiconductor material are moved to the conduction band. These photons in the occurrence, the light will have the energy greater than the bandgap of the semiconductor material to move the electrons to valence band to the conduction band.
Therefore, therefore the light will strike the sufficient energy on the device and there is more electrons are eager to the conduct, therefore, the result will be in a large number of charge carriers. Hence the result of this process, when the circuit is closed more current is flowing through the device and the resistance of the device is decreased.
Construction of LDR
The construction of the light dependent resistor consists of light sensitive material which is deposited on an insulating substrate like a ceramic. To get the desired resistance & power rating the metal is deposited in the pattern of zigzag. This zigzag pattern separates the metal deposited areas into two regions and on both sides of the pattern the Ohmic contact is prepared.
The resistance of the contacts is very less to make sure that the resistance, mainly changes due to result of light. Generally to construct the light dependent resistance materials are used like cadmium sulfide, cadmium selenide, indium antimonide and cadmium sulphonide. The utilization of lead and cadmium are neglected because they are very destructive to the environment.
Circuit Diagram of Light Dependent Resistor
The light dependent circuit is an electronic circuit and the components required to build the circuit are LDR, relay, Darlington pair, diode and resistor. The voltage supply is set for the load and the necessary DC voltage of the LDR circuit is from the battery or a bridge rectifier circuit. The step down transformer is used in the bridge rectifier circuit and it transforms the voltage from the 230V to 12V.
In this circuit the diodes connected with the bridge form and it is used to alter the AC voltage into DC. To change 12V DC to 6V DC the voltage regulator is used and to the entire circuit the 6v DC is supplied. From the two bridge rectifiers 230V is supplied and the load is kept continuously in constant operation of the light sensor circuit.
The light sensor has low resistance in the morning time of 100ohms. Therefore the power supply passes through the LDR & ground through the variable resistor. Due to this, the resistance exists by the light dependent resistor in the daytime or when the light fall on the LDR, therefore it is comparatively very less to the resistance of the remaining parts of the circuit. As we know the principle of current, that the flow of current always flow in the path of low resistance.
Thus, the relay coil will not get enough supply to get strengthen. Hence the light is switched OFF in the day time. In the same process in the night time the resistance of the LDR increases to 200Mohms. Therefore, due to the high resistance of the resistor, the current flow is low or almost zero. In the low resistance the flow of current increases with the base voltage of Darlington pair to get more than 1.4V. Therefore the Darlington pair transistor is triggered with the relay coil obtain sufficient supply to get energized. Hence the light glows in the night times.
Characteristics of LDR
The LDR’s are light dependent devices and the resistance is decreased when the light falls on the light dependent circuit. If the LDR is in the dark room, its resistance is very high and it is called as a dark resistance. The light dependent resistance is high as 1012 ohms and if the device is permitted to absorb light its resistance will be decreased completely and if a constant voltage is applied to it. The intensity of light is increased, the current will increase. The following graph represent between the resistance and illumination curve for a LDR.
Applications of Light Dependent Resistor
- The price of the LDR is very less and it is very simple in structure
- In the light sensor the resistors are used frequently
- These resistors are used to sense the presence & absence of light like alarm circuits, alarm clock, light intensity meters and etc.
Advantages of LDR
- The price of the LDR is less and there are different sizes & shapes which are available in the market
- In the practical LDR there are different sizes are available and the most popular size is 100mm phase diameter
- For the operation of LDR it requires less power and voltage
Disadvantages of LDR
Extremely they are inexact with the reaction of time about tens, hundreds of milliseconds.
In this article we have discussed about tutorial on light dependent resistor and its applications. I hope by reading this article you have gained some basic information on the LDR working. If you have any queries about this article or about the electrical projects for engineering students, please feel free to comment in the below section. Here is the question for you, what are the functions of the light dependent resistor?