Temperature Sensors Quiz

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Questions and Answers

What determines the resistance of a Light Dependent Resistor (LDR)?

The resistance of an LDR is determined by the intensity of light; it is inversely proportional to that intensity.

What is the primary function of a photodiode?

A photodiode converts light into electrical current, particularly sensitive to longer wavelengths like red and infrared.

How does a phototransistor operate in response to light?

A phototransistor functions by allowing light to reach its base-collector junction, which controls its electrical output.

Explain the difference between absolute and incremental rotary encoders.

Absolute encoders provide a unique digital word for each position, while incremental encoders generate pulses corresponding to movement. Signup and view all the answers

What role does a light-sensitive switch play in electronics?

A light-sensitive switch uses an LDR to detect light, which then alters its resistance to trigger a transistor and activate a relay. Signup and view all the answers

Describe the basic operation of a photoelectric sensor.

A photoelectric sensor emits a light beam which, when interrupted or reflected by a target, signals a change in state. Signup and view all the answers

What is the significance of using transparent casings in photodiodes and phototransistors?

Transparent casings allow light to reach the sensitive areas of the devices, enhancing their responsiveness. Signup and view all the answers

How does the construction of a phototransistor differ from a regular transistor?

A phototransistor is encased in a transparent housing, as opposed to regular transistors which are typically not situated in such casings. Signup and view all the answers

What is the primary function of a thermistor?

A thermistor changes its resistance exponentially with temperature to measure temperature. Signup and view all the answers

How does a Wheatstone bridge circuit relate to thermistors?

A Wheatstone bridge circuit connects thermistors to produce an output voltage proportional to the resistance change. Signup and view all the answers

What is the output behavior of the LM35 temperature sensor?

The LM35 provides an output voltage that is linearly proportional to the Celsius temperature. Signup and view all the answers

Describe the construction and working principle of a bimetallic strip.

A bimetallic strip consists of two metals with different thermal expansion coefficients, causing it to deform with temperature changes. Signup and view all the answers

Why is measuring voltage across a p-n junction more preferable than measuring current?

Measuring voltage is more stable and provides a clearer indication of temperature variation than current measurement. Signup and view all the answers

What types of environments can optical sensors detect electromagnetic radiation?

Optical sensors can detect electromagnetic radiation across the broad optical range, from far infrared to ultraviolet. Signup and view all the answers

What is the significance of having calibrated sensors in degrees centigrade like the LM35?

Calibrated sensors like the LM35 simplify temperature readings by providing direct Celsius measurements. Signup and view all the answers

Explain the role of thermal expansion in the functioning of a bimetallic strip.

Thermal expansion causes the two metals in a bimetallic strip to differ in length, resulting in mechanical deformation. Signup and view all the answers

What is the primary function of a thrubeam type sensor?

To measure the change in light quantity caused by a target crossing the optical axis. Signup and view all the answers

How does a retroreflective type sensor operate?

It sends light to a reflector, which bounces it back to the receiver, and detects interruptions from targets. Signup and view all the answers

What are the different types of proximity sensors mentioned?

Inductive, capacitive, photoelectric, and magnetic sensors. Signup and view all the answers

Describe the basic principle of operation for a reed switch.

It consists of two ferromagnetic reeds that close the circuit when a magnetic field is present. Signup and view all the answers

What happens to the electrons in a magneto resistor when subjected to a magnetic field?

They start moving in an indirect motion, leading to increased collisions and higher resistance. Signup and view all the answers

What is the purpose of a Linear Variable Differential Transformer (LVDT)?

To measure linear displacement accurately. Signup and view all the answers

In what scenarios would capacitive sensors be best utilized?

For detecting both metallic and non-metallic objects, including liquids and plastics. Signup and view all the answers

What distinguishes inductive sensors from other types of proximity sensors?

They specifically detect the presence of metallic objects. Signup and view all the answers

What are the primary components of an LVDT and how does it function?

An LVDT consists of primary and secondary windings and a movable iron core. It functions by inducing voltages in the secondary coil in response to excitation in the primary coil. Signup and view all the answers

How does a Hall Effect sensor generate its output voltage?

A Hall Effect sensor generates an output voltage called Hall Voltage when the magnetic flux density around it exceeds a certain pre-set threshold. The output signal depends on the magnetic field density surrounding the device. Signup and view all the answers

Describe the function of a mechanical switch as a displacement sensor.

A mechanical switch acts as a displacement sensor by returning one bit of information: whether it is in a 'touched' or 'not touched' state. It typically uses a lever mechanism to create a mechanical contact that closes an electrical connection. Signup and view all the answers

What is a strain gauge and how is it configured for measuring strain?

A strain gauge is a transducer that measures strain in a mechanical component, typically made of a metal foil grid. It is configured in a Wheatstone bridge circuit to obtain voltage output based on the strain experienced by the component. Signup and view all the answers

Explain the basic working principle of an accelerometer.

An accelerometer measures acceleration, which is the rate of change of velocity of an object. It detects changes in motion and provides data regarding acceleration in various directions. Signup and view all the answers

What is the purpose of the large metallic pads in a strain gauge?

The large metallic pads in a strain gauge serve as connection points for lead wires, allowing for easy attachment with solder. These pads facilitate the electrical connections necessary for the measurement process. Signup and view all the answers

What type of signal is required to excite an LVDT for measurement?

An LVDT must be excited by an AC signal to induce a corresponding AC response in the secondary output. This is essential for accurate displacement measurement. Signup and view all the answers

Identify and briefly describe the three types of strain gauge assemblies.

The three types of strain gauge assemblies are uniaxial, which measures strain in one direction; biaxial, which measures strain in two perpendicular directions; and strain gauge rosette, which allows measurement of strain in multiple directions using multiple gauges. Signup and view all the answers

What is the key principle behind how piezoelectric accelerometers generate charge?

They generate charge in response to a force or deformation applied to piezo crystals. Signup and view all the answers

How does an inertial accelerometer measure acceleration?

It uses a mass-spring-damper system where the mass vibrates due to acceleration, resulting in frequency variation. Signup and view all the answers

What components can be used to construct a simple magnetic accelerometer?

A permanent magnet acting as a mass along with a Hall element or a magnetoresistive sensor. Signup and view all the answers

In what manner does a load cell convert force into an electronic signal?

It converts the load or force into a change in voltage, current, or frequency. Signup and view all the answers

What are MEMS and what primary functions do they serve?

MEMS are tiny integrated devices combining mechanical and electrical components, used for sensing, controlling, and acting at the micro scale. Signup and view all the answers

What defines the operational mechanism of an inertial accelerometer?

It operates on the principle of mass movement within a spring-damper system that responds to external accelerations. Signup and view all the answers

How are mechanical microstructures integrated into MEMS devices?

They are fabricated onto silicon chips using integrated circuit processing techniques. Signup and view all the answers

What role does the Hall element play in a magnetic accelerometer?

It senses the magnetic field changes caused by the motion of the permanent magnet. Signup and view all the answers

Study Notes

Thermistors

A type of semiconductor device whose resistance changes exponentially with temperature.
Thermistors are available in various shapes and sizes.
Thermistors have smaller operating ranges than RTDs.

p-n Junction Temperature Sensors

A forward-biased p-n junction exhibits temperature dependence in its current flow.
This current can be measured to determine temperature.
Alternatively, the voltage across the diode can be measured (usually preferred).
LM35 series are precision integrated-circuit p-n junction temperature sensors with output voltage linearly proportional to Celsius temperature.
LM35 sensors have three pins: Vcc (positive supply), Ground, and the analog output pin.
LM35 sensors are:
- Calibrated directly in degrees Celsius.
- Have a linear + 10.0 mV/˚C scale factor.
- Operate from 4 to 30 volts.

Bimetallic Strip

Composed of two or more metal layers with different coefficients of thermal expansion.
The strip can be straight or coiled for compactness.
Deforms due to temperature changes caused by differing thermal expansions.
Deflection can be correlated to the strip's temperature.
Used in thermostats to activate heating/cooling systems by making and breaking an electrical contact.

Optical Sensors

Detect electromagnetic radiation in the optical range (far Infrared to Ultraviolet).
Use direct transduction (light to electrical quantity) in photovoltaic sensors.
Use indirect transduction (light to temperature variation, then to electrical quantity).

Light Dependent Resistor (LDR)

A type of photo sensor that is cheap and robust.
Resistance varies continuously and inversely proportional to light intensity.

Photo-diode

Similar construction to a conventional PN-junction diode, but with a transparent casing or lens to focus light onto the PN junction for greater sensitivity.
Responds better to longer wavelengths (red and infrared) than visible light.

Photo-transistor

A light-sensitive transistor.
Typically, a photobipolar transistor is a bipolar transistor housed in a transparent case to allow light to reach the base–collector junction.

Optical Encoder

Converts motion into digital pulses.
Counts pulses to measure relative or absolute position measurements.
Available as linear or rotary configurations, with rotary being most common.
Absolute encoders have a unique digital word for each shaft position.
Incremental encoders generate pulses with rotation, measuring relative displacement.

Photoelectric Sensors

Emit a visible or infrared light beam.
Reflective type detects reflected light beam from a target.
Thrubeam type measures light changes from a target crossing the optical axis.
Retroreflective type uses a reflector to return emitted light to the receiving element, interruption by a target triggers detection.

Proximity Sensor

Detects the presence of an object without contact.
Employs magnetic, electrical capacitance, inductance, and eddy current methods.
Types:
- Inductive Sensors: Detect metallic objects.
- Capacitive Sensors: Detect metallic and non-metallic objects (liquids, plastics, woods).
- Photoelectric Sensors: Use light-sensitive elements.
- Magnetic Sensors: Detect permanent magnets.

REED SWITCH

Two flat ferromagnetic reeds sealed in an inert atmosphere within a glass capsule.
In the presence of a magnetic field, the reeds attract each other, closing the magnetic and electric circuit.

MAGNETO RESISTOR

Resistance increases with increasing magnetic field strength.
When subjected to magnetic force, free electrons become excited and move indirectly causing collisions, leading to increased resistance.

Linear Variable Differential Transformer/Transducer (LVDT)

Transducer for measuring linear displacement.
Consists of primary and secondary windings and a movable iron core.
Acts like a transformer, with voltages induced in the secondary coil when the primary coil is excited.
Must be excited by an AC signal to induce an AC response in the secondary.
Core position is determined by measuring the secondary response.

HALL EFFECT SENSOR

Activated by an external magnetic field.
Output signal is a function of the magnetic field density around the device.
Generates an output voltage (Hall Voltage) when magnetic flux density exceeds a threshold.

Mechanical Sensors

MECHANICAL SWITCH

Simplest displacement sensor, providing one bit of information (touching or not touching).
A typical microswitch includes a lever that, when depressed, creates a mechanical contact, closing an electrical connection.

STRAIN GAUGE

Most common transducer for measuring strain in mechanical components.
Consists of a thin metal foil (usually constantan) deposited as a grid pattern on a plastic backing (polyimide).
The foil pattern has metallic pads for attaching lead wires.
Typically small (5 to 15 mm long).
Connected to a Wheatstone bridge circuit to obtain voltage output.
Types:
- Uniaxial: Measures strain along a single axis.
- Biaxial: Measures strain along two perpendicular axes.
- Strain gauge rosette: Arranges three strain gauges to measure strain along multiple axes.

Accelerometers

Measure acceleration (rate of change of velocity).
Piezoelectric Accelerometer:
- Uses piezoelectric crystals (quartz, silicon dioxide, barium titanate, PZT) that produce a charge in response to applied force or deformation.
Inertial Accelerometer:
- Small mass-spring-damper system with high natural frequency.
- As an object moves, mass inside vibrates, with faster speed resulting in higher vibration or frequency.
Magnetic Accelerometers:
- Uses a permanent magnet as a mass on a spring or cantilever beam, position sensed by a Hall effect or magnetoresistive sensor.

Load Cell

Transducer that converts load or force into an electronic signal (voltage, current, or frequency change).

Potentiometer

A variable resistor with a movable wiper that slides along a resistive element.
The wiper's position changes resistance, which can be used for position sensing or voltage control.

MEMS (Micro-electromechanical Systems)

Technology used to create tiny integrated devices or systems combining mechanical and electrical components.
Fabricated using integrated circuit (IC) batch processing techniques.
Range in size from a few micrometers to millimeters.
Sense, control, and actuate at the micro-scale, generating effects on the macro scale.
Consist of mechanical microstructures, microsensors, microactuators, and microelectronics integrated on a silicon chip.

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Description

Test your knowledge on various temperature sensors including thermistors, p-n junction temperature sensors, and bimetallic strips. This quiz covers their properties, operational principles, and applications. Explore the differences and functionalities of each type.