Theory - 76.1 :- What is a Sensor? And Its Types

What is a Sensor?

There are numerous definitions as to what a sensor is but I would like to define a Sensor as an input device which provides an output (signal) with respect to a specific physical quantity (input).

The term “input device” in the definition of a Sensor means that it is part of a bigger system which provides input to a main control system (like a Processor or a Microcontroller).

Another unique definition of a Sensor is as follows: It is a device that converts signals from one energy domain to electrical domain. The definition of the Sensor can be better understood if we take an example in to consideration.

The simplest example of a sensor is an LDR or a Light Dependent Resistor. It is a device, whose resistance varies according to intensity of light it is subjected to. When the light falling on an LDR is more, its resistance becomes very less and when the light is less, well, the resistance of the LDR becomes very high.

We can connect this LDR in a voltage divider (along with other resistor) and check the voltage drop across the LDR. This voltage can be calibrated to the amount of light falling on the LDR. Hence, a Light Sensor.

Now that we have seen what a sensor is, we will proceed further with the classification of Sensors.

Classification of Sensors

There are several classifications of sensors made by different authors and experts. Some are very simple and some are very complex. The following classification of sensors may already be used by an expert in the subject but this is a very simple classification of sensors.

In the first classification of the sensors, they are divided in to Active and Passive. Active Sensors are those which require an external excitation signal or a power signal.

Passive Sensors, on the other hand, do not require any external power signal and directly generates output response.

The other type of classification is based on the means of detection used in the sensor. Some of the means of detection are Electric, Biological, Chemical, Radioactive etc.

The next classification is based on conversion phenomenon i.e., the input and the output. Some of the common conversion phenomena are Photoelectric, Thermoelectric, Electrochemical, Electromagnetic, Thermotic, etc.

The final classification of the sensors are Analog and Digital Sensors. Analog Sensors produce an analog output i.e., a continuous output signal (usually voltage but sometimes other quantities like Resistance etc.) with respect to the quantity being measured.

Digital Sensors, in contrast to Analog Sensors, work with discrete or digital data. The data in digital sensors, which is used for conversion and transmission, is digital in nature.

Different Types of Sensors

The following is a list of different types of sensors that are commonly used in various applications. All these sensors are used for measuring one of the physical properties like Temperature, Resistance, Capacitance, Conduction, Heat Transfer etc.

• Temperature Sensor
• Proximity Sensor
• Accelerometer
• IR Sensor (Infrared Sensor)
• Pressure Sensor
• Light Sensor
• Ultrasonic Sensor
• Smoke, Gas and Alcohol Sensor
• Touch Sensor
• Color Sensor
• Humidity Sensor
• Position Sensor
• Magnetic Sensor (Hall Effect Sensor)
• Microphone (Sound Sensor)
• Tilt Sensor
• Flow and Level Sensor
• PIR Sensor
• Touch Sensor
• Strain and Weight Sensor

We will see about few of the above-mentioned sensors in brief. More information about the sensors will be added subsequently. A list of projects using the above sensors is given at the end of the page.

Temperature Sensor

One of the most common and most popular sensors is the Temperature Sensor. A Temperature Sensor, as the name suggests, senses the temperature i.e., it measures the changes in the temperature.

 

There are different types of Temperature Sensors like Temperature Sensor ICs (like LM35, DS18B20), Thermistors, Thermocouples, RTD (Resistive Temperature Devices), etc.

Temperature Sensors can be analog or digital. In an Analog Temperature Sensor, the changes in the Temperature correspond to change in its physical property like resistance or voltage. LM35 is a classic Analog Temperature Sensor.

Coming to the Digital Temperature Sensor, the output is a discrete digital value (usually, some numerical data after converting analog value to digital value). DS18B20 is a simple Digital Temperature Sensor.

Temperature Sensors are used everywhere like computers, mobile phones, automobiles, air conditioning systems, industries etc.

Proximity Sensors

A Proximity Sensor is a non-contact type sensor that detects the presence of an object. Proximity Sensors can be implemented using different techniques like Optical (like Infrared or Laser), Sound (Ultrasonic), Magnetic (Hall Effect), Capacitive, etc.

 

Some of the applications of Proximity Sensors are Mobile Phones, Cars (Parking Sensors), industries (object alignment), Ground Proximity in Aircrafts, etc.

Infrared Sensor (IR Sensor)

IR Sensors or Infrared Sensor are light based sensor that are used in various applications like Proximity and Object Detection. IR Sensors are used as proximity sensors in almost all mobile phones.

 

There are two types of Infrared or IR Sensors: Transmissive Type and Reflective Type. In Transmissive Type IR Sensor, the IR Transmitter (usually an IR LED) and the IR Detector (usually a Photo Diode) are positioned facing each other so that when an object passes between them, the sensor detects the object.

The other type of IR Sensor is a Reflective Type IR Sensor. In this, the transmitter and the detector are positioned adjacent to each other facing the object. When an object comes in front of the sensor, the infrared light from the IR Transmitter is reflected from the object and is detected by the IR Receiver and thus the sensor detects the object.

Different applications where IR Sensor is implemented are Mobile Phones, Robots, Industrial assembly, automobiles etc.

Ultrasonic Sensor

An Ultrasonic Sensor is a non-contact type device that can be used to measure distance as well as velocity of an object. An Ultrasonic Sensor works based on the properties of the sound waves with frequency greater than that of the human audible range.

 

Using the time of flight of the sound wave, an Ultrasonic Sensor can measure the distance of the object (similar to SONAR). The Doppler Shift property of the sound wave is used to measure the velocity of an object..

 

Light Sensor

Sometimes also known as Photo Sensors, Light Sensors are one of the important sensors. A simple Light Sensor available today is the Light Dependent Resistor or LDR. The property of LDR is that its resistance is inversely proportional to the intensity of the ambient light i.e., when the intensity of light increases, its resistance decreases and vise-versa.

 

By using LDR is a circuit, we can calibrate the changes in its resistance to measure the intensity of Light. There are two other Light Sensors (or Photo Sensors) which are often used in complex electronic system design. They are Photo Diode and Photo Transistor. All these are Analog Sensors.

There are also Digital Light Sensors like BH1750, TSL2561, etc., which can calculate intensity of light and provide a digital equivalent value.

Smoke and Gas Sensors

One of the very useful sensors in safety related applications are Smoke and Gas Sensors. Almost all offices and industries are equipped with several smoke detectors, which detect any smoke (due to fire) and sound an alarm.

Gas Sensors are more common in laboratories, large scale kitchens and industries. They can detect different gases like LPG, Propane, Butane, Methane (CH4), etc.

Now-a-days, smoke sensors (which often can detect smoke as well gas) are also installed in most homes as a safety measure.

The “MQ” series of sensors are a bunch of cheap sensors for detecting CO, CO2, CH4, Alcohol, Propane, Butane, LPG etc. You can use these sensors to build your own Smoke Sensor Application.

Alcohol Sensor

As the name suggests, an Alcohol Sensor detects alcohol. Usually, alcohol sensors are used in breathalyzer devices, which determine whether a person is drunk or not. Law enforcement personnel uses breathalyzers to catch drunk-and-drive culprits.

 Alcohol-Sensor-MQ3

A simple tutorial on HOW TO MAKE ALCOHOL BREATHALYZER CIRCUIT?

Touch Sensor

We do not give much importance to touch sensors but they became an integral part of our life. Whether you know or not, all touch screen devices (Mobile Phones, Tablets, Laptops, etc.) have touch sensors in them. Another common application of touch sensor is trackpads in our laptops.

 

Touch Sensors, as the name suggests, detect touch of a finger or a stylus. Often touch sensors are classified into Resistive and Capacitive type. Almost all modern touch sensors are of Capacitive Types as they are more accurate and have better signal to noise ratio.

Color Sensor

A Color Sensor is an useful device in building color sensing applications in the field of image processing, color identification, industrial object tracking etc. The TCS3200 is a simple Color Sensor, which can detect any color and output a square wave proportional to the wavelength of the detected color.

Humidity Sensor

If you see Weather Monitoring Systems, they often provide temperature as well as humidity data. So, measuring humidity is an important task in many applications and Humidity Sensors help us in achieving this.

Often all humidity sensors measure relative humidity (a ratio of water content in air to maximum potential of air to hold water). Since relative humidity is dependent on temperature of air, almost all Humidity Sensors can also measure Temperature.

 

Humidity Sensors are classified into Capacitive Type, Resistive Type and Thermal Conductive 

Type. DHT11 and DHT22 are two of the frequently used Humidity Sensors in DIY Community (the former is a resistive type while the latter is capacitive type).

Tilt Sensor

Often used to detect inclination or orientation, Tilt Sensors are one of the simplest and inexpensive sensors out there. Previously, tilt sensors are made up of Mercury (and hence they are sometimes called as Mercury Switches) but most modern tilt sensors contain a roller ball.

 

  

Theory - 76 :- Relay Principle ,its Types & Applications

A relay is an electrically operated switch. Many relays use an electromagnet to mechanically operate a switch, but other operating principles are also used, such as solid-state relays.

Relays are used where it is necessary to control a circuit by a separate low-power signal, or where several circuits must be controlled by one signal.

“A relay is a device designed to cause a sudden predicted change in a single

or multiple electrical output circuits when certain conditions are satisfied by

the electrical circuit that contains the relay device”.

 

Symbol

The above figure shows the most commonly used symbol of a relay. A1 and A2 represent its coil and 11, 12 & 14 represent its contacts.

 

 Relay Symbol 

Where do we use relays?

Relays have a wide range of application. You can find them everywhere: home appliances, automobiles, industries, and even in copy machines. In some applications, they are used for switching or control of circuit (as in timer-based lighting control) whereas in others they are used to sense and protect circuits (as in the case of earth fault protection relays). Therefore, it is difficult to specify their area of application area.

 

Principle of operation

They are basically classified into two types based on their working principle as electro-mechanical and solid-state relays. Let us discuss the principle of operation of each one of them in detail.

 

Operation of electromechanical relays

 

Electromechanical relays transfer signals between its contact through a mechanical motion. It consists of two sections: the first is the electromagnet section and the other is the armature and mechanical contacts section. The electromagnet section consists of a set of coil wound over a magnetic core.

 

When an input voltage (almost equal to the rated voltage of the coil) is applied to the coil, it gets magnetized and attracts the armature. The mechanical contacts are attached to the armature. Hence, when the armature is pulled towards the electromagnet, the contact closes. When the input voltage applied to the coil is removed, the armature is brought back to its original position by the spring release.

Operation of Solid-state relays

Solid-state relays are commonly known as SSRs. Unlike electromechanical type, they do not posses any mechanically moving parts. On the other hand, it consists of semiconductor and electronic components within. In solid-state relays, the electromagnetic section is replaced by optocoupler and required driver circuits and the output contact section is replaced by a TRIAC or transistor plus snubber and driver circuits.

 

When the rated voltage is applied to the input section, current flows through the optocoupler. The output of the optocoupler is used to operate the switching circuit of TRIAC or transistor. Switching circuit applies a gate pulse to the TRIAC and the TRIAC starts conducting. Similarly, when the applied input voltage is removed, the optocoupler turns off the TRIAC switching circuit and which, in turn, stops the gate pulse to the TRIAC and the TRAIC stops conducting.

 

In the following sections, we shall discuss in detail the parts and operation of electromechanical and solid-state relays in detail.

 

Parts of an electromechanical relay

A typical electromechanical relay consists of the following components:

 

1. Electromagnetic coil
2. Armature
3. Core
4. Movable contacts
5. Spring return arrangement

Electromagnetic coil

The electromagnetic coil is the most important part of an electromechanical relay. It consists of a set of copper windings over a magnetic core. As you know, the flow of current through the coil produces a magnetic field. Therefore, when voltage is applied to the coil, it becomes an electromagnet and attracts the armature.

 

Armature

An armature is a movable piece of metal, balanced using a pivot.

 

Core

Core is the metallic part over which the coil is wound.

 

Movable contacts and fixed contact

Contacts are the conducting parts inside the relay, that open or close when voltage is applied to its coil. The contact that is attracted by the electromagnet is called movable contact and that is stationary and connected to the terminals are called fixed contacts.

 

Spring arrangement

Spring arrangements are also present in them, such as to bring the armature and the contacts back to the original position when the coil is de-energized.

 

Parts of a solid-state relay

As discussed earlier, solid-state relays do not have any movable parts within. In order to explain their internal parts, here, we have split it to the following sections:

 

• Input circuit section
• Electrical isolation
• Driver circuitry
• Output section

1. Input Circuit Section                                            The input circuit consists of diodes transistors gates and resistors required to drive the optocoupler.
2. Electrical Isolation                                                  Unlike electromagnetic relays, the input and output sections of an SSR do not have any physical contacts. Galvanic separation is provided between them using optocouplers. 
3. Driver Circuit                                                        Driver circuits consist of components required to turn on the TRIAC or transistors or thyristors in the output circuit. The o/p of the optocoupler is      conditioned and the gate pulse needed to trigger the transistor is generated.                         
4. Output Section                                                          The output section consists of semiconductor devices such as transistor or TRIAC or thyristor as an alternative to relay contacts.

Application

Relays have a wide range of applications starting from washing machines at homes to the telecommunication systems at the International space station, they can be found everywhere. The following are a few key applications:

1. They are used in electronic circuits and home appliances for isolating low voltage or DC circuits from high voltage AC circuits.
2. They are the backbone of industrial process automation systems. They are used in combination with PLCs for process control. They are one of the key components in an automation cabinet.
3. Used for signaling and control in railway networks.
4. In motor control circuits for motor switching, protection as well as control.
5. In substations and power distribution centers for sensing various faults and operating the circuit breaker.

Relay selection considerations

The following factors must be considered while selecting a relay for any application.

• Nominal Voltage: The voltage at which the coil is designed to operate.
• Rated Power: The power consumed by the coil at normal room temperature.
• Contact Rating: The current carrying capacity and voltage rating of their contacts
• Contact Mechanism: The number of contacts required and the contact configuration (NO/NC/changeover).
• Environmental Protection: the degree of sealing required, meaning, whether the external casing of relay is necessary or not?
• Insulation Resistance: Insulation resistance between any two sets of contacts and that between the contacts and the coil.

PR - 69 :- Study of Gears, Belts, stepper motor and Drive

 

We can say that a gear is a kind of machine element which has teeth cut around a cylindrical or cone-shaped surface with equal spacing? They are used to transmit rotations and forces from the driving shaft to the driven shaft when a pair of these elements have meshed. There are different types of gear. They have a different basis of classification. The history of gears is old and is mentioned by Archimedes to be in use in ancient Greece in B.C.

Gears can be classified as parallel shaft gears, intersecting shaft gears and non-intersecting non-parallel shaft gears depending on the position of their axes.

Different Types and Applications of Gears

Internal Gear

These gears have teeth cut on the inside part of cones and cylinders and are used to pair with external gears. These are used in shaft couplings which are of gear types and planetary gear drives. Due to trimming problems and interference such as trochoid and involute, there lies one disadvantage with this gear, which is an unequal number of internal and external gears.

Screw Gear

Screw gears or sometimes called crossed helical gears are helical gears used in motion transmission between non-intersecting shafts. In parallel shafts, the helical gears used have the same helix angle but in opposite directions. It consists of the same hand helical gears at an angle of 45 degrees on the non-intersecting and non-parallel shafts. It is used for small power transmission.

Worm Gear

It consists of two elements, a screw-shaped cut on the shaft called a worm and the other one is a mating gear called a worm wheel. These two together on a non-intersecting shaft form worm gear. material is used for worm and a soft one for worm wheels as it is necessary to reduce friction due to sliding contact of surfaces. They can have a cylindrical shape and also an hourglass type which increases the contact ratio but reduces the production. 

Mitre Gear

These are basic bevel gears with a speed ratio of 1. The direction of power transmission is changed by them without changing speed. They can be both straight and spiral. With spiral mitre gear thrust bearing is also used as it produces thrust force in the axial direction. Mitre gears with shaft angles other than 90° are called angular mitre gears.

Bevel Gear

These have a cone shape at their pitch surface and teeth are cut along the cone. They transmit force between two shafts that intersect at a point. Various kinds of bevel gears are helical bevel gears, spiral bevel gears, straight bevel gears, mitre gears, angular bevel gears, zero gears, hypoid gears and crowns bevel gears.

Spiral Bevel Gear

Bevel gears with curved tooth lines are called spiral bevel gears. They are superior to straight bevel gears in efficiency, strength, vibration and noise due to higher contact ratio but are difficult to produce. Since teeth are curved, it produces thrust force in the axial direction. These gears with zero twisting angles are called zero l bevel gears.

Spur Gear

Spur gears are included in the parallel shaft gear group. They are cylindrical gears having tooth lines straight and parallel to the shaft. Cylindrical gears are gears with cylindrical pitch surfaces. In meshing pairs, the larger one is called gear and the smaller one is pinion. They achieve high accuracy and are relatively easy to produce.

Gear Rack

A gear rack consists of same sized and shaped teeth cut at equal distances along a flat surface or a straight rod. It is a cylindrical gear having a radius of pitch infinity. It converts rotational motion into linear motion by meshing with a cylindrical gear pinion. Straight tooth racks and helical tooth racks are its broader classification.

Helical Gear

These gears can transmit high loads. They are very quiet and are cylindrical gear with winding tooth lines. Its two subdivisions are left-hand twist and right-hand twist.

Type of Gear	Characteristics
Spur	Most common type of gear Circular gear body Straight teeth cut or inserted parallel to the gear’s shaft Used for parallel axes configuration Mated with spur gears, internal gears, or gear racks High precision and efficiency (A) Easy to manufacture (A) Does not produce thrust force (A) Capable of handling high speed and high loads (A) Gear teeth experience high stress due to tooth design (D) Noise production during high speeds (D)
Helical	Circular gear body Teeth twisted at an angle around gear body Used for parallel axes configuration Available in right-hand and left-hand designs Available in single and double helical designs Gradual tooth engagement and less impact loading (A) Quieter, smoother operation (A) Capable of handling greater loads (A) Lower efficiency (D) Higher design complexity, greater cost of manufacturing (D) Single helical design products thrust force (D), double helical does not (A)
Bevel	Cone-shaped gear body Used for intersecting axes configuration Available in straight, spiral, and Zerol® bevel tooth designs Straight: simplest bevel gear design and easiest to manufacture (A); high impact, noise level, and stress (D) Spiral: gradual tooth engagement and less impact loading, noise, and vibration (A); higher design complexity and greater cost of manufacturing (D) Zerol®: Quieter and smoother than straight bevel, able to rotate in both directions unlike spiral bevel (A)
Worm	Pair comprised of a circular gear and a screw-shaped gear Used for non-parallel, non-intersecting axes configuration Large gear ratios and gear reduction (A) Quiet, smooth operation (A) Self-locking mechanism (A) Low transmission efficiency (D) Large amounts of friction (D)
Rack and Pinion	Pair comprised of a gear rack and cylindrical gear Used for parallel axes configuration Rack mated with spur or helical gear Converts rotational motion to linear motion or vice versa Simple design, easy to manufacture (A) Capable of handling greater loads (A) Transmission cannot continue infinitely in one direction (D) Large amount of backlash between mated teeth (D) Gear teeth experience high friction and stress due to tooth design (D)

Table 2 – Industries and Applications of Gears by Type

Type of Gear	Common Industries and Applications
Spur	Clocks Pumps Watering systems Household appliances Clothes washing and drying machines Power plants Material handling systems Aerospace and aircrafts Railways and trains
Helical	Same as spur gears but with greater loads and higher speeds (see above) Automobiles (transmission systems)
Bevel	Pumps Power plants Material handling systems Aerospace and aircrafts Railways and trains Automobiles
Worm	Instruments Lifts and elevators Material handling systems Automobiles (steering systems)
Rack         and       Pinion	Weighing scales Material handling and transfer systems Railways and trains Automobiles (steering systems)