IEEE Report on Advanced Sensors EC Department, GTU Gaurav Maniar, Karan Raithatha email@example.com firstname.lastname@example.org DIET, INDIAAbstract – This report explains different commonly There are many different types of transducersused six advanced autonomous sensors. It also available in the marketplace, and the choice ofdescribes their working, application and uses. which one to use really depends upon the quantity I. INTRODUCTION being measured or controlled, with the more Simple stand alone electronic circuits can be common types given in the table below.made to repeatedly flash a light or play a musicalnote, but in order for an electronic circuit or system A. Common Transducersto perform any useful task or function it needs to be Input type transducers or sensors, produce aable to communicate with the "real world" whether proportional output voltage or signal in response tothis is by reading an input signal from an changes in the quantity that they are measuring (the"ON/OFF" switch or by activating some form of stimulus) and the type or amount of the outputoutput device to illuminate a single light and to do signal depends upon the type of sensor being used.this we use Transducers. Generally, all types of sensors can be classed as Transducers can be used to sense a wide range of two kinds, passive and active.different energy forms such as movement, Active sensors require some form of externalelectrical signals, radiant energy, thermal or power to operate, called an excitation signal whichmagnetic energy etc, and there are many different is used by the sensor to produce the output signal.types of both analogue and digital input and output Active sensors are self-generating devices becausedevices available to choose from. The type of input their own properties change in response to anor output transducer being used, really depends external effect and produce an output voltage, forupon the type of signal or process being "Sensed" example, 1 to 10v DC or an output current such asor "Controlled" but we can define a transducer as a 4 to 20mA DC. For example, a strain gauge is adevice that converts one physical quantity into pressure-sensitive resistor. It does not generate anyanother. electrical signal, but by passing a current through it Devices which perform an input function are (excitation signal), its resistance can be measuredcommonly called Sensors because they "sense" a by detecting variations in the current and/or voltagephysical change in some characteristic that changes across it relating these changes to the amount ofin response to some excitation, for example heat or strain or force.force and covert that into an electrical signal. Unlike the active sensor, a passive sensor doesDevices which perform an output function are not need any additional energy source and directlygenerally called Actuators and are used to control generates an electric signal in response to ansome external device, for example movement. Both external stimulus. For example, a thermocouple orsensors and actuators are collectively known photodiode. Passive sensors are direct sensorsas Transducers because they are used to convert which change their physical properties, such asenergy of one kind into energy of another kind, for resistance, capacitance or inductance etc. As wellexample, a microphone (input device) converts as analogue sensors, Digital Sensors produce asound waves into electrical signals for the amplifier discrete output representing a binary number orto amplify, and a loudspeaker (output device) digit such as a logic level "0" or a logic level "1".converts the electrical signals back into soundwaves and an example of this is given below. II. ANALOGUE AND DIGITAL SENSORS A. Analogue Sensors Analogue Sensors produce a continuous output signal or voltage which is generally proportional to the quantity being measured. Physical quantities such as Temperature, Speed, Pressure, Displacement, Strain etc are all analogue quantities as they tend to be continuous in nature. For example, the temperature of a liquid can beFig. 1 Simple Input/output System using Sound Transducers measured using a thermometer or thermocouple
which continuously responds to temperature In our simple example above, the speed of thechanges as the liquid is heated up or cooled down. rotating shaft is measured by using a digital LED/Otto-detector sensor. The disc which is fixed to a rotating shaft, has a number of transparent slots within its design. As the disc rotates with the speed of the shaft, each slot passes by the sensor inturn producing an output pulse representing a logic level "1". These pulses are sent to a register of counter and finally to an output display to show the speed or revolutions of the shaft. By increasing the number of slots or "windows" within the disc more output pulses can be produced giving a greater resolution and accuracy as fractions of a revolution can be detected. Then this type of sensor arrangement could be used for positional control. Compared to analogue signals, digital signals or quantities have very high accuracies and can beFig. 2 Thermocouple used to produce an Analogue Signal both measured and "sampled" at a very high clock speed. The accuracy of the digital signal is Analogue sensors tend to produce output signals proportional to the number of bits used to representthat are changing smoothly and continuously which the measured quantity. For example, using aare very small in value so some form of processor of 8 bits, will produce an accuracy ofamplification is required. Then circuits which 0.195% (1 part in 512). While using a processor ofmeasure analogue signals usually have a slow 16 bits gives an accuracy of 0.0015%, (1 part inresponse and/or low accuracy. Also analogue 65,536) or 130 times more accurate. This accuracysignals can be easily converted into digital type can be maintained as digital quantities aresignals for use in microcontroller systems by the manipulated and processed very rapidly, millionsuse of analogue-to-digital converters, or ADCs. of times faster than analogue signals. In most cases, sensors and more specificallyB. Digital Sensors analogue sensors generally require an external As its name implies, Digital Sensors produce a power supply and some form of additionaldiscrete output signal or voltage that is a digital amplification or filtering of the signal in order torepresentation of the quantity being measured. produce a suitable electrical signal which is capableDigital sensors produce a Binary output signal in of being measured or used. One very good way ofthe form of a logic "1" or a logic "0", ("ON" or achieving both amplification and filtering within a"OFF"). This means then that a digital signal only single circuit is to use Operational Amplifiers asproduces discrete (non-continuous) values which seen before.may be outputted as a single "bit", (serialtransmission) or by combining the bits to produce a C. Signal Conditioningsingle "byte" output (parallel transmission). As we saw in the Operational Amplifier tutorial, op-amps can be used to provide amplification of signals when connected in either inverting or non- inverting configurations. The very small analogue signal voltages produced by a sensor such as a few milli-volts or even pico-volts can be amplified many times over by a simple op-amp circuit to produce a much larger voltage signal of say 5v or 5mA that can then be used as an input signal to a microprocessor or analogue-to-digital based system. Therefore, an amplification of a sensors output signal has to be made with a voltage gain up to 10,000 and a current gain up to 1,000,000 with the amplification of the signal being linear with the output signal being an exact reproduction of the input, just changed in amplitude. Then amplification is part of signal conditioning. So when using analogue sensors, generally some form of amplification (Gain), impedance matching, isolation between the input and output or perhapsFig. 3 Light Sensor used to produce an Digital Signal filtering (frequency selection) may be required
before the signal can be used and this is robots wheel to determine its distance travelledconveniently performed by Operational Amplifiers. along the ground. Either way, Position Sensors canAlso, when measuring very small physical changes detect the movement of an object in a straight linethe output signal of a sensor can become using Linear Sensors or by its angular movement"contaminated" with unwanted signals or voltages using Rotational Sensors.that prevent the actual signal required from beingmeasured correctly. These unwanted signals are A. The Potentiometercalled "Noise". This Noise or Interference can be The most commonly used of all the "Positioneither greatly reduced or even eliminated by using Sensors", is the potentiometer because it is ansignal conditioning or filtering techniques as we inexpensive and easy to use position sensor. It has adiscussed in the Active Filter tutorial. By using wiper contact linked to a mechanical shaft that caneither a Low Pass, or a High Pass or even Band be either angular (rotational) or linear (slider type)Pass filter the "bandwidth" of the noise can be in its movement, and which causes the resistancereduced to leave just the output signal required. For value between the wiper/slider and the two endexample, many types of inputs from switches, connections to change giving an electrical signalkeyboards or manual controls are not capable of output that has a proportional relationship betweenchanging state rapidly and so low-pass filter can be the actual wiper position on the resistive track andused. When the interference is at a particular its resistance value. In other words, resistance isfrequency, for example mains frequency, narrow proportional to position.band reject or Notch filters can be used to producefrequency selective filters. Where some randomnoise still remains after filtering it may benecessary to take several samples and then averagethem to give the final value so increasing thesignal-to-noise ratio. Fig. 5 Potentiometer Potentiometers come in a wide range of designs and sizes such as the commonly available round rotational type or the longer and flat linear slider types. When used as a positional sensor the moveable object is connected directly to the shaft or slider of the potentiometer and a DC reference voltage is applied across the two outer fixed connections forming the resistive element while the output signal is taken from the wiper terminal of Fig. 4 Op-amp Filters the sliding contact as shown below thus producing a potential or voltage divider type circuit output. Either way, both amplification and filtering play Then for example, if you apply a voltage of say 10van important role in interfacing microprocessor and across the resistive element of the potentiometerelectronics based systems to "real world" the maximum output voltage would be 10 volts andconditions. Now Positional Sensors which measure the wiper will vary the output signal from 0 to 10the position and/or displacement of physical objects volts, with 5 volts indicating that the wiper or slidermeaning the movement from one position to is at the half-way centre position.another for a specific distance or angle would beintroduced. III. POSITION SENSORS In this tutorial we will look at a variety ofdevices which are classed as Input Devices and aretherefore called "Sensors" and in particular thosesensors which are Positional in nature which meansthat they are referenced either to or from somefixed point or position. As their name implies, thesetypes of sensors provide a "position" feedback. Onemethod of determining a position, is to use either"distance", which could be the distance betweentwo points such as the distance travelled or movedaway from some fixed point, or by "rotation"(angular movement). For example, the rotation of a Fig. 6 Simple Positional Sensing Circuit
While resistive potentiometer position sensors If the soft iron magnetic core armature is exactlyhave many advantages: low cost, low tech, easy to in the centre of the tube and the windings, "nulluse etc, as a position sensor they also have many position", the two induced emfs in the twodisadvantages: wear due to moving parts, low secondary windings cancel each other out as theyaccuracy, low repeatability, and limited frequency are 180oout of phase, so the resultant output voltageresponse. But one main disadvantage of using the is zero. As the core is displaced slightly to one sidepotentiometer as a positional sensor is that the or the other from this null or zero position, therange of movement of its wiper or slide (and hence induced voltage in one of the secondaries will bethe output signal obtained) is limited to the physical become greater than that of the other secondary andsize of the potentiometer being used. For example a an output will be produced. The polarity of thesingle turn rotational potentiometer generally only output signal depends upon the direction andhas a fixed electrical rotation between about 240 to displacement of the moving core. The greater the330o however, multi-turn pots of up to 3600o of movement of the soft iron core from its central nullelectrical rotation are also available. Most types of position the greater will be the resulting outputpotentiometers use carbon film for their resistive signal. The result is a differential voltage outputtrack, but these types are electrically noisy (the which varies linearly with the cores position.crackle on a radio volume control), and also have a Therefore, the output signal has both amplitude thatshort mechanical life. Wire-wound pots also known is a linear function of the cores displacement and aas rheostats, in the form of either a straight wire or polarity that indicates direction of movement. Thewound coil resistive wire can also be used, but wire phase of the output signal can be compared to thewound pots suffer from resolution problems as primary coil excitation phase enabling suitabletheir wiper jumps from one wire segment to the electronic circuits such as the AD592 LVDTnext producing a logarithmic (LOG) output Sensor Amplifier to know which half of the coil theresulting in errors in the output signal. These too magnetic core is in and thereby know the directionsuffer from electrical noise. of travel. For high precision low noise applicationsconductive plastic resistance element type polymerfilm or cermets type potentiometers are nowavailable. These pots have a smooth low frictionelectrically linear (LIN) resistive track giving thema low noise, long life and excellent resolution andare available as both multi-turn and single turndevices. A typical application for this type of highaccuracy position sensor is in computer gamejoysticks, steering wheels, industrial and robotapplications.B. Inductive Position Sensors One type of positional sensor that does not sufferfrom mechanical wear problems is the "LinearVariable Differential Transformer" or LVDT forshort. This is an inductive type position sensorwhich works on the same principle as the ACtransformer that is used to measure movement. It isa very accurate device for measuring lineardisplacement and whose output is proportional tothe position of its moveable core. It basically consists of three coils wound on ahollow tube former, one forming the primary coiland the other two coils forming identicalsecondary‟s connected electrically together inseries but 180o out of phase either side of theprimary coil. A moveable soft iron ferromagneticcore (sometimes called an "armature") which isconnected to the object being measured slides ormoves up and down inside the tube. A small ACreference voltage called the "excitation signal"(2 - 20V rms, 2 - 20kHz) is applied to the primarywinding which inurn induces an EMF signal intothe two adjacent secondary windings. Fig. 7 The Linear Variable Differential Transformer
When the armature is moved from one end to the An inductive proximity sensor has four mainother through the centre position the output components; The oscillator which produces thevoltages changes from maximum to zero and back electromagnetic field, the coil which generates theto maximum again but in the process changes its magnetic field, the detection circuit whichphase angle by 180 degs. This enables the LVDT detects any change in the field when an objectto produce an output AC signal whose magnitude enters it and the output circuit which produces therepresents the amount of movement from the centre output signal, either with normally closed (NC) orposition and whose phase angle represents the normally open (NO) contacts. Inductive proximitydirection of movement of the core. A typical sensors allow for the detection of metallic objectsapplication of this type of sensor would be a in front of the sensor head without any physicalpressure transducers, were the pressure being contact of the object itself being detected. Thismeasured pushes against a diaphragm to produce a makes them ideal for use in dirty or wetforce. Advantages of the linear variable differential environments. The "sensing" range of proximitytransformer, or LVDT compared to a resistive sensors is very small, typically 0.1mm to 12mm.potentiometer are that its linearity that is its voltageoutput to displacement is excellent, very goodaccuracy, good resolution, high sensitivity as wellas frictionless operation and is sealed againsthostile environments.C. Inductive Proximity Sensors Another type of inductive sensor in common use Fig. 9 Proximity Sensoris the Inductive Proximity Sensor also calledan Eddy current sensor. While they do notactually measure displacement or angular rotation As well as industrial applications, inductivethey are mainly used to detect the presence of an proximity sensors are also used to control theobject in front of them or within a close proximity, changing of traffic lights at junctions and crosshence the name proximity sensors. roads. Rectangular inductive loops of wire are Proximity sensors, are non-contact devices that buried into the tarmac road surface and when a caruse a magnetic field for detection with the simplest or other road vehicle passes over the loop, themagnetic sensor being the reed switch. In an metallic body of the vehicle changes the loopsinductive sensor, a coil is wound around an iron inductance and activates the sensor thereby alertingcore within an electromagnetic field to form an the traffic lights controller that there is a vehicleinductive loop. When a ferromagnetic material is waiting.placed within the eddy current field generated One main disadvantage of these types of sensorsaround the sensor, such as a ferromagnetic metal is that they are "Omni-directional", that is they willplate or metal screw, the inductance of the coil sense a metallic object either above, below or to thechanges significantly. The proximity sensors side of it. Also, they do not detect non-metallicdetection circuit detects this change producing an objects althoughCapacitive Proximityoutput voltage. Therefore, inductive proximity Sensors and Ultrasonic Proximity Sensors aresensors operate under the electrical principle available. Other commonly available magneticof Faradays Law of inductance. position sensor include: reed switches, hall effect1) sensors and variable reluctance sensors. IV. TEMPERATURE SENSOR The most commonly used type of all the sensors are those which detect Temperature or heat. These types of temperature sensor vary from simple ON/OFF thermostatic devices which control a domestic hot water system to highly sensitive semiconductor types that can control complex process control plants. We remember from our school science classes that the movement of molecules and atoms produces heat (kinetic energy) and the more movement, the more heat is generated. Temperature Sensors measure the amount of heat energy or even coldness that is generated by an object or system, and can "sense" Fig. 8 Inductive Proximity Sensors or detect any physical change to that temperature producing either an analogue or digital output.
There are many different types of Temperature The thermostat consists of two thermally differentSensor available and all have different metals stuck together back to back. When it is coldcharacteristics depending upon their actual the contacts are closed and current passes throughapplication. Temperature sensors consist of two the thermostat. When it gets hot, one metal expandsbasic physical types: more than the other and the bonded bi-metallic strip bends up (or down) opening the contacts 1.) Contact Temperature Sensor: These types of preventing the current from flowing.temperature sensor are required to be in physicalcontact with the object being sensed and useconduction to monitor changes in temperature.They can be used to detect solids, liquids or gasesover a wide range of temperatures. 2.) Non-contact Temperature Sensor: Thesetypes of temperature sensor use convection andradiation to monitor changes in temperature. They Fig. 11 On/Off Thermostatcan be used to detect liquids and gases that emitradiant energy as heat rises and cold settles to the There are two main types of bi-metallic stripsbottom in convection currents or detect the radiant based mainly upon their movement when subjectedenergy being transmitted from an object in the form to temperature changes, "snap-action" types thatof infra-red radiation. produce an instantaneous "ON/OFF" or "OFF/ON" type action on the electrical contacts and the slower The two basic types of contact or even non- "creep-action" types that gradually change theircontact temperature sensors can also be sub-divided position as the temperature changes. Snap-actioninto the following three groups of thermostats are commonly used in homes forsensors, Electro- controlling the temperature of ovens, irons,mechanical, Resistive and Electronic and all immersion hot water tanks and on walls to controlthree types are discussed below. the domestic heating system. Creeper types generally consist of a bi-metallicA. The Thermostat coil or spiral that slowly unwinds or coils-up as the The Thermostat is a contact type electro- temperature changes. Generally, creeper type bi-mechanical temperature sensor or switch, that metallic strips are more sensitive to temperaturebasically consists of two different metals such as changes than the standard snap ON/OFF types asnickel, copper, tungsten or aluminium etc, that are the strip is longer and thinner making them idealbonded together to form a Bi-metallic strip. The for use in temperature gauges and dials etc.different linear expansion rates of the two One main disadvantage of the standard snap-dissimilar metals produce a mechanical bending action type thermostats when used as a temperaturemovement when the strip is subjected to heat. The sensor is that they have a large hysteresis rangebi-metallic strip is used as a switch in the from when the electrical contacts open until whenthermostat and is used extensively to control hot they close for example, set to 20oC but may notwater heating elements in boilers, furnaces, hot open until 22oC or close again until 18oC. So thewater storage tanks as well as in vehicle radiator range of temperature swing can be quite high.cooling systems. Commercially available bi-metallic thermostats for home use do have temperature adjustment screws that allow for a desired set-point and even its hysteresis level to be pre-set and are available over a wide operating range. B. The Thermistor The Thermistor is another type of temperature sensor, whose name is a combination of the words THERM-ally sensitive res-ISTOR. A thermistor is a type of resistor which changes its physical resistance with changes in temperature. 2) Fig. 10 The Bi-metallic Thermostat
Fig. 12 Thermistor At 25oC Thermistors are generally made from ceramictype semiconductor materials such as oxides of Fig. 13 Circuit diagram for examplenickel, manganese or cobalt coated in glass whichmakes them easily damaged. Most types ofthermistors have a Negative TemperatureCoefficient of resistance or (NTC), that is theirresistance value goes DOWN with an increase inthe temperature but some with a PositiveTemperature Coefficient, (PTC), their At 100oCresistance value goes UP with an increase intemperature are also available. Their mainadvantage is their speed of response to any changesin temperature, accuracy and repeatability. Thermistors are made of a ceramic typesemiconductor material using metal oxide by changing the fixed resistor value of R2 (in ourtechnology such as manganese, cobalt and nickel, example 1kΩ) to a potentiometer or preset, aetc. The semiconductor material is generally voltage output can be obtained at a predeterminedformed into small pressed discs or balls which are temperature set point for example, 5v output athermetically sealed to give a relatively fast 60oC and by varying the potentiometer a particularresponse to any changes in temperature. They are output voltage level can be obtained over a widerrated by their resistive value at room temperature temperature range.(usually at 25oC), their time constant (the time to It needs to be noted however, that thermistorsreact to the temperature change) and their power are non-linear devices and their standard resistancerating with respect to the current flowing through values at room temperature is different betweenthem. Like resistors, thermistors are available with different thermistors, which is due mainly to theresistance values at room temperature from 10s of semiconductor materials they are made from.MΩ down to just a few Ohms, but for sensing The Thermistor, have an exponential change withpurposes those types with values in the kilo-ohms temperature and therefore have a Beta temperatureare generally used. constant ( β ) which can be used to calculate its Thermistors are passive resistive devices which resistance for any given temperature point.means we need to pass a current through it to However, when used with a series resistor such asproduce a measurable voltage output. Then in a voltage divider network or Whetstone Bridgethermistors are generally connected in series with a type arrangement, the current obtained in responsesuitable biasing resistor to form a potential divider to a voltage applied to the divider/bridge network isnetwork and the choice of resistor gives a voltage linear with temperature. Then, the output voltageoutput at some pre-determined temperature point or across the resistor becomes linear with temperature.value for example: The following thermistor has a resistance value C. Resistive Temperature Detectors (RTD)of 10KΩ at 25oC and a resistance value of 100Ω at Another type of electrical resistance temperature100oC. Calculate the voltage drop across the sensor is the Resistance Temperaturethermistor and hence its output voltage (Vout) for Detector orRTD. RTDs are precision temperatureboth temperatures when connected in series with a sensors made from high-purity conducting metals1kΩ resistor across a 12v power supply. such as platinum, copper or nickel wound into a coil and whose electrical resistance changes as a function of temperature, similar to that of the
thermistor. Also available are thin-film RTDs. temperatures, a voltage is developed across theThese devices have a thin film of platinum paste is junction which is used to measure the temperaturedeposited onto a white ceramic substrate. sensor as shown below. Fig. 14 RTD Resistive temperature detectors have positivetemperature coefficients (PTC) but unlike thethermistor their output is extremely linearproducing very accurate measurements oftemperature. However, they have poor sensitivity,that is a change in temperature only produces avery small output change for example, 1Ω/oC. Themore common types of RTDs are made fromplatinum and are called Platinum Resistance Fig. 15 Construction of ThermocoupleThermometer or PRTs with the most commonlyavailable of them all the Pt100 sensor, which has a The principle of operation is that the junction ofstandard resistance value of 100Ω at 0oC. However, the two dissimilar metals such as copper andPlatinum is expensive and one of the main constantan, produces a "thermo-electric" effect thatdisadvantages of this type of device is its cost. produces a constant potential difference of only a Like the thermistor, RTDs are passive resistive few millivolts (mV) between them. The voltagedevices and by passing a constant current through difference between the two junctions is called thethe temperature sensor it is possible to obtain an "Seebeck effect" as a temperature gradient isoutput voltage that increases linearly with generated along the conducting wires producing antemperature. A typical RTD has a base resistance emf. Then the output voltage from a thermocoupleof about 100Ω at 0oC, increasing to about 140Ω at is a function of the temperature changes. If both the100oC with an operating temperature range of junctions are at the same temperature the potentialbetween -200 to +600oC. difference across the two junctions is zero in other Because the RTD is a resistive device, we need words, no voltage output as V1 = V2. However,to pass a current through them and monitor the when the junctions are connected within a circuitresulting voltage. However, any variation in and are both at different temperatures a voltageresistance due to self heat of the resistive wires as output will be detected relative to the difference inthe current flows through it, I2R, (Ohms Law) temperature between the two junctions, V1 - V2.causes an error in the readings. To avoid this, the This difference in voltage will increase withRTD is usually connected into a Whetstone Bridge temperature until the junctions peak voltage level isnetwork which has additional connecting wires for reached and this is determined by thelead-compensation and/or connection to a constant characteristics of the two dissimilar metals used.current source. Thermocouples can be made from a variety of different materials enabling extreme temperaturesD. The Thermocouple of between -200oC to over +2000oC to be The Thermocouple is by far the most commonly measured. With such a large choice of materialsused type of all the temperature sensing devices and temperature range, internationally recogniseddue to its simplicity, ease of use and their speed of standards have been developed complete withresponse to changes in temperature, due mainly to thermocouple colour codes to allow the user totheir small size. Thermocouples also have the choose the correct thermocouple sensor for awidest temperature range of all the temperature particular application. The British colour code forsensors from below -200oC to well over 2000oC. standard thermocouples is given below. Thermocouples are thermoelectric sensors that The three most common thermocouple materialsbasically consists of two junctions of dissimilar used above for general temperature measurementmetals, such as copper and constantan that are are,welded or crimped together. One junction is kept at Iron-Constantan (Type-J),a constant temperature called the reference (Cold) Copper-Constantan (Type-T),junction, while the other the measuring (Hot) Nickel-Chromium (Type K).junction. When the two junctions are at different
The output voltage from a thermocouple is very photons have converting light energy into electricalsmall, only a few mill-volts (mV) for a 10oC energy.change in temperature difference and because ofthis small voltage output some form of 2) Photo Conductive Cells: These photodevicesamplification is generally required. vary their electrical resistance when subjected to light. Photoconductivity results from light hitting a semiconductor material which controls the current flow through it. Thus, more light increase the current for a given applied voltage. The most common photoconductive material is Cadmium Sulphide used in LDR photocells. 3) Photo Voltaic Cells: These photodevices generate an emf in proportion to the radiant light energy received and is similar in effect to 3) Fig. 16 Thermocouple Amplification photoconductivity. Light energy falls on to two semiconductor materials sandwiched together The type of amplifier, either discrete or in the creating a voltage of approximately 0.5V. The mostform of an Operational Amplifier needs to be common photovoltaic material is Selenium used incarefully selected, because good drift stability is solar cells.required to prevent recalibration of thethermocouple at frequent intervals. This makes the 4) Photo Junction Devices: These photodeviceschopper and instrumentation type of amplifier are mainly true semiconductor devices such as thepreferable for most temperature sensing photodiode or phototransistor which use light toapplications. control the flow of electrons and holes across their Other types of Temperature Sensor not PN-junction. Photojunction devices are specificallymentioned here include, Semiconductor Junction designed for detector application and lightSensors, Infra-red and Thermal Radiation Sensors, penetration with their spectral response tuned to theMedical type Thermometers, Indicators and Colour wavelength of incident light.Changing Inks or Dyes. A. The Photoconductive Cell V. LIGHT SENSORS A Photoconductive light sensor does not produce A Light Sensor generates an output signal electricity but simply changes its physicalindicating the intensity of light by measuring the properties when subjected to light energy. The mostradiant energy that exists in a very narrow range of common type of photoconductive device isfrequencies basically called "light", and which the Photo resistor which changes its electricalranges in frequency from "Infrared" to "Visible" up resistance in response to changes in the lightto "Ultraviolet" light spectrum. The light sensor is a intensity. Photo resistors arepassive devices that convert this "light energy" Semiconductor devices that use light energy towhether visible or in the infrared parts of the control the flow of electrons, and hence the currentspectrum into an electrical signal output. Light flowing through them. The commonlysensors are more commonly known as used Photoconductive Cell is called the Light"Photoelectric Devices" or "Photo Sensors" becuse Dependant Resistor LDR.the convert light energy (photons) into electricity(electrons). 1) The Light Dependant Resistor: Photoelectric devices can be grouped into two As its name implies, the Light Dependantmain categories, those which generate electricity Resistor (LDR) is made from a piece of exposedwhen illuminated, such as Photo- semiconductor material such as cadmium sulphidevoltaics or Photo-emissives etc, and those which that changes its electrical resistance from severalchange their electrical properties in some way such thousand Ohms in the dark to only a few hundredas Photo-resistors or Photo-conductors. This Ohms when light falls upon it by creating hole-leads to the following classification of devices. electron pairs in the material. The net effect is an improvement in its conductivity with a decrease in 1) Photo Emissive Cells: These are photodevices resistance for an increase in illumination. Also,which release free electrons from a light sensitive photo resistive cells have a long response timematerial such as caesium when struck by a photon requiring many seconds to respond to a change inof sufficient energy. The amount of energy the the light intensity.photons have depends on the frequency of the lightand the higher the frequency, the more energy the
Materials used as the semiconductor substrateinclude, lead sulphide (PbS), lead selenide (PbSe),indium antimonide (InSb) which detect light in theinfra-red range with the most commonly used of allphoto resistive light sensors being CadmiumSulphide (Cds). Cadmium sulphide is used in themanufacture of photoconductive cells because itsspectral response curve closely matches that of thehuman eye and can even be controlled using asimple torch as a light source. Typically then, it hasa peak sensitivity wavelength (λp) of about 560nmto 600nm in the visible spectral range. Fig. 19 LDR Switch This basic light sensor circuit is of a relay output light activated switch. A potential divider circuit is formed between the photo resistor, LDR and the resistor R1. When no light is present ie in darkness, the resistance of the LDR is very high in the Mega ohms range so zero base bias is applied to the Fig. 17 Typical LDR transistor TR1 and the relay is de-energized or "OFF". As the light level increases the resistance of the 2.) The Light Dependant Resistor Cell LDR starts to decrease causing the base bias voltage at V1 to rise. At some point determined by the potential divider network formed with resistor R1, the base bias voltage is high enough to turn the transistor TR1 "ON" and thus activate the relay which inturn is used to control some external circuitry. As the light level falls back to darkness again the resistance of the LDR increases causing the base voltage of the transistor to decrease, turning the transistor and relay "OFF" at a fixed light level determined again by the potential divider network. By replacing the fixed resistor R1 with a potentiometer VR1, the point at which the relay turns "ON" or "OFF" can be pre-set to a particular light level. This type of simple circuit shown above Fig. 18 Light Dependent Resistor Cell has a fairly low sensitivity and its switching point may not be consistent due to variations in either The most commonly used photo resistive light temperature or the supply voltage. A more sensitivesensor is the ORP12 Cadmium Sulphide precision light activated circuit can be easily madephotoconductive cell. This light dependant resistor by incorporating the LDR into a "Wheatstonehas a spectral response of about 610nm in the Bridge" arrangement and replacing the transistoryellow to orange region of light. The resistance of with an Operational Amplifier as shown.the cell when unilluminated (dark resistance) is 4)very high at about 10MΩs which falls to about100Ωs when fully illuminated (lit resistance). Toincrease the dark resistance and therefore reducethe dark current, the resistive path forms a zigzagpattern across the ceramic substrate. The CdSphotocell is a very low cost device often used inauto dimming, darkness or twilight detection forturning the street lights "ON" and "OFF", and forphotographic exposure meter type applications. One simple use of a Light Dependant Resistor, isas a light sensitive switch as shown below. Fig. 20 Light Level Sensing Circuit
In this basic circuit the light dependant The construction of the Photodiode light sensorresistor, LDR1 and the potentiometer VR1 form is similar to that of a conventional PN-junctionone arm of a simple Wheatstone bridge network diode except that the diodes outer casing is eitherand the two fixed resistors R1 and R2 forming the transparent or has a clear lens to focus the lightother arm. Both sides of the bridge form potential onto the PN junction for increased sensitivity. Thedivider networks whose outputs V1 and V2 are junction will respond to light particularly longerboth connected to the inverting and non-inverting wavelengths such as red and infrared rather thanvoltage inputs respectively of the operational visible light.amplifier. The configuration of the operational This characteristic can be a problem for diodesamplifier is as a Differential Amplifier also known with transparent or glass bead bodies such as theas a voltage comparator with its output signal being 1N4148 signal diode. LEDs can also be used asthe difference between the two input signals or photodiodes as they can both emit and detect lightvoltages, V2 - V1. The feedback resistor Rf can be from their junction. All PN-junctions are lightchosen to give a suitable amplifier voltage gain if sensitive and can be used in a photo-conductiverequired. unbiased voltage mode with the PN-junction of the The resistor combination R1 and R2 form a fixed photodiode always "Reverse Biased" so that onlyreference voltage input V2, set by the ratio of the the diodes leakage or dark current can flow.two resistors and the LDR - VR1 combination a The current-voltage characteristic of avariable voltage input V1. As with the previous photodiode with no light on its junction (darkcircuit the output from the operational amplifier is mode) is very similar to a normal signal orused to control a relay, which is protected by a free rectifying diode. When the photodiode is forwardwheel diode,D1. When the light level sensed by the biased, there is an exponential increase in theLDR and its output voltage falls below the current, the same as for a normal diode. When areference voltage at V2the output from the op-amp reverse bias is applied, a small reverse saturationchanges activating the relay and switching the current appears which causes an increase of theconnected load. Likewise as the light level depletion region, which is the sensitive part of theincreases the output will switch back turning "OFF" junction. Photodiodes can also be connected in athe relay. current mode using a fixed bias voltage across the The operation of this type of light sensor circuit junction. The current mode is very linear over acan also be reversed to switch the relay "ON" when wide range.the light level exceeds the reference voltage leveland vice versa by reversing the positions of the B. Photo-diode Construction and Characteristicslight sensor LDR and the potentiometer VR1. Thepotentiometer can be used to "pre-set" theswitching point of the differential amplifier to anyparticular light level making it ideal as a lightsensor circuit. VI. PHOTOJUNCTION DEVICES Photojunction Devices are basically PN-Junction light sensors or detectors made fromsilicon semiconductor PN-junctions which aresensitive to light and which can detect both visiblelight and infrared light levels. Photo-junctiondevices are specifically made for sensing light andthis class of photoelectric light sensors includesthe Photodiode and the Phototransistor. Fig. 22 Photo DiodeA. The Photodiode. When used as a light sensor, a photodiodes dark current (0 lux) is about 10uA for geranium and 1uA for silicon type diodes. When light falls upon the junction more hole/electron pairs are formed and the leakage current increases. This leakage current increases as the illumination of the junction increases. Thus, the photodiodes current is directly proportional to light intensity falling onto the PN- Fig. 21 Photo-diode junction. One main advantage of photodiodes when used as light sensors is their fast response to
changes in the light levels, but one disadvantage of provide current gain and are much more sensitivethis type of photo device is the relatively small than the photodiode with currents are 50 to 100current flow even when fully lit. times greater than that of the standard photodiode The following circuit shows a photo-current-to- and any normal transistor can be easily convertedvoltage convertor circuit using an operational into a phototransistor light sensor by connecting aamplifier as the amplifying device. The output photodiode between the collector and base.voltage (Vout) is given as Vout = Ip × Rf and Phototransistors consist mainly of a bipolar NPNwhich is proportional to the light intensity Transistor with its large base region electricallycharacteristics of the photodiode. This type of unconnected, although some phototransistors allowcircuit also utilizes the characteristics of an a base connection to control the sensitivity, andoperational amplifier with two input terminals at which uses photons of light to generate a baseabout zero voltage to operate the photodiode current which inturn causes a collector to emitterwithout bias. This zero-bias op-amp configuration current to flow. Most phototransistors are NPNgives a high impedance loading to the photodiode types whose outer casing is either transparent orresulting in less influence by dark current and a has a clear lens to focus the light onto the basewider linear range of the photocurrent relative to junction for increased sensitivity.the radiant light intensity. Capacitor Cf is used toprevent oscillation or gain peaking and to set theoutput bandwidth (1/2πRC). Fig. 25 Photo-transistor Construction and Characteristics 5) Fig. 23 Photo-diode Amplifier Circuit In the NPN transistor the collector is biased Photodiodes are very versatile light sensors that positively with respect to the emitter so that thecan turn its current flow both "ON" and "OFF" in base/collector junction is reverse biased. therefore,nanoseconds and are commonly used in cameras, with no light on the junction normal leakage orlight meters, CD and DVD-ROM drives, TV dark current flows which is very small. When lightremote controls, scanners, fax machines and falls on the base more electron/hole pairs arecopiers etc, and when integrated into operational formed in this region and the current produced byamplifier circuits as infrared spectrum detectors for this action is amplified by the transistor. Thefiber optic communications, burglar alarm motion sensitivity of a phototransistor is a function of thedetection circuits and numerous imaging, laser DC current gain of the transistor.scanning and positioning systems etc. D. Photo-DarlingtonC. The Phototransistor Fig. 24 Photo-transistor Fig. 26 Darlington An alternative photo-junction device to the Photo Darlington transistors use a second bipolarphotodiode is the Phototransistor which is basically NPN transistor to provide additional amplificationa photodiode with amplification. The or when higher sensitivity of a photo detector isPhototransistor light sensor has its collector-base required due to low light levels or selectivePN-junction reverse biased exposing it to the sensitivity, but its response is slower than that of anradiant light source. Phototransistors operate the ordinary NPN phototransistor.same as the photodiode except that they can
Photo Darlington devices consist of a normal the dark. When illuminated the light energy causesphototransistor whose emitter output is coupled to electrons to flow through the PN junction and anthe base of a larger bipolar NPN transistor. Because individual solar cell can generate an open circuita Darlington transistor configuration gives a current voltage of about 0.58v (580mV). Solar cells have again equal to a product of the current gains of two "Positive" and a "Negative" side just like a battery.individual transistors, a photo Darlington device Individual solar cells can be connected together inproduces a very sensitive detector. series to form solar panels which increases the Typical applications of Phototransistors light output voltage or connected together in parallel tosensors are in opto-isolators, slotted opto switches, increase the available current. Commerciallylight beam sensors, fiber optics and TV type remote available solar panels are rated in Watts, which iscontrols, etc. Infrared filters are sometimes required the product of the output voltage and current (Voltswhen detecting visible light. times Amps) when fully lit. Another type of photo junction semiconductorlight sensor worth a mention is the Photo-thyristor.This is a light activated thyristor or SiliconControlled Rectifier, SCR that can be used as alight activated switch in AC applications. Howevertheir sensitivity is usually very low compared tophotodiodes or phototransistors, as to increase theirsensitivity to light they are made thinner around thegate junction which inturn limits the amount ofcurrent that they can switch. Then for highercurrent AC applications they are used as pilotdevices in opto-couplers to switch larger moreconventional thyristors.E. Photovoltaic Cells. The most common type of photovoltaic lightsensor is the Solar Cell. Solar cells convert lightenergy directly into DC electrical energy in the 6) Fig. 28 Characteristics of Photovoltaic Cellform of a voltage or current to a resistive load suchas a light, battery or motor. Then photovoltaic cellsare similar to a battery because they supply DCpower. Unlike the other photo devices above whichuse light intensity even from a torch to operate,photovoltaic cells work best using the suns radiantenergy. Solar cells are used in many different typesof applications to offer an alternative power sourcefrom conventional batteries, such as in calculators,satellites and now in homes offering a form ofrenewable power. Fig. 29 Internal Process of Photovoltaic Cell The amount of available current from a solar cell depends upon the light intensity, the size of the cell and its efficiency which is generally very low at around 15 to 20%. To increase the overall efficiency of the cell commercially available solar cells use polycrystalline silicon or amorphous silicon, which have no crystalline structure, and can Fig. 27 Photovoltaic Cell generate currents of between 20 to 40mA per cm2. Other materials used include Gallium Arsenide, Photovoltaic cells are made from single crystal Copper Indium Diselenide and Cadmium Telluride.silicon PN junctions, the same as photodiodes with These different materials each have a differenta very large light sensitive region but are used spectrum band response, and so can be "tuned" towithout the reverse bias. They have the same produce an output voltage at different wavelengthscharacteristics as a very large photodiode when in of light.
VII. MOTION SENSORS Fig. 29 Motion Detector A motion detector is a device for motion Fig. 30 PIRdetection. That is, it is a device that contains a All objects above absolute zero emit energy inphysical mechanism or electronic sensor that the form of radiation. Usually infrared radiation isquantifies motion that can be either integrated with invisible to the human eye but can be detected byor connected to other devices that alert the user of electronic devices designed for such a purpose. Thethe presence of a moving object within the field of term passive in this instance means that the PIRview. They form a vital component of device does not emit an infrared beam but merelycomprehensive security systems, for both homes passively accepts incoming infrared radiation.and businesses. “Infra” meaning below our ability to detect it An electronic motion detector contains a motion visually, and “Red” because this color representssensor that transforms the detection of motion into the lowest energy level that our eyes can sensean electric signal. This can be achieved by before it becomes invisible. Thus, infrared meansmeasuring optical or acoustical changes in the field below the energy level of the color red, and appliesof view. Most motion detectors can detect up to 15 to many sources of invisible energy.– 25 meters (50–80ft). A motion detector may be connected to a burglar B. Ultrasonic Sensoralarm that is used to alert the home owner or Ultrasonic sensors (also knownsecurity service after it detects motion. Such a as transceivers when they both send and receive)detector may also trigger a red light camera or work on a principle similaroutdoor lighting. to radar or sonar which evaluate attributes of a An occupancy sensor is a motion detector that is target by interpreting the echoes from radio orintegrated with a timing device. It senses when sound waves respectively. Ultrasonic sensorsmotion has stopped for a specified time period in generate high frequency sound waves and evaluateorder to trigger a light extinguishing signal. These the echo which is received back by the sensor.devices prevent illumination of unoccupied Sensors calculate the time interval between sendingspaces like public toilets. They are widely used for the signal and receiving the echo to determine thesecurity purposes. distance to an object.A. Passive Infrared Sensor A Passive Infrared sensor (PIR sensor) isan electronic device that measures infrared (IR)light radiating from objects in its field of view. PIRsensors are often used in the construction of PIR-based motion detectors. Apparent motion isdetected when an infrared source withone temperature, such as a human, passes in frontof an infrared source with another temperature,such as a wall. This is not to say that the sensordetects the heat from the object passing in front ofit but that the object breaks the field which thesensor has determined as the "normal" state. Anyobject, even one the exact same temperature as thesurrounding objects will cause the PIR to activate if Fig. 31 Ultrasonic Sensorit moves in the field of the sensors.
This technology can be used for measuring: wind ACKNOWLEDGMENTspeed and direction (anemometer), fullness of a We are sincerely thankful to Mr. Divyang Vyastank and speed through air or water. For measuring & Mr. Kuldeep Vyas- Faculty at DIET for thisspeed or direction a device uses multiple detectors report. We thank them for their total support &and calculates the speed from the relative distances UNENDING help to us during the entire report.to particulates in the air or water. To measure the We are also thankful to our friends who haveamount of liquid in a tank, the sensor measures the helped us very much during the report for any kinddistance to the surface of the fluid. Further of information, data, format, etc. Last but not theapplications include: humidifiers, sonar, medical least; we are thankful to our college & its libraryultrasonography, burglar alarms and non- for providing us the needful and supportingdestructive testing. material for our report. Systems typically use a transducer whichgenerates sound waves in the ultrasonic range,above 18,000 hertz, by turning electrical energy REFERENCESinto sound, then upon receiving the echo turn the  http://www.wikipedia.orgsound waves into electrical energy which can be  http://www.advancedsensors.co.ukmeasured and displayed.  http://www.sensors-research.com  http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=61 0263C. Dual Technology Motion Detector  Jeffrey Cole & Steven Dubowsky, The Application of Many modern motion detectors use a Advanced Robotics and Sensor Technologies.combination of different technologies. These dual-technology detectors benefit with each type ofsensor, and false alarms are reduced. Placement ofthe sensors can be strategically mounted so as tolessen the chance of pets activating alarms. Often, PIR technology will be paired withanother model to maximize accuracy and reduceenergy usage. PIR draws less energy thanmicrowave detection, and so many sensors arecalibrated so that when the PIR sensor is tripped, itactivates a microwave sensor. If the latter alsopicks up an intruder, then the alarm is sounded. Asinterior motion detectors do not „see‟ throughwindows or walls, motion-sensitive outdoorlighting is often recommended to enhancecomprehensive efforts to protect your property. False alarms are those usually caused bytechnical errors such as electrical and mechanicalfailures. Nuisance alarms are system activations notcommonly caused by attackers or intruders butrather from windblown debris, animals, insects andfoliage. Sequencing alarm systems to trip the alertmechanism only when both alarm sensors havebeen activated will reduce nuisance alarms, butmay also cause the probability of detection todecrease. VIII. CONCLUSIONS This repots explains the application and uses ofdifferent types of sensors to make our work easyand to get accurate result. This report describescommonly used six different advanced sensors.This autonomous or sensor technology have animportant impact on every small and big field.