Unit – 4 Transducers and sensors:Definition and types of transducers

Definition and types of transducers. Basic characteristics of an electrical transducer, factors governing the selection of a transducer, Resistive transducer-potentiometer, Strain gauge and types (general description), Resistance thermometer-platinum resistance thermometer. Thermistor. Inductive Transducer-general principles, Linear Variable Differential Transducer (LDVT)- principle and construction, Capacitive Transducer, Piezo-electric transducer, Photoelectric transducer, Photovoltaic cell, photo diode and phototransistor – principle and working.

1. BSc in Physics 6
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
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PHY C16 – T: Electronic Instrumentation & Sensors(Theory)

Unit – 4: Transducers and Sensors

Definitions

1. Transducers:
 A transducer is a device that converts one form of energy into another.
 Transducers convert physical phenomena (like sound, light, temperature, etc.) into
electrical signals or vice versa.

 The process of conversion of one form to another is known as transduction.
 Thus, we can define a transducer as a device that converts non-electrical energy to
electrical energy.
 It consists of a sensor and a transduction element connected in series.

 Sensor: Detects a specific physical parameter and converts it into an electrical signal.
 Transduction Element: Processes the electrical signal from the sensor and converts it
into another form of energy or action.
 Examples of Transducers: Microphone, Loudspeaker

2. Sensor (Input Transducer):
 A sensor is a type of transducer that converts physical parameters (e.g., temperature,
sound, light) into electrical signals that can be measured and analyzed.

2. BSc in Physics 6
th
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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 Sensors are input devices because they take information from the environment and send
it to a system for processing.
 Examples of Sensors:
 Temperature Sensor: Thermistors.
 Light Sensor: Photodiodes, Phototransistor
 Pressure Sensor: Strain gauges, piezoelectric sensors.


3. Actuator (Output Transducer):
 An actuator is a type of transducer that converts electrical signals into physical movement
or other physical phenomena.

 Actuators are output devices because they take commands from a system and perform
physical action
 Examples of Actuators
 Electric Motor: Converts electrical energy into rotational mechanical movement.
 Speaker: Converts electrical signals into sound waves (acoustic energy).
Note:
 sensors and actuators, transducers convert physical phenomena (like sound, light,
temperature, etc.) into electrical signals or vice versa.
 While all sensors are transducers (since they convert physical stimuli into electrical
signals), not all transducers are sensors. Transducers can be broadly categorized into input
transducers (sensors) and output transducers (actuators).

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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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Example of a Mobile Phone Call System Using Sound Transducers:
The following diagram demonstrates how a mobile phone uses sound transducers to capture,
process, transmit, and output voice signals during a phone call.


1. Microphone (Sensor/Input Device):
o Function: Converts sound waves (acoustic energy) into electrical signals.
o Example: In a mobile phone, the microphone picks up the user's voice and converts
it into an electrical signal that can be processed and transmitted.

2. Amplifier (Transduction Element):
o Function: Processes and amplifies the electrical signal from the microphone to a
level suitable for transmission and driving the speaker.
o Example: The amplifier increases the signal strength to ensure the sound is loud
enough when output through the speaker.

3. Speaker (Actuator/Output Device):
o Function: Converts the amplified electrical signal back into sound waves (acoustic
energy).
o Example: In a mobile phone, the speaker outputs the received and processed voice
signal, making it audible to the listener.
.

4. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
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Types of Transducers
They can be classified main 5 types based on various criteria such as construction, energy source
requirement, output type, physical phenomenon, and transduction phenomenon.

I) Classification Based on Construction:
a) Mechanical Transducer: Converts physical quantities into mechanical signals.

Example: Mechanical Strain Gauge
 Measures strain (deformation) in a material by changing its electrical resistance.

b) Electrical Transducer: Converts physical quantities into electrical signals.

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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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Example: Potentiometer
 Measures displacement by varying resistance.

c) Optical Transducer: Converts physical quantities into optical signals.

Example: Photoelectric Transducer (Photodiode and Phototransistor)
 Converts light into electrical signals.

II) Types of Transducers Based on Source of Energy Requirement:
a) Active Transducer: These transducers generate electrical output directly from the physical
quantity without requiring an external power source.

Example: Photovoltaic/Solar Cell
 Converts light energy directly into electrical energy.

b) Passive Transducer: These transducers require an external power source to operate and
produce an output.

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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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Example: Resistance Thermometer (RTD):
 Measures temperature by changing resistance.
III) Types of Transducers Based on Its Output:
a) Analog Transducer: Produces a continuous output signal proportional to the measured
quantity.

Example: Thermistor
 Measures temperature through resistance variation.

b) Digital Transducer: Produces a discrete output signal, often in the form of digital data.


Example: Photo Diode
 Converts light into electrical signals that can be interpreted digitally.

IV) Types of Transducers Based on Physical Phenomenon:
a) Primary Transducer: A primary transducer converts the physical quantity into a mechanical
signal. They include mechanical as well as mechanical devices. The output of the primary
transducer is further converted by the secondary transducer.

7. BSc in Physics 6
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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b) Secondary Transducer: A secondary transducer converts the mechanical signal of the primary
transducer into an electrical signal. They are electrical circuits whose output signal magnitude is
proportional to the mechanical signal.
Example


VI) Classification Based on Transduction Phenomenon
a) Resistive Transducers
 Operate based on the change in resistance due to a physical phenomenon (like strain or
position).
 Examples include Potentiometers and Strain Gauges.
b) Temperature Transducers
 Convert temperature variations into electrical signals.
 Examples include Resistance Thermometers (RTDs), Thermistors, and Thermocouples.
c) Inductive Transducers
 Use changes in inductance to detect variations in a physical quantity.
 Examples include Inductive Proximity Sensors and Linear Variable Differential
Transformers (LVDTs).
d) Capacitive Transducers
 Utilize changes in capacitance to measure physical quantities.
 Examples include Capacitive Proximity Sensors and Capacitive Pressure Sensors.

8. BSc in Physics 6
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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f) Piezoelectric Transducers
 Generate electrical signals in response to mechanical pressure or stress.
 Examples include Piezoelectric Crystals used in accelerometers and ultrasound
transducers.
g) Photoelectric Transducers
 Convert light energy into electrical signals.
 Examples include Photovoltaic Cells, Photodiodes, and Phototransistors.

Basic Characteristics of an Electrical Transducer
An electrical transducer must have the following parameters:
1. Linearity: The relationship between a physical parameter and the resulting electrical signal
must be linear.

2. Sensitivity (S) is defined as the change in electrical output (Δy) per unit change in the
physical parameter (Δx). S=Δx/Δy (for example V/°C for a temperature sensor).

9. BSc in Physics 6
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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3. Dynamic Range:
 The operating range of the transducer should be wide, to permit its use under a wide range
of measurement conditions.
 It is typically expressed in terms of the minimum and maximum values of the physical
parameter x that the transducer can accurately measure.

4. Repeatability
 Repeatability ensures that the transducer provides consistent output for the same input
under the same conditions over time.
 It is often quantified by the variation in output signals measured over repeated
measurements of the same input.

5. Physical Size: The physical size of the transducer should be minimized to minimize its impact
on the system and environment.

Factors Governing the Selection of a Transducer
When selecting a transducer, several key factors should be considered to ensure optimal
performance for the intended application:
1. Operating Range: Define the range of input values (e.g., temperature, pressure) the
transducer can accurately measure. It should match the application's requirements.
2. Sensitivity: This refers to the transducer's ability to detect small changes in the input signal.
Higher sensitivity is crucial for applications requiring precise measurements.
3. Frequency Response and Resonant Frequency: Evaluate how the transducer responds
to varying frequencies of the input signal. The resonant frequency indicates the peak
response; operating near this frequency can yield better performance.
4. Environmental Compatibility: Consider the operating environment (e.g., temperature
extremes, humidity, exposure to chemicals). The transducer should be durable and reliable
under these conditions.

10. BSc in Physics 6
th
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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5. Minimum Sensitivity: Determine the lowest detectable input level. This is important for
applications requiring detection of very small signals.
6. Accuracy: Assess how close the output signal is to the actual input value. Higher accuracy
is necessary for critical measurements.
7. Usage and Ruggedness: The transducer should withstand physical stresses, vibrations,
and potential impacts, especially in industrial applications.
8. Electrical Parameters: Review the electrical characteristics such as impedance, power
requirements, and output signal type (analog, digital) to ensure compatibility with other
system components.
9. Response Time: This is the time taken by the transducer to respond to changes in the input
parameter. Faster response times are crucial in dynamic applications where rapid changes
occur.
10. Physical Size and Mounting: The physical dimensions of the transducer should fit within
the available space in the application. Considerations such as mounting options and ease
of installation are also important.
11. Cost: The cost of the transducer should be balanced with its performance requirements and
the budget constraints of the application. Factors such as maintenance and operational costs
should also be considered.
I)Resistive Transducer
 Resistive transducers are devices that change their electrical resistance in response to a
physical phenomenon such as mechanical strain, temperature change, or displacement.
 The following are common types of Resistive Transducer
i) Potentiometer
ii) Strain Gauge
iii) Platinum Resistance Thermometer
iv) Thermistor

11. BSc in Physics 6
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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i) Potentiometer
 A potentiometer is a type of resistive transducer that measures the position of a wiper
sliding along a resistive element, converting mechanical motion into an electrical signal.

Construction:
 It consists of a resistive element (either linear or circular) and a sliding contact (wiper).

Working Principle
 Resistance Element: This is the component that changes resistance based on the position
of a sliding contact (wiper).
 Sliding Contact (Wiper): This moves along the resistance element, creating a voltage
output that corresponds to its position.
 Operation: As the wiper moves along the resistive path, it divides the total resistance into
two parts, producing an output voltage that varies linearly with the wiper position,
depending on the input voltage applied across the ends of the resistive element.

Types of Potentiometers
1. Translatory (Linear) Potentiometers: These potentiometers measure linear
displacement. They typically consist of a straight resistive track with a sliding wiper that
moves along the length of the track.

12. BSc in Physics 6
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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Examples:
 Linear position sensors in industrial automation systems.
 Slide potentiometers used in audio mixing consoles.

2. Rotational Potentiometers: These are designed to measure angular displacement. They
usually feature a circular resistive path where the wiper moves in a rotational manner.

Examples:
 Rotary knobs used in consumer electronics, such as volume controls and tuning
dials.
 Encoders in robotic joints to measure angular position.

3. Helipot (Helical Potentiometers): Helipots are multi-turn potentiometers that consist of a
helical (coiled) resistive element. They can measure both translatory and rotational
movements, allowing for greater range and resolution.

13. BSc in Physics 6
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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Examples:
 Precision control systems requiring fine adjustments, such as in aerospace applications.
 Multi-turn volume controls in high-end audio systems.

Advantages of Potentiometers
 Cost-Effective: Generally inexpensive and readily available.
 Simplicity: Easy to install and use.
 Large Displacement Measurement: Capable of measuring substantial positional changes.
 High Electrical Efficiency: Provides sufficient output for many control applications.

Disadvantages of Potentiometers
 Mechanical Wear: The sliding contact can wear out over time, leading to inaccuracies.
 Output Noise: Mechanical movement may introduce noise and misalignment issues.
 Force Requirement: Linear potentiometers may require considerable force to move the
wiper.

Applications
 Volume Controls: In audio equipment.
 Position Sensors: In robotics and automation.
 Feedback Systems: In various engineering applications to provide real-time data on
displacement.

14. BSc in Physics 6
th
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
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ii) Strain Gauge
Strain Gauges are devices used to measure strain on an object. They work based on the principle
that a change in length or diameter of a conductor, when subjected to strain, leads to a change in
its electrical resistance.

Construction
 Material: Typically made from fine wire or metallic foil arranged in a grid pattern.
 Carrier/Base: The wire or foil is attached to a thin backing material (like paper, bakelite,
or teflon) for mounting.
 Adhesive: Strong adhesive bonds the gauge to the material under test, ensuring accurate
strain transfer.

Working Principle
 When the material deforms (stretches or compresses), the strain gauge also deforms.
 This deformation alters the length and cross-sectional area of the wire or foil.
 The output resistance change is proportional to the strain, allowing for precise
measurements.

15. BSc in Physics 6
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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Types of Strain Gauges
1) Wire strain gauges (Bonded and Unbonded, Grid type, Rossette type, Torque type, Helical
type)
2) Foil strain gauges
3) Semiconductor strain gauges

1)Wire Strain Gauges
Wire strain gauges are devices that measure strain through changes in electrical resistance,
typically made from fine wire arranged in various patterns for sensitivity and precision.

a) Bonded Strain Gauges
 Bonded strain gauges are affixed to the surface of a material using a specialized adhesive.
This ensures that any deformation of the substrate (the material to which the gauge is
attached) is transferred to the gauge itself.

 Typically made from a fine wire (often made of constantan or other alloy) arranged in a
specific pattern, which maximizes sensitivity and measurement accuracy.
 Common substrates include metals, composites, and other materials, allowing for a wide
range of applications.

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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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b) Unbonded Strain Gauges:
 These gauges are not fixed to the substrate, allowing them to measure strain without
direct contact. This configuration can be useful in specific experimental setups or for
measuring strain in fluids or gases.

 Often consist of a wire loop that can stretch or compress freely, making them sensitive
to strain without being constrained by a substrate.

c) Grid Type
 The wire is arranged in a grid pattern (typically a zig-zag or serpentine shape) to
increase sensitivity and gauge length. This design allows for effective strain
measurement across a specified area.

 Commonly used for general-purpose strain measurement in various engineering
applications.

17. BSc in Physics 6
th
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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d) Rosette Type:
 Consists of multiple grids oriented at specific angles (typically 0°, 45°, and 90°) to
measure strain in multiple directions simultaneously. This is particularly useful for
analyzing stress in complex loading conditions.

 Ideal for components subjected to multi-directional stresses, such as in structural
testing.

e) Torque Type:
 Designed to measure torque by being wrapped around a shaft. The strain gauge detects
the twisting action and converts it to a readable electrical signal.

18. BSc in Physics 6
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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 Widely used in automotive and mechanical engineering to monitor the performance of
rotating machinery.
e) Helical Type:
 This type is constructed in a helical shape, allowing it to measure torsional strain
effectively. The helical design can capture the effects of twisting and bending.

 Useful in applications where torsional stress is significant, such as in rotating shafts
or structural components under twisting loads.

2)Foil Strain Gauges
Construction:
 Made from a thin metallic foil pattern etched onto a flexible backing material. The foil is
typically made of materials like constantan or nickel.
 The gauge is bonded to the surface of the material being measured.

19. BSc in Physics 6
th
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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Working Principle:
 Similar to wire gauges, foil strain gauges detect changes in resistance as they deform. The
change in resistance occurs due to the elongation or compression of the foil pattern when
the material bends or stretches.
 They provide high accuracy and stability and are widely used due to their ease of
installation and sensitivity.
3)Semiconductor Strain Gauges
Construction:
 Made from silicon or other semiconductor materials. The gauge is typically integrated into
a microelectronic circuit.
 They may be mounted on a flexible substrate or directly on the surface of the material.

Working Principle:
 Semiconductor strain gauges have a much higher gauge factor (sensitivity) compared to
metallic gauges, meaning they produce a greater change in resistance per unit strain.
 The working principle is based on the piezoresistive effect, where the electrical resistivity
of the semiconductor material changes significantly under mechanical stress.
 This high sensitivity makes them ideal for applications where small strains need to be
measured.

20. BSc in Physics 6
th
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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Advantages of Semiconductor Strain Gauges
1. High Sensitivity: Semiconductor strain gauges have a much higher gauge factor
(sensitivity) than metallic gauges, allowing for the detection of very small strains.
2. Compact Size: They can be made very small, enabling use in applications where space is
limited.
3. Fast Response Time: Semiconductor gauges typically respond quickly to changes in
strain, making them suitable for dynamic measurements.
4. Temperature Stability: They can be designed to have good temperature coefficients,
improving measurement accuracy over a range of temperatures.
5. Cost-Effectiveness: Although often more expensive than metallic gauges, the high
sensitivity may reduce the need for additional amplification and signal processing,
potentially lowering overall costs.
Disadvantages of Semiconductor Strain Gauges
1. Nonlinearity: Their response to strain can be nonlinear, complicating calibration and
interpretation.
2. Temperature Sensitivity: They can be more sensitive to temperature changes than
metallic gauges, requiring careful temperature compensation in measurements.
3. Brittleness: Semiconductor materials are generally more brittle than metallic ones, making
them susceptible to damage under mechanical stress or impact.
4. Complex Circuitry: The signal output often requires more complex electronic circuitry
for conditioning and processing, which can increase system complexity.
5. Limited Range: They may have a more limited range of operation compared to some
metallic strain gauges, particularly under extreme conditions.

21. BSc in Physics 6
th
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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iii) Platinum Resistance Thermometer(PTR)
A Platinum Resistance Thermometer (PRT) is a highly accurate temperature sensor that utilizes
the predictable change in electrical resistance of platinum with temperature variations. It is widely
used in scientific and industrial applications for precise temperature measurements.
Construction:

 Sensing Element: The core component is a thin wire or film of platinum, typically
wound around a ceramic or glass substrate. The purity of platinum and its resistance-
temperature characteristics are critical for accuracy.
 Wiring: The platinum wire is connected to lead wires, allowing for the measurement
of resistance. In some designs, a 3-wire or 4-wire configuration is used to minimize
errors due to lead resistance.
 Encapsulation: The sensing element is often encapsulated in a protective sheath (made
of materials like stainless steel or glass) to protect it from environmental conditions
while ensuring good thermal conductivity.

22. BSc in Physics 6
th
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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Working Principle
 The working principle of a PRT is based on the fact that the electrical resistance of platinum
increases linearly with temperature.
 The resistance R at a given temperature T can be described by the formula
R(T)=R0(1+α(T−T0))
where:
R0 is the resistance at a reference temperature T0,
α is the temperature coefficient of resistance for platinum.
 As the temperature changes, the resistance of the platinum wire changes predictably. By
measuring the resistance using a precision ohmmeter or Wheatstone bridge, the
corresponding temperature can be accurately determined.
Advantages of Platinum Resistance Thermometers (PRTs)
1. High Accuracy: PRTs offer excellent precision in temperature measurement, often with
an accuracy of ±0.1 °C or better.
2. Wide Temperature Range: They can operate effectively over a broad temperature range,
typically from -200 °C to +850 °C, depending on the construction.
3. Stability and Repeatability: PRTs exhibit minimal drift over time, providing consistent
measurements under the same conditions.
4. Linear Response: The relationship between resistance and temperature is nearly linear,
simplifying calibration and interpretation of readings.
5. Long Lifespan: With proper handling and installation, PRTs can last for many years,
making them cost-effective in the long term.

23. BSc in Physics 6
th
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PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

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Disadvantages of Platinum Resistance Thermometers (PRTs)
1. Cost: PRTs are generally more expensive than other temperature sensors, such as
thermocouples or thermistors.
2. Fragility: The sensing element can be delicate, especially if not properly protected, making
them susceptible to physical damage.
3. Response Time: PRTs can have slower response times compared to other temperature
sensors, especially in high-speed applications.
4. Complexity of Installation: They may require more complex installation and wiring,
particularly in configurations using multiple wires to minimize lead resistance.

iv) Thermistor
 Thermistor (THERMally sensitive resISTOR) are non-metallic resistors (semiconductor
material), made by sintering mixtures of metallic oxides such as manganese, nickel, cobalt,
copper and uranium.
 Thermistors are widely used in applications such as home appliances, automotive systems,
and industrial equipment due to their high sensitivity and precision.

24. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 24
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

Construction:

 Material: Thermistors are usually made from ceramic materials or metal oxides that exhibit
specific resistance-temperature characteristics.
 Shape: They can be formed into different shapes, such as beads, discs, or thin films, depending
on the intended application.
 Encapsulation: Thermistors are often encapsulated in protective materials like glass or epoxy
to shield them from environmental factors, ensuring durability and reliability.
 Electrical Leads: They typically have two or more leads for electrical connections, allowing
integration into circuits for temperature sensing.
Working Principle

25. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 25
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

 Thermistors operate based on their resistance changing with temperature.
 NTC (Negative Temperature Coefficient) thermistors decrease in resistance as temperature
increases, while PTC (Positive Temperature Coefficient) thermistors increase in resistance
with rising temperature.
 When connected in a circuit, the change in resistance alters the voltage across the
thermistor.
 This voltage change is measured and converted to temperature using calibration data, often
facilitated by a microcontroller or analog circuitry.

Types of Thermistors
a) NTC (Negative Temperature Coefficient) Thermistors
 Resistance decreases as temperature increases. This property allows for precise
temperature measurement, especially in the lower temperature ranges.
 Typically made from metal oxides such as manganese, nickel, cobalt, and copper, often
mixed to create a ceramic composite that exhibits the desired thermistor characteristics.
 Widely used in temperature measurement and control systems, including: Digital
thermometers, Battery management systems, Temperature compensation circuits

b) PTC (Positive Temperature Coefficient) Thermistors
 Resistance increases with rising temperature, often displaying a sharp transition at a
specific temperature, known as the "switching temperature."
 Commonly made from barium titanate and other ceramic materials, which provide the
necessary characteristics for PTC behavior.
 Frequently employed in applications such as: Over-temperature protection circuits,
Resettable fuses, Motor protection circuits

26. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 26
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

Advantages of Thermistors
1. High Sensitivity Thermistors, especially NTC types, exhibit significant resistance changes
with small temperature variations, making them highly sensitive.
2. Fast Response Time: They can quickly respond to temperature changes, which is
beneficial for applications requiring rapid feedback.
3. Compact Size: Thermistors can be made small, allowing for integration into compact
electronic devices and systems.
4. Cost-Effective: Generally, thermistors are less expensive than some other temperature
sensing devices, such as RTDs and certain types of thermocouples.
5. Good Linearity: NTC thermistors can be designed to provide a relatively linear response
over a limited temperature range, simplifying calibration and measurement.
Disadvantages of Thermistors
1. Limited Temperature Range: Thermistors typically operate effectively only within a
certain temperature range (usually from -55°C to 125°C for NTCs), which may not suit all
applications.
2. Non-Linear Response: The resistance-temperature characteristic can be nonlinear,
especially outside the optimal range, complicating temperature measurement and
calibration.
3. Temperature Drift: Over time, thermistors may experience drift, leading to changes in
resistance characteristics, which can affect accuracy.
4. Fragility: Thermistors, particularly those made of ceramic materials, can be brittle and
susceptible to damage under mechanical stress.
5. Self-Heating: When a current flows through the thermistor, it can generate heat, potentially
affecting temperature readings, especially in low-power applications.

27. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 27
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

II) Inductive Transducer
 Inductive transducers operate based on the principle of electromagnetic induction. They
convert mechanical motion or changes in physical parameters into electrical signals using
inductance variations.

 When an object moves in the vicinity of an inductive coil, it alters the magnetic field, which
induces a change in the inductance of the coil. This change can be measured and correlated
to the physical quantity being sensed, such as displacement, pressure, or force.
 The inductance of the coil depends upon the reluctance of the magnetic circuits. The self-
inductance of the coil is given by

 The reluctance of the iron part is negligible compared to that of the air gap.

28. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 28
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

Linear Variable Differential Transformer (LVDT)
 The Linear Variable Differential Transformer (LVDT) is an electromechanical device used
to measure linear displacement with high accuracy and precision.
 It operates on the principle of electromagnetic mutual induction, converting mechanical
motion into an electrical signal that is proportional to the displacement, can be used to
measure displacement, pressure, and force.

Construction:
 LVDT consists of one primary coil and two secondary coils wounded on a cylindrical core.
The core is made up of a ferromagnetic material such as iron core and is freely movable
inside the coil and this movement measures the physical quantity.

 The primary winding is connected to an AC source. The two secondary winding S1 and
S2 have an equal number of turns and are set up in series opposition. So the e.m.f induced
in these winding are 180° out of phase with each other and thus the net effect is cancelled.
 The entire assembly is enclosed in a protective casing to ensure durability and
environmental protection, allowing for easy integration into various systems.

29. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 29
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

Working Principle:
 When an alternating voltage input is given in the primary winding, an alternating emf is
induced in the secondary winding(S1 and S2).
 Suppose V1 is the voltage induced across S1 and V2 is the voltage induced across S2. The
overall output voltage across the secondary winding(V0) is the difference between V1 and
V2. So the differential output is Vo =V1-V2
 The value of V0 depends on the position of the movable core influences the coupling
between the primary coil and the two secondary coils. Three possible cases are illustrated
in the following figure.

Case 1: When the core is positioned at its null position
When the core is positioned at the centre , voltages induced across winding S1 and S2 are equal(but
in reverse-phase). Then, the resultant voltage V0=0. In this case, we say there is no displacement.


Case 2: When The Core is Moving Towards S1
 When the core of LVDT moves towards the second winding S1 then the flux linkage S1
will be more as compared to S2. The EMF induced in S1 will be more than the EMF of S2.
 Hence v1 is greater than v2 & net differential voltage Vo(V1-V2) will be positive. The
means output voltage Vo will be in phase with input AC voltage.

30. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 30
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

Case 3 : When The Core Moving Towards S2
 When the core of LVDT moves towards secondary winding S2 then the flux linkage with
S2 will be more than S1. It means the EMF induced in S2 will be more than the induced
EMF of S1

 Hence V2 is greater than V1 & net differential voltage Vo (V1-V2) will be negative. It
means the output voltage will be out of phase input AC voltage.
 Hence the difference between the two secondary voltages provides a linear output that is
proportional to the core's displacement.

31. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 31
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

Advantages of LVDT
1. High Precision: LVDTs provide very accurate measurements with minimal hysteresis and
non-linearity.
2. No Contact: Since the core moves without physical contact with the coils, wear and tear
are minimized, leading to a long operational lifespan.
3. Wide Range: They can measure a broad range of displacements, from a fraction of a
millimeter to several centimeters.
4. Robustness: They are resistant to environmental factors such as dust, moisture, and
vibration, especially when properly housed.
Disadvantages of LVDT
1. Sensitivity to External Magnetic Fields: LVDTs can be affected by stray magnetic fields,
which may lead to inaccurate readings.
2. Requires AC Power: They need an AC power source for operation, which may limit their
use in battery-operated applications.
3. Size: While compact, LVDTs may be bulkier than some other displacement sensors, such
as potentiometers.
Applications of LVDT
1. Industrial Automation: Used for position feedback in hydraulic and pneumatic actuators.
2. Aerospace: Employed in aircraft control systems for precise measurement of control
surface positions.
3. Robotics: Utilized for joint and actuator position sensing.
4. Civil Engineering: Applied in structural health monitoring and displacement
measurements in bridges and buildings.

32. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 32
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

III)Capacitive Transducer
 A capacitive transducer is a type of sensor that converts a physical quantity (like
displacement, pressure, or temperature) into an electrical signal based on the principle of
capacitance.
 It utilizes the variation in capacitance caused by changes in distance between its plates or
variations in the dielectric material between them.

Construction:


The basic construction of a capacitive transducer consists o
 Two conductive plates: These act as the capacitor plates, which can be flat, cylindrical,
or spherical.
 Dielectric material: The space between the plates can be filled with air or a dielectric
material, which influences the capacitance.
 Support structure: Holds the plates and ensures proper alignment.
 Electrical connection: Allows the output signal to be transmitted.

33. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 33
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

Working Principle:
 When the physical quantity being measured changes, it affects either the area of the plates,
the distance between them, or the dielectric constant, leading to a change in capacitance.

 This change in capacitance can be measured and converted into an electrical signal.
 The capacitance CCC of a capacitor is given by the formula:
C = ƐA/d
Where:
C = capacitance
ε = permittivity of the dielectric material
A = area of one of the plates
d = distance between the plates


The capacitive transducer uses the following three effects.
1. Variation in capacitance of transducer is because of the overlapping of capacitor plates.

34. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 34
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

2. The change in capacitance is because of the change in distances between the plates.

3. The capacitance changes because of dielectric constant.

In the diagram above, the distance between two plates is denoted with ‘d’ and the width of
every plate is denoted with ‘b’ & the dielectric material’s displacement is denoted with ‘x’
& the overlapped region is denoted with ‘I’.

∆C = Ɛobx/d (Ɛr-1)

Advantages of Capacitive Transducer
 High Sensitivity: Capacitance changes are often very sensitive to variations in distance or
material properties.
 Wide Range of Measurements: Can be used for measuring various physical quantities.
 No Mechanical Wear: Lack of moving parts reduces maintenance and wear.
 Fast Response Time: Can respond quickly to changes.

35. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 35
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

Disadvantages of Capacitive Transducer
 Sensitivity to Environmental Factors: Performance can be affected by temperature,
humidity, and other environmental conditions.
 Calibration Needs: May require regular calibration for accurate measurements.
 Limited Range: May not be suitable for very large displacements or extreme conditions.
Applications of Capacitive Transducer
 Displacement Sensors: Used in applications like position sensing and proximity detection.
 Pressure Sensors: Utilized in various pressure measurement systems.
 Touch Sensors: Employed in touch screens and other user interfaces.
 Liquid Level Measurement: Used to monitor fluid levels in tanks.
 Medical Devices: Applied in devices for measuring physiological parameters.

IV) Piezo-Electric Transducer
 A piezoelectric transducer is a device that converts mechanical stress into electrical energy
(or vice versa) using the piezoelectric effect.

 This effect occurs in certain materials that generate an electric charge in response to applied
mechanical force.

36. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 36
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

Construction:

The basic construction of a piezoelectric transducer includes:
 Piezoelectric Material: Common materials include quartz (Rochelle salt and Barium
titanate), ceramics (Lead Zirconate Titanate (PZT)), and certain polymers.
 Electrodes: Metal electrodes are attached to the surface of the piezoelectric material to
collect the generated electrical charges.
 Housing: A protective casing that supports and encloses the transducer.

Working Principle:
 When mechanical stress is applied to the piezoelectric material, it causes a displacement of
charge within the material’s crystal structure, leading to a generation of voltage. The
generated voltage is proportional to the applied mechanical stress, allowing for precise
measurement.
V=d⋅σ
Where:
V = Voltage generated
d = Piezoelectric coefficient (units: C/N)
σ = Applied mechanical stress (units: N/m²)

37. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 37
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

 Conversely, applying an electric field to the material can induce mechanical deformation.
S=d⋅E
Where:
S = Mechanical strain (dimensionless)
d = Piezoelectric coefficient (C/N)
E = Electric field (units: V/m)
 The relationship can also be represented in terms of charge Q generated, which is related
to the applied stress and the area A of the piezoelectric element:
Q=d⋅A⋅σ
Where:
Q = Electric charge (C)
A = Area of the piezoelectric element (m²).[uum,
Advantages of Piezo-Electric Transducer
 High Sensitivity: Capable of detecting very small changes in pressure or force.
 Wide Frequency Range: Suitable for applications involving high-frequency signals.
 Robustness: Generally durable and can operate in harsh environments.
 No External Power Required: Generates its own electrical signal from mechanical stress.

Disadvantages of Piezo-Electric Transducer
 Non-linear Response: The output may not be linear across all ranges of applied stress.
 Temperature Sensitivity: Performance can be affected by temperature changes.
 Limited Displacement Measurement: Generally not effective for large displacements or
static measurements.

Applications of Piezo-Electric Transducer
 Medical Devices: Used in ultrasound transducers and pressure sensors.
 Industrial Sensors: Employed for monitoring vibrations and pressures in machinery.
 Consumer Electronics: Found in microphones and speakers.
 Automotive Applications: Used in airbag sensors and piezoelectric actuators.
 Structural Health Monitoring: Utilized for detecting stress and strain in structures.

38. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 38
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

V)Photoelectric Transducer
A photoelectric transducer is a device that converts light energy into electrical energy using the
photoelectric effect. It is commonly used in applications such as light sensors, optical
communication, and safety systems.
Construction:

 Material: Typically made from semiconductors or metals that exhibit the photoelectric
effect.
 Electrodes: Conductive surfaces that collect the emitted electrons.
 Housing: A protective casing to shield the internal components from environmental
factors.
Working Principle:
 When light (photons) strikes the surface of the photoelectric material, it transfers energy to
electrons.
 If the energy of the photons is sufficient to overcome the material's work function, electrons
are emitted from the surface.
 The emitted electrons can be collected by electrodes, creating a measurable electric current
proportional to the intensity of the light.

39. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 39
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

 The photoelectric transducer absorbs the radiation of light which falls on
their semiconductor material.
 The absorption of light energises the electrons of the material, and hence the electrons
start moving.
The mobility of electrons produces one of the three effects.
1. The resistance of the material change
2. The output current of the semiconductor change
3. The output voltage of the semiconductor changes.
 Photoelectric Transducer can be categorized as photo emissive, photo-conductive or photo-
voltai, photo diode and phototransistor.

a) Photovoltaic Cell
 A photovoltaic (PV) cell, commonly known as a solar cell, is a device that converts light
energy directly into electrical energy through the photovoltaic effect.

 It is widely used in solar panels for renewable energy applications, harnessing sunlight to
generate electricity.

40. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 40
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

Construction:

 Semiconductor Material: Typically made from silicon (monocrystalline, polycrystalline,
or amorphous), which has favorable electronic properties.
 P-N Junction: Formed by doping one layer of silicon with phosphorus (n-type) and
another with boron (p-type), creating a junction that facilitates charge separation.
 Antireflective Coating: A thin layer that reduces reflection of light, allowing more
photons to enter the cell.
 Transparent Conductive Layer: Often made from materials like indium tin oxide (ITO),
allowing light to pass through while conducting electricity.
 Back Contact: Conductive layer at the rear side that collects and transmits the generated
electric current.
Working Principle
o When sunlight hits the PV cell, photons are absorbed by the semiconductor material, particularly
at the p-n junction.
o The absorbed energy excites electrons, creating electron-hole pairs (free electrons and positive
holes).
o The electric field at the p-n junction drives the electrons toward the n-type layer and the holes
toward the p-type layer.
o This movement of charge carriers generates a direct current (DC) as the electrons flow through an
external circuit from the n-type layer to the p-type layer, allowing the cell to power electrical
device

41. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 41
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

b) Photo Diode
 A photodiode is a semiconductor device that converts light into electrical current.
 It operates on the principle of the photoelectric effect, making it essential in various
applications such as optical communication, light sensing, and medical devices.

Construction:

 Semiconductor Material: Commonly made from silicon, but other materials like
germanium or InGaAs may also be used for specific applications.
 P-N Junction: Formed by doping two layers of semiconductor: one with p-type (positive)
and the other with n-type (negative) material.
 Optical Window: Allows light to enter the device, enhancing sensitivity.
 Electrical Contacts: Metal contacts on the p and n sides to collect the generated current.

42. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 42
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

Working Principle
 When light photons strike the photodiode, they are absorbed in the semiconductor material.
 The absorbed energy excites electrons, creating electron-hole pairs.
 In reverse-bias mode, the electric field at the p-n junction separates the electron-hole pairs,
driving electrons toward the n-side and holes toward the p-side.

 This movement generates a measurable current that is proportional to the intensity of the
incident light.

Advantages of Photo Diode
 High Sensitivity: Capable of detecting low levels of light.
 Fast Response Time: Quick response to changes in light intensity, making it suitable for
high-speed applications.
 Compact Size: Small and lightweight, making them easy to integrate into various devices.

Disadvantages of Photo Diode
 Temperature Sensitivity: Performance can be affected by temperature changes.
 Non-linear Response: The current-voltage characteristics can be non-linear, requiring
calibration.
 Limited Detection Range: Not suitable for very high light intensities without saturation.

43. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 43
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

c) Phototransistor
 A phototransistor is a semiconductor device that combines the functions of a photodetector
and an electronic switch.
 It converts light energy into electrical energy and amplifies the resulting electrical signal,
making it suitable for various light-sensing applications.
 The sensitivity of a photo diode can be increased by as much as 100 times by adding a
junction, resulting in an NPN device.

Construction:

 Transistor Structure: Similar to a conventional bipolar junction transistor (BJT) but
includes a light-sensitive base region.
 P-N Junctions: Comprises a p-type and n-type semiconductor forming the emitter, base,
and collector regions.
 Optical Window: Allows light to enter the base region, enhancing sensitivity to light.
 Electrical Contacts: Metal contacts on the emitter and collector for current flow.

44. BSc in Physics 6
th
Semester
PHY C16 – T: Electronic Instrumentation & Sensors Unit – 4: Transducers and Sensors

Notes by Mr. Chandrakantha T S, Dept. of PG Studies & Research in Electronics Kuvempu University, Jnanasahyadri Shankaraghatta,2023-24
P a g e | 44
For more Notes visit :https://sites.google.com/view/chandrakanthats/teaching

Working Principle
o When light photons strike the phototransistor, they are absorbed in the base region.
o The absorbed light energy creates electron-hole pairs within the base material.
o The generated electrons are attracted to the collector, allowing a larger current to flow from
the collector to the emitter. The output current is proportional to the light intensity.
o The phototransistor can operate as a switch, turning on or off in response to light exposure.

Advantages of Phototransistor
 Light Amplification: Provides signal amplification, allowing for greater sensitivity than
standard photodiodes.
 Wide Dynamic Range: Effective across various light levels, from low to high intensity.
 Compact Size: Small form factor makes them suitable for integration into electronic
devices.
Disadvantages of Phototransistor
 Slower Response Time: Generally slower than photodiodes due to the additional
amplification process.
 Temperature Sensitivity: Performance may vary with temperature changes.
 Less Linear Response: Output can be less linear compared to photodiodes, requiring
calibration in certain applications.
Applications of Phototransistor
 Optical Switches: Used in automatic lighting systems and remote controls.
 Light Sensing: Employed in cameras, smoke detectors, and security systems.
 Communication Systems: Utilized in fiber optic systems for light signal detection.
 Consumer Electronics: Found in devices like digital clocks and appliances that respond
to ambient light.
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