The document summarizes different advanced autonomous sensors. It describes six common types of transducers: temperature, speed, pressure, displacement, strain, and position sensors. Both analog and digital sensors are discussed. Analog sensors produce a continuous output signal proportional to the measured quantity, while digital sensors produce a discrete output representing a binary number. The accuracy of digital sensors is proportional to the number of bits used to represent the measured quantity.

1. IEEE Report on Advanced Sensors
EC Department, GTU
Gaurav Maniar, Karan Raithatha
gaurav019@facebook.com
karan.raithatha@gmail.com
DIET, INDIA
Abstract – This report explains different commonly There are many different types of transducers
used six advanced autonomous sensors. It also available in the marketplace, and the choice of
describes 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 musical
note, but in order for an electronic circuit or system A. Common Transducers
to perform any useful task or function it needs to be
Input type transducers or sensors, produce a
able to communicate with the "real world" whether
proportional output voltage or signal in response to
this 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 output
output 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 external
electrical signals, radiant energy, thermal or
power to operate, called an excitation signal which
magnetic 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 because
devices available to choose from. The type of input
their own properties change in response to an
or output transducer being used, really depends
external effect and produce an output voltage, for
upon the type of signal or process being "Sensed"
example, 1 to 10v DC or an output current such as
or "Controlled" but we can define a transducer as a
4 to 20mA DC. For example, a strain gauge is a
device that converts one physical quantity into
pressure-sensitive resistor. It does not generate any
another.
electrical signal, but by passing a current through it
Devices which perform an input function are (excitation signal), its resistance can be measured
commonly called Sensors because they "sense" a by detecting variations in the current and/or voltage
physical change in some characteristic that changes across it relating these changes to the amount of
in 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 does
Devices which perform an output function are
not need any additional energy source and directly
generally called Actuators and are used to control
generates an electric signal in response to an
some external device, for example movement. Both
external stimulus. For example, a thermocouple or
sensors and actuators are collectively known
photodiode. Passive sensors are direct sensors
as Transducers because they are used to convert
which change their physical properties, such as
energy of one kind into energy of another kind, for
resistance, capacitance or inductance etc. As well
example, a microphone (input device) converts
as analogue sensors, Digital Sensors produce a
sound waves into electrical signals for the amplifier
discrete output representing a binary number or
to amplify, and a loudspeaker (output device)
digit such as a logic level "0" or a logic level "1".
converts the electrical signals back into sound
waves 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 be
Fig. 1 Simple Input/output System using Sound Transducers measured using a thermometer or thermocouple

2. which continuously responds to temperature In our simple example above, the speed of the
changes 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 be
Fig. 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 represent
that are changing smoothly and continuously which the measured quantity. For example, using a
are very small in value so some form of processor of 8 bits, will produce an accuracy of
amplification is required. Then circuits which 0.195% (1 part in 512). While using a processor of
measure analogue signals usually have a slow 16 bits gives an accuracy of 0.0015%, (1 part in
response and/or low accuracy. Also analogue 65,536) or 130 times more accurate. This accuracy
signals can be easily converted into digital type can be maintained as digital quantities are
signals for use in microcontroller systems by the manipulated and processed very rapidly, millions
use of analogue-to-digital converters, or ADC's. of times faster than analogue signals.
In most cases, sensors and more specifically
B. Digital Sensors analogue sensors generally require an external
As its name implies, Digital Sensors produce a power supply and some form of additional
discrete output signal or voltage that is a digital amplification or filtering of the signal in order to
representation of the quantity being measured. produce a suitable electrical signal which is capable
Digital sensors produce a Binary output signal in of being measured or used. One very good way of
the 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 as
produces discrete (non-continuous) values which seen before.
may be outputted as a single "bit", (serial
transmission) or by combining the bits to produce a C. Signal Conditioning
single "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 perhaps
Fig. 3 Light Sensor used to produce an Digital Signal filtering (frequency selection) may be required

3. before the signal can be used and this is robots wheel to determine its distance travelled
conveniently performed by Operational Amplifiers. along the ground. Either way, Position Sensors can
Also, when measuring very small physical changes detect the movement of an object in a straight line
the 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 being
measured correctly. These unwanted signals are A. The Potentiometer
called "Noise". This Noise or Interference can be The most commonly used of all the "Position
either greatly reduced or even eliminated by using Sensors", is the potentiometer because it is an
signal conditioning or filtering techniques as we inexpensive and easy to use position sensor. It has a
discussed in the Active Filter tutorial. By using wiper contact linked to a mechanical shaft that can
either 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 resistance
reduced to leave just the output signal required. For value between the wiper/slider and the two end
example, many types of inputs from switches, connections to change giving an electrical signal
keyboards or manual controls are not capable of output that has a proportional relationship between
changing state rapidly and so low-pass filter can be the actual wiper position on the resistive track and
used. When the interference is at a particular its resistance value. In other words, resistance is
frequency, for example mains frequency, narrow proportional to position.
band reject or Notch filters can be used to produce
frequency selective filters. Where some random
noise still remains after filtering it may be
necessary to take several samples and then average
them to give the final value so increasing the
signal-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 10v
an important role in interfacing microprocessor and across the resistive element of the potentiometer
electronics based systems to "real world" the maximum output voltage would be 10 volts and
conditions. Now Positional Sensors which measure the wiper will vary the output signal from 0 to 10
the position and/or displacement of physical objects volts, with 5 volts indicating that the wiper or slider
meaning the movement from one position to is at the half-way centre position.
another for a specific distance or angle would be
introduced.
III. POSITION SENSORS
In this tutorial we will look at a variety of
devices which are classed as Input Devices and are
therefore called "Sensors" and in particular those
sensors which are Positional in nature which means
that they are referenced either to or from some
fixed point or position. As their name implies, these
types of sensors provide a "position" feedback. One
method of determining a position, is to use either
"distance", which could be the distance between
two points such as the distance travelled or moved
away from some fixed point, or by "rotation"
(angular movement). For example, the rotation of a Fig. 6 Simple Positional Sensing Circuit

4. While resistive potentiometer position sensors If the soft iron magnetic core armature is exactly
have many advantages: low cost, low tech, easy to in the centre of the tube and the windings, "null
use etc, as a position sensor they also have many position", the two induced emf's in the two
disadvantages: wear due to moving parts, low secondary windings cancel each other out as they
accuracy, low repeatability, and limited frequency are 180oout of phase, so the resultant output voltage
response. But one main disadvantage of using the is zero. As the core is displaced slightly to one side
potentiometer as a positional sensor is that the or the other from this null or zero position, the
range of movement of its wiper or slide (and hence induced voltage in one of the secondaries will be
the output signal obtained) is limited to the physical become greater than that of the other secondary and
size of the potentiometer being used. For example a an output will be produced. The polarity of the
single turn rotational potentiometer generally only output signal depends upon the direction and
has a fixed electrical rotation between about 240 to displacement of the moving core. The greater the
330o however, multi-turn pots of up to 3600o of movement of the soft iron core from its central null
electrical rotation are also available. Most types of position the greater will be the resulting output
potentiometers use carbon film for their resistive signal. The result is a differential voltage output
track, 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 that
short mechanical life. Wire-wound pots also known is a linear function of the cores displacement and a
as rheostats, in the form of either a straight wire or polarity that indicates direction of movement. The
wound coil resistive wire can also be used, but wire phase of the output signal can be compared to the
wound pots suffer from resolution problems as primary coil excitation phase enabling suitable
their wiper jumps from one wire segment to the electronic circuits such as the AD592 LVDT
next producing a logarithmic (LOG) output Sensor Amplifier to know which half of the coil the
resulting in errors in the output signal. These too magnetic core is in and thereby know the direction
suffer from electrical noise. of travel.
For high precision low noise applications
conductive plastic resistance element type polymer
film or cermets type potentiometers are now
available. These pots have a smooth low friction
electrically linear (LIN) resistive track giving them
a low noise, long life and excellent resolution and
are available as both multi-turn and single turn
devices. A typical application for this type of high
accuracy position sensor is in computer game
joysticks, steering wheels, industrial and robot
applications.
B. Inductive Position Sensors
One type of positional sensor that does not suffer
from mechanical wear problems is the "Linear
Variable Differential Transformer" or LVDT for
short. This is an inductive type position sensor
which works on the same principle as the AC
transformer that is used to measure movement. It is
a very accurate device for measuring linear
displacement and whose output is proportional to
the position of its moveable core.
It basically consists of three coils wound on a
hollow tube former, one forming the primary coil
and the other two coils forming identical
secondary‟s connected electrically together in
series but 180o out of phase either side of the
primary coil. A moveable soft iron ferromagnetic
core (sometimes called an "armature") which is
connected to the object being measured slides or
moves up and down inside the tube. A small AC
reference voltage called the "excitation signal"
(2 - 20V rms, 2 - 20kHz) is applied to the primary
winding which inurn induces an EMF signal into
the two adjacent secondary windings. Fig. 7 The Linear Variable Differential Transformer

5. When the armature is moved from one end to the An inductive proximity sensor has four main
other through the centre position the output components; The oscillator which produces the
voltages changes from maximum to zero and back electromagnetic field, the coil which generates the
to maximum again but in the process changes its magnetic field, the detection circuit which
phase angle by 180 deg's. This enables the LVDT detects any change in the field when an object
to produce an output AC signal whose magnitude enters it and the output circuit which produces the
represents the amount of movement from the centre output signal, either with normally closed (NC) or
position and whose phase angle represents the normally open (NO) contacts. Inductive proximity
direction of movement of the core. A typical sensors allow for the detection of metallic objects
application of this type of sensor would be a in front of the sensor head without any physical
pressure transducers, were the pressure being contact of the object itself being detected. This
measured pushes against a diaphragm to produce a makes them ideal for use in dirty or wet
force. Advantages of the linear variable differential environments. The "sensing" range of proximity
transformer, or LVDT compared to a resistive sensors is very small, typically 0.1mm to 12mm.
potentiometer are that its linearity that is its voltage
output to displacement is excellent, very good
accuracy, good resolution, high sensitivity as well
as frictionless operation and is sealed against
hostile environments.
C. Inductive Proximity Sensors
Another type of inductive sensor in common use
Fig. 9 Proximity Sensor
is the Inductive Proximity Sensor also called
an Eddy current sensor. While they do not
actually measure displacement or angular rotation As well as industrial applications, inductive
they are mainly used to detect the presence of an proximity sensors are also used to control the
object in front of them or within a close proximity, changing of traffic lights at junctions and cross
hence 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 car
use a magnetic field for detection with the simplest or other road vehicle passes over the loop, the
magnetic sensor being the reed switch. In an metallic body of the vehicle changes the loops
inductive sensor, a coil is wound around an iron inductance and activates the sensor thereby alerting
core within an electromagnetic field to form an the traffic lights controller that there is a vehicle
inductive loop. When a ferromagnetic material is waiting.
placed within the eddy current field generated One main disadvantage of these types of sensors
around the sensor, such as a ferromagnetic metal is that they are "Omni-directional", that is they will
plate or metal screw, the inductance of the coil sense a metallic object either above, below or to the
changes significantly. The proximity sensors side of it. Also, they do not detect non-metallic
detection circuit detects this change producing an objects althoughCapacitive Proximity
output voltage. Therefore, inductive proximity Sensors and Ultrasonic Proximity Sensors are
sensors operate under the electrical principle available. Other commonly available magnetic
of Faraday's Law of inductance. position sensor include: reed switches, hall effect
1) 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.

6. There are many different types of Temperature The thermostat consists of two thermally different
Sensor available and all have different metals stuck together back to back. When it is cold
characteristics depending upon their actual the contacts are closed and current passes through
application. Temperature sensors consist of two the thermostat. When it gets hot, one metal expands
basic 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 physical
contact with the object being sensed and use
conduction to monitor changes in temperature.
They can be used to detect solids, liquids or gases
over a wide range of temperatures.
2.) Non-contact Temperature Sensor: These
types of temperature sensor use convection and
radiation to monitor changes in temperature. They Fig. 11 On/Off Thermostat
can be used to detect liquids and gases that emit
radiant energy as heat rises and cold settles to the There are two main types of bi-metallic strips
bottom in convection currents or detect the radiant based mainly upon their movement when subjected
energy being transmitted from an object in the form to temperature changes, "snap-action" types that
of 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 their
contact temperature sensors can also be sub-divided position as the temperature changes. Snap-action
into the following three groups of thermostats are commonly used in homes for
sensors, Electro- controlling the temperature of ovens, irons,
mechanical, Resistive and Electronic and all immersion hot water tanks and on walls to control
three types are discussed below. the domestic heating system.
Creeper types generally consist of a bi-metallic
A. 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 temperature
basically consists of two different metals such as changes than the standard snap ON/OFF types as
nickel, copper, tungsten or aluminium etc, that are the strip is longer and thinner making them ideal
bonded 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 temperature
movement when the strip is subjected to heat. The sensor is that they have a large hysteresis range
bi-metallic strip is used as a switch in the from when the electrical contacts open until when
thermostat and is used extensively to control hot they close for example, set to 20oC but may not
water heating elements in boilers, furnaces, hot open until 22oC or close again until 18oC. So the
water 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

7. Fig. 12 Thermistor
At 25oC
Thermistors are generally made from ceramic
type semiconductor materials such as oxides of Fig. 13 Circuit diagram for example
nickel, manganese or cobalt coated in glass which
makes them easily damaged. Most types of
thermistor's have a Negative Temperature
Coefficient of resistance or (NTC), that is their
resistance value goes DOWN with an increase in
the temperature but some with a Positive
Temperature Coefficient, (PTC), their At 100oC
resistance value goes UP with an increase in
temperature are also available. Their main
advantage is their speed of response to any changes
in temperature, accuracy and repeatability.
Thermistors are made of a ceramic type
semiconductor material using metal oxide by changing the fixed resistor value of R2 (in our
technology such as manganese, cobalt and nickel, example 1kΩ) to a potentiometer or preset, a
etc. The semiconductor material is generally voltage output can be obtained at a predetermined
formed into small pressed discs or balls which are temperature set point for example, 5v output at
hermetically sealed to give a relatively fast 60oC and by varying the potentiometer a particular
response to any changes in temperature. They are output voltage level can be obtained over a wider
rated 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 thermistor's
react to the temperature change) and their power are non-linear devices and their standard resistance
rating with respect to the current flowing through values at room temperature is different between
them. Like resistors, thermistors are available with different thermistor's, which is due mainly to the
resistance values at room temperature from 10's of semiconductor materials they are made from.
MΩ down to just a few Ohms, but for sensing The Thermistor, have an exponential change with
purposes those types with values in the kilo-ohms temperature and therefore have a Beta temperature
are 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 as
produce a measurable voltage output. Then in a voltage divider network or Whetstone Bridge
thermistors are generally connected in series with a type arrangement, the current obtained in response
suitable biasing resistor to form a potential divider to a voltage applied to the divider/bridge network is
network and the choice of resistor gives a voltage linear with temperature. Then, the output voltage
output 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 temperature
100oC. Calculate the voltage drop across the sensor is the Resistance Temperature
thermistor and hence its output voltage (Vout) for Detector orRTD. RTD's are precision temperature
both temperatures when connected in series with a sensors made from high-purity conducting metals
1kΩ 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

8. thermistor. Also available are thin-film RTD's. temperatures, a voltage is developed across the
These devices have a thin film of platinum paste is junction which is used to measure the temperature
deposited onto a white ceramic substrate. sensor as shown below.
Fig. 14 RTD
Resistive temperature detectors have positive
temperature coefficients (PTC) but unlike the
thermistor their output is extremely linear
producing very accurate measurements of
temperature. However, they have poor sensitivity,
that is a change in temperature only produces a
very small output change for example, 1Ω/oC. The
more common types of RTD's are made from
platinum and are called Platinum Resistance Fig. 15 Construction of Thermocouple
Thermometer or PRT's with the most commonly
available of them all the Pt100 sensor, which has a The principle of operation is that the junction of
standard resistance value of 100Ω at 0oC. However, the two dissimilar metals such as copper and
Platinum is expensive and one of the main constantan, produces a "thermo-electric" effect that
disadvantages of this type of device is its cost. produces a constant potential difference of only a
Like the thermistor, RTD's are passive resistive few millivolts (mV) between them. The voltage
devices and by passing a constant current through difference between the two junctions is called the
the temperature sensor it is possible to obtain an "Seebeck effect" as a temperature gradient is
output voltage that increases linearly with generated along the conducting wires producing an
temperature. A typical RTD has a base resistance emf. Then the output voltage from a thermocouple
of about 100Ω at 0oC, increasing to about 140Ω at is a function of the temperature changes. If both the
100oC with an operating temperature range of junctions are at the same temperature the potential
between -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 circuit
resulting voltage. However, any variation in and are both at different temperatures a voltage
resistance due to self heat of the resistive wires as output will be detected relative to the difference in
the 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 with
RTD is usually connected into a Whetstone Bridge temperature until the junctions peak voltage level is
network which has additional connecting wires for reached and this is determined by the
lead-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 temperatures
D. 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 materials
used type of all the temperature sensing devices and temperature range, internationally recognised
due to its simplicity, ease of use and their speed of standards have been developed complete with
response to changes in temperature, due mainly to thermocouple colour codes to allow the user to
their small size. Thermocouples also have the choose the correct thermocouple sensor for a
widest temperature range of all the temperature particular application. The British colour code for
sensors from below -200oC to well over 2000oC. standard thermocouples is given below.
Thermocouples are thermoelectric sensors that The three most common thermocouple materials
basically consists of two junctions of dissimilar used above for general temperature measurement
metals, 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

9. The output voltage from a thermocouple is very photons have converting light energy into electrical
small, only a few mill-volts (mV) for a 10oC energy.
change in temperature difference and because of
this small voltage output some form of 2) Photo Conductive Cells: These photodevices
amplification 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 most
form of an Operational Amplifier needs to be common photovoltaic material is Selenium used in
carefully selected, because good drift stability is solar cells.
required to prevent recalibration of the
thermocouple at frequent intervals. This makes the 4) Photo Junction Devices: These photodevices
chopper and instrumentation type of amplifier are mainly true semiconductor devices such as the
preferable for most temperature sensing photodiode or phototransistor which use light to
applications. control the flow of electrons and holes across their
Other types of Temperature Sensor not PN-junction. Photojunction devices are specifically
mentioned here include, Semiconductor Junction designed for detector application and light
Sensors, Infra-red and Thermal Radiation Sensors, penetration with their spectral response tuned to the
Medical 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 physical
indicating the intensity of light by measuring the properties when subjected to light energy. The most
radiant energy that exists in a very narrow range of common type of photoconductive device is
frequencies basically called "light", and which the Photo resistor which changes its electrical
ranges in frequency from "Infrared" to "Visible" up resistance in response to changes in the light
to "Ultraviolet" light spectrum. The light sensor is a intensity. Photo resistors are
passive devices that convert this "light energy" Semiconductor devices that use light energy to
whether visible or in the infrared parts of the control the flow of electrons, and hence the current
spectrum into an electrical signal output. Light flowing through them. The commonly
sensors 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 Dependant
main categories, those which generate electricity Resistor (LDR) is made from a piece of exposed
when illuminated, such as Photo- semiconductor material such as cadmium sulphide
voltaics or Photo-emissives etc, and those which that changes its electrical resistance from several
change their electrical properties in some way such thousand Ohms in the dark to only a few hundred
as 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 time
material such as caesium when struck by a photon requiring many seconds to respond to a change in
of sufficient energy. The amount of energy the the light intensity.
photons have depends on the frequency of the light
and the higher the frequency, the more energy the

10. Materials used as the semiconductor substrate
include, lead sulphide (PbS), lead selenide (PbSe),
indium antimonide (InSb) which detect light in the
infra-red range with the most commonly used of all
photo resistive light sensors being Cadmium
Sulphide (Cds). Cadmium sulphide is used in the
manufacture of photoconductive cells because its
spectral response curve closely matches that of the
human eye and can even be controlled using a
simple torch as a light source. Typically then, it has
a peak sensitivity wavelength (λp) of about 560nm
to 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 sensitive
sensor is the ORP12 Cadmium Sulphide precision light activated circuit can be easily made
photoconductive cell. This light dependant resistor by incorporating the LDR into a "Wheatstone
has a spectral response of about 610nm in the Bridge" arrangement and replacing the transistor
yellow 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 10MΩ's which falls to about
100Ω's when fully illuminated (lit resistance). To
increase the dark resistance and therefore reduce
the dark current, the resistive path forms a zigzag
pattern across the ceramic substrate. The CdS
photocell is a very low cost device often used in
auto dimming, darkness or twilight detection for
turning the street lights "ON" and "OFF", and for
photographic exposure meter type applications.
One simple use of a Light Dependant Resistor, is
as a light sensitive switch as shown below. Fig. 20 Light Level Sensing Circuit

11. In this basic circuit the light dependant The construction of the Photodiode light sensor
resistor, LDR1 and the potentiometer VR1 form is similar to that of a conventional PN-junction
one arm of a simple Wheatstone bridge network diode except that the diodes outer casing is either
and the two fixed resistors R1 and R2 forming the transparent or has a clear lens to focus the light
other arm. Both sides of the bridge form potential onto the PN junction for increased sensitivity. The
divider networks whose outputs V1 and V2 are junction will respond to light particularly longer
both connected to the inverting and non-inverting wavelengths such as red and infrared rather than
voltage inputs respectively of the operational visible light.
amplifier. The configuration of the operational This characteristic can be a problem for diodes
amplifier is as a Differential Amplifier also known with transparent or glass bead bodies such as the
as a voltage comparator with its output signal being 1N4148 signal diode. LED's can also be used as
the difference between the two input signals or photodiodes as they can both emit and detect light
voltages, V2 - V1. The feedback resistor Rf can be from their junction. All PN-junctions are light
chosen to give a suitable amplifier voltage gain if sensitive and can be used in a photo-conductive
required. unbiased voltage mode with the PN-junction of the
The resistor combination R1 and R2 form a fixed photodiode always "Reverse Biased" so that only
reference 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 a
variable voltage input V1. As with the previous photodiode with no light on its junction (dark
circuit the output from the operational amplifier is mode) is very similar to a normal signal or
used to control a relay, which is protected by a free rectifying diode. When the photodiode is forward
wheel diode,D1. When the light level sensed by the biased, there is an exponential increase in the
LDR and its output voltage falls below the current, the same as for a normal diode. When a
reference voltage at V2the output from the op-amp reverse bias is applied, a small reverse saturation
changes activating the relay and switching the current appears which causes an increase of the
connected load. Likewise as the light level depletion region, which is the sensitive part of the
increases the output will switch back turning "OFF" junction. Photodiodes can also be connected in a
the 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 a
can also be reversed to switch the relay "ON" when wide range.
the light level exceeds the reference voltage level
and vice versa by reversing the positions of the B. Photo-diode Construction and Characteristics
light sensor LDR and the potentiometer VR1. The
potentiometer can be used to "pre-set" the
switching point of the differential amplifier to any
particular light level making it ideal as a light
sensor circuit.
VI. PHOTOJUNCTION DEVICES
Photojunction Devices are basically PN-
Junction light sensors or detectors made from
silicon semiconductor PN-junctions which are
sensitive to light and which can detect both visible
light and infrared light levels. Photo-junction
devices are specifically made for sensing light and
this class of photoelectric light sensors includes
the Photodiode and the Phototransistor. Fig. 22 Photo Diode
A. 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

12. changes in the light levels, but one disadvantage of provide current gain and are much more sensitive
this type of photo device is the relatively small than the photodiode with currents are 50 to 100
current 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 converted
voltage convertor circuit using an operational into a phototransistor light sensor by connecting a
amplifier 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 NPN
which is proportional to the light intensity Transistor with its large base region electrically
characteristics of the photodiode. This type of unconnected, although some phototransistors allow
circuit also utilizes the characteristics of an a base connection to control the sensitivity, and
operational amplifier with two input terminals at which uses photons of light to generate a base
about zero voltage to operate the photodiode current which inturn causes a collector to emitter
without bias. This zero-bias op-amp configuration current to flow. Most phototransistors are NPN
gives a high impedance loading to the photodiode types whose outer casing is either transparent or
resulting in less influence by dark current and a has a clear lens to focus the light onto the base
wider linear range of the photocurrent relative to junction for increased sensitivity.
the radiant light intensity. Capacitor Cf is used to
prevent oscillation or gain peaking and to set the
output 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 the
can 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 or
light meters, CD and DVD-ROM drives, TV dark current flows which is very small. When light
remote controls, scanners, fax machines and falls on the base more electron/hole pairs are
copiers etc, and when integrated into operational formed in this region and the current produced by
amplifier circuits as infrared spectrum detectors for this action is amplified by the transistor. The
fiber optic communications, burglar alarm motion sensitivity of a phototransistor is a function of the
detection circuits and numerous imaging, laser DC current gain of the transistor.
scanning and positioning systems etc.
D. Photo-Darlington
C. The Phototransistor
Fig. 24 Photo-transistor
Fig. 26 Darlington
An alternative photo-junction device to the
Photo Darlington transistors use a second bipolar
photodiode is the Phototransistor which is basically
NPN transistor to provide additional amplification
a photodiode with amplification. The
or when higher sensitivity of a photo detector is
Phototransistor light sensor has its collector-base
required due to low light levels or selective
PN-junction reverse biased exposing it to the
sensitivity, but its response is slower than that of an
radiant light source. Phototransistors operate the
ordinary NPN phototransistor.
same as the photodiode except that they can

13. Photo Darlington devices consist of a normal the dark. When illuminated the light energy causes
phototransistor whose emitter output is coupled to electrons to flow through the PN junction and an
the base of a larger bipolar NPN transistor. Because individual solar cell can generate an open circuit
a Darlington transistor configuration gives a current voltage of about 0.58v (580mV). Solar cells have a
gain 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 in
produces a very sensitive detector. series to form solar panels which increases the
Typical applications of Phototransistors light output voltage or connected together in parallel to
sensors are in opto-isolators, slotted opto switches, increase the available current. Commercially
light beam sensors, fiber optics and TV type remote available solar panels are rated in Watts, which is
controls, etc. Infrared filters are sometimes required the product of the output voltage and current (Volts
when detecting visible light. times Amps) when fully lit.
Another type of photo junction semiconductor
light sensor worth a mention is the Photo-thyristor.
This is a light activated thyristor or Silicon
Controlled Rectifier, SCR that can be used as a
light activated switch in AC applications. However
their sensitivity is usually very low compared to
photodiodes or phototransistors, as to increase their
sensitivity to light they are made thinner around the
gate junction which inturn limits the amount of
current that they can switch. Then for higher
current AC applications they are used as pilot
devices in opto-couplers to switch larger more
conventional thyristors.
E. Photovoltaic Cells.
The most common type of photovoltaic light
sensor is the Solar Cell. Solar cells convert light
energy directly into DC electrical energy in the 6) Fig. 28 Characteristics of Photovoltaic Cell
form of a voltage or current to a resistive load such
as a light, battery or motor. Then photovoltaic cells
are similar to a battery because they supply DC
power. Unlike the other photo devices above which
use light intensity even from a torch to operate,
photovoltaic cells work best using the suns radiant
energy. Solar cells are used in many different types
of applications to offer an alternative power source
from conventional batteries, such as in calculators,
satellites and now in homes offering a form of
renewable 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 different
a very large light sensitive region but are used spectrum band response, and so can be "tuned" to
without the reverse bias. They have the same produce an output voltage at different wavelengths
characteristics as a very large photodiode when in of light.

14. VII. MOTION SENSORS
Fig. 29 Motion Detector
A motion detector is a device for motion Fig. 30 PIR
detection. That is, it is a device that contains a
All objects above absolute zero emit energy in
physical mechanism or electronic sensor that
the form of radiation. Usually infrared radiation is
quantifies motion that can be either integrated with
invisible to the human eye but can be detected by
or connected to other devices that alert the user of
electronic devices designed for such a purpose. The
the presence of a moving object within the field of
term passive in this instance means that the PIR
view. They form a vital component of
device does not emit an infrared beam but merely
comprehensive 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 represents
sensor that transforms the detection of motion into
the lowest energy level that our eyes can sense
an electric signal. This can be achieved by
before it becomes invisible. Thus, infrared means
measuring optical or acoustical changes in the field below the energy level of the color red, and applies
of 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 Sensor
alarm that is used to alert the home owner or Ultrasonic sensors (also known
security 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 similar
outdoor 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 or
integrated with a timing device. It senses when sound waves respectively. Ultrasonic sensors
motion has stopped for a specified time period in generate high frequency sound waves and evaluate
order 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 sending
spaces like public toilets. They are widely used for the signal and receiving the echo to determine the
security purposes. distance to an object.
A. Passive Infrared Sensor
A Passive Infrared sensor (PIR sensor) is
an electronic device that measures infrared (IR)
light radiating from objects in its field of view. PIR
sensors are often used in the construction of PIR-
based motion detectors. Apparent motion is
detected when an infrared source with
one temperature, such as a human, passes in front
of an infrared source with another temperature,
such as a wall. This is not to say that the sensor
detects the heat from the object passing in front of
it but that the object breaks the field which the
sensor has determined as the "normal" state. Any
object, even one the exact same temperature as the
surrounding objects will cause the PIR to activate if Fig. 31 Ultrasonic Sensor
it moves in the field of the sensors.

15. This technology can be used for measuring: wind ACKNOWLEDGMENT
speed and direction (anemometer), fullness of a We are sincerely thankful to Mr. Divyang Vyas
tank and speed through air or water. For measuring & Mr. Kuldeep Vyas- Faculty at DIET for this
speed 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 have
amount of liquid in a tank, the sensor measures the helped us very much during the report for any kind
distance to the surface of the fluid. Further of information, data, format, etc. Last but not the
applications include: humidifiers, sonar, medical least; we are thankful to our college & its library
ultrasonography, burglar alarms and non- for providing us the needful and supporting
destructive testing. material for our report.
Systems typically use a transducer which
generates sound waves in the ultrasonic range,
above 18,000 hertz, by turning electrical energy REFERENCES
into sound, then upon receiving the echo turn the [1] http://www.wikipedia.org
sound waves into electrical energy which can be [2] http://www.advancedsensors.co.uk
measured and displayed. [3] http://www.sensors-research.com
[4] http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=61
0263
C. Dual Technology Motion Detector [5] 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 of
sensor, and false alarms are reduced. Placement of
the sensors can be strategically mounted so as to
lessen the chance of pets activating alarms.
Often, PIR technology will be paired with
another model to maximize accuracy and reduce
energy usage. PIR draws less energy than
microwave detection, and so many sensors are
calibrated so that when the PIR sensor is tripped, it
activates a microwave sensor. If the latter also
picks up an intruder, then the alarm is sounded. As
interior motion detectors do not „see‟ through
windows or walls, motion-sensitive outdoor
lighting is often recommended to enhance
comprehensive efforts to protect your property.
False alarms are those usually caused by
technical errors such as electrical and mechanical
failures. Nuisance alarms are system activations not
commonly caused by attackers or intruders but
rather from windblown debris, animals, insects and
foliage.
Sequencing alarm systems to trip the alert
mechanism only when both alarm sensors have
been activated will reduce nuisance alarms, but
may also cause the probability of detection to
decrease.
VIII. CONCLUSIONS
This repots explains the application and uses of
different types of sensors to make our work easy
and to get accurate result. This report describes
commonly used six different advanced sensors.
This autonomous or sensor technology have an
important impact on every small and big field.