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The term “Temperature” is a physical parameter and it can be measured in units of degrees. It is a critical part of any method to measure precise temperature. The usual applications needful accurate temperature quantities include medical, electrical or electronic studies, biology research, materials research, etc. A Temperature Sensor is used to measure the heat energy amount that let us detect a physical variation in temperature, generating either an analog or digital output.

Types of Basic Physical Temperature Sensors

In general, there are two types of sensing methods are available, namely contact type temperature sensors and non-contact type temperature sensors.

Contact Type Temperature Sensor

The contact type temperature sensor is in physical contact with the substance or object. These sensors are used to measure the temperature of solids, liquids or gases.

Non-contact Type Temperature Sensor

Non-contact type temperature sensors are used to detect temperature by interrupting a part of the emitted IR energy of the substance and detecting its intensity. These sensors can be used to measure only the temperature of solids and liquids. It is not potential to use them on gases due to their transparent nature.

Types of Temperature Sensors

The different types of temperature sensors available in the market can vary from simple ON/OFF thermostatic devices to highly sensitive semiconductor devices. The two basic types of physical temperature sensors like contact and non-contact are further categorized into a voltage, resistive, and electromechanical sensors. The most commonly used temperature sensors are classified into three types such as Thermistors, thermocouples, and resistance temperature detectors.These sensors vary from each other in terms of working parameters. For modest temperature series applications, solid state sensors are also accessible which give the advantage of simple interface and incorporated signal conditioning.

Thermistor

The thermistor is a temperature sensing device that changes the physical resistance with the temperature. Generally, these devices are made from ceramic material semiconductor like cobalt, manganese. It is shaped into small pressed hermetically closed discs that give a comparatively fast response to any temperature variations.

NTC Type Thermistor (Epcos)

Due to the properties of a semiconductor material, these devices have an NTC (negative temperature coefficient), i.e. the resistance falls with the rise in temperature. But, there are also Thermistors available with PTC (positive temperature coefficient), their resistance rises with the rise in temperature.

Application of a Thermistor

The thermistor is rated by their resistance value at room temperature, the time constant, and power rating. The thermistor is a passive resistive device, that requires current to generate an o/p voltage. Generally, they are connected in series with an appropriate biasing resistor to form a potential divider network.

Consider a thermistor with a value of resistance 2.5KΩ at 30°C of and 30Ω at 60°C. Here, the thermistor is connected in series with a 1kΩ resistor across a 5V power supply.

Here, the output voltage of this Thermistor can be calculated as

At 30°C, RNTC = 2500Ω;

Vout=1000/ (2500+1000)*5= 1.42V

At 60°C, RNTC = 30Ω;

Vout=1000/(30+1000)*5 = 4.85V

By substituting the specific resistor value with a potentiometer, we can get an o/p voltage at a prearranged temperature.

Though, it is significant to note that typical resistance values are dissimilar at room temperature for different types of thermistors since they are non-linear. Thermistor has an exponential transform with temperature; so it has a temperature constant (Beta) that is used to analyze its resistance for a given temperature. Though in a voltage divider network, the current which is obtained from the applied voltage is linear with temperature, therefore the o/p voltage across the resistor & temperature are linearly related.

Advantages of Thermistors

The advantages of Thermistors include the following.

• The speed of response is better to changes in accuracy, temperature, and repeatability.
• Compared to RTDs, it is inexpensive
• The range of higher resistance ranges from 2,000 to 10,000 ohms
• Sensitivity is Much higher, i.e., ~200 Ω/°C within a partial temperature range of up to 300 °C

Resistive Temperature Detectors

The term RTD stands for “Resistive Temperature Detector” is an electrical resistance temperature sensor that is made of coils of metals or films like platinum whose electrical resistance is a role of temperature.

Resistive Temperature Detectors have PTC (positive temperature coefficients) and not like Thermistors, they offer precise temperature measurements since they have linear output. But, they have poor sensitivity generating a small o/p change, for instance, 1Ω/°C for a temperature change. Pt100 has been the most frequently available sensor with a typical resistance value of 100Ω at 0°C. The main drawback is its high cost.

How to Use Resistive Temperature Detectors?

Resistive temperature detectors are passive resistor devices such as Thermistors and the flow of current through the sensor to get an o/p voltage is linearly connected to the temperature. But, an error can happen in the reading due to the variation in resistance caused by the self-heating of the flow of current through the resistive wires. To overcome this difficulty, a resistive temperature detector is connected to a resistive bridge n/w with additional connecting wires for lead compensation and/or adding of a constant current source.

Advantages of RTDs

The advantages of resistive temperature detectors include the following.

• The temperature range ranges from -200 to 650°C
• It offers a high o/p for a current drop
• RTDs are more linear compared to Thermistors and thermocouples.

Thermocouples

Thermocouples are the most commonly used temperature sensors due to some features like a wide temperature range from below -200°C to over 2000°C, accurate, and are relatively low-cost.

Construction and Working

The construction of a Thermocouple can be done with two different metals that are soldered together producing a slight potential difference (mV) as a function of temperature. One junction is kept at a constant temperature named the reference junction, while the other is the measuring junction. With the temperature difference between the two junctions, a voltage is established across the junction which is used to measure the temperature. The Seebeck effect can be defined as the voltage difference between the two junctions.

If both junctions are at the similar temperature, the potential difference through the junctions is zero, i.e. V1 = V2. However, when the junctions are at dissimilar temperatures connected in a circuit, the o/p voltage is comparable to the temperature difference between the two junctions, i.e. V1 – V2.

Types of Thermocouples

Thermocouples are existing in different materials and temperature ranges; therefore there are different kinds of thermocouples available for precise applications as set by international standards such as E, J, K, N, T, and U. Where, type J &K are the most frequently used thermocouples.

Advantages of Thermocouple

• Temperature ranges are high
• Rough, withstand shock and vibration
• Offers instant response to temperature changes

How to Use a Thermocouple?

Thermocouple gives a few millivolts output voltage for a temperature change of 10°C. So, it is mandatory to increase the output voltage.

It is essential to carefully choose an amplifier to get good drift stability for stopping recalibration of the thermocouple. This makes an Op-Amp (operational amplifier) desirable for most applications.

Thus, this is all about different types of temperature sensors.We hope that you have got a better understanding of this concept.Furthermore, any doubts regarding this concept or to implement any temperature sensor based projects please comment in the comment section below.

 

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?