A sensor is a device that detects and responds to some type of input from the physical environment. It measures an input quantity and converts it into a signal which can be read by an observer or by an instrument. A transducer is a device that converts one form of energy to another. Common sensors include temperature sensors, light sensors, motion sensors, pressure sensors, and humidity sensors. Sensors are used in a wide variety of applications including weather stations, security systems, automobiles, medical devices, traffic control, and many more.

1. SENSORS AND
TRANSDUSERS
Nikhilesh Vaidya,
T.Y.
Instrumentation Engg.,
S.G.G.S.I.E.&T., Nanded.

2. SENSORS
 A sensor is a device that, as its name signifies, measures a physical quantity and
then converts it into signals that can be read by the user or by any other
instrument.
 It detects events or changes in quantities and provides a corresponding output,
generally as an electrical or optical signal; for example,
a thermocouple converts temperature to an output voltage. But a mercury-in-
glass thermometer is also a sensor; it converts the measured temperature into
expansion and contraction of a liquid which can be read on a calibrated glass
tube.
 Light,motion,temperature,magneticfields, gravity, humidity, moisture, vibratio
n, pressure, electrical fields, sound, and other physical aspects of the external
environment

3. TRANSDUSERS
 A transducer is a device that converts one form of energy to another form of
energy. Energy types
include electrical, mechanical, electromagnetic(including light), chemical, acou
stic, and thermal energy. Usually a transducer converts a signal in one form of
energy to a signal in another[1] (for example, a loudspeaker converts an electric
signal to sound).
 Actuator , microphones , Resistance temperature detector, Geophone &
combination transducers have both functions; they both detect and create
action, example is an antenna, a transducer of radio waves.

4. Evangelista Torricelli
invented the barometer
in 1643. Torricelli’s tube
1] Barometer

5. The pressure of the air
molecules pushed the mercury
up into the glass tube.
Torricelli’s barometer used a
glass column suspended in a
bowl of mercury.

6. Barometer
• He concluded that the
weight of the mercury in the
tube was equal to the weight
of the air pressing down on
the mercury in the dish.
• Water is 13.6 times less
dense than mercury.
So, if we were to use water in a
barometer, it would be 13.6
times taller than a mercury
barometer.
So it would be 10.3 meters tall!

7. Barometer
 Airplanes use a special
type of barometer, called
an altimeter, to measure
altitude
 Higher atmospheric
pressure is usually
associated with fair
weather and clearing
skies.
 Tornadoes and hurricanes
can produce very low
barometric readings.

8. Barometers
Mount Everest
Sea level On top of Mount EverestSea level
fraction
of 1 atm
average altitude
(m) (ft)
1 0 0
1/2 5,486 18,000
1/3 8,376 27,480
1/10 16,132 52,926
1/100 30,901 101,381
1/1000 48,467 159,013
1/10000 69,464 227,899
1/100000 96,282 283,076

9. Bimetallic strip
 A bimetallic strip is used
to convert a temperature
change into mechanical
displacement.
 change in L = * Lo * (T
To)
Lower temperature Higher temperature

11. Fire Alarm If the fire breaks
out, the heat from
the fire will cause
the bimetallic strips
to bend upwards
and complete the
circuit. The alarm
bell then ring.
Bimetallic strip & its application

12. Smoke Sensors/detectors
A smoke detector is a device that senses smoke,
typically as an indicator of fire. Commercial and
residential security devices issue a signal to a fire
alarm control panel as part of a fire alarm system,
while household detectors, known as smoke alarms,
generally issue a local audible or visual alarm from the
detector itself.
TYPES:
 Optical
 Ionization

13. Optical
 Uses incandescent bulb or Light-emitting diode
,a lens, and a photoelectric receiver (typically a
photodiode)
 Two types
 Scattered light
 Blocked light

14. Scattered Light Method
 Under normal operation, the light passes the
photodetector
 Smoke scatters the light
 Photodetector catches scattered light
http://www.smokealarmdetectors.com/images/photoelectric%20smoke%20detector.jpg

15. Blocked Light Method
 A tight beam of light shines from source to detector
 Smoke blocks path, triggering alarm
 This type can span large rooms
http://www.sas.org/tcs/weeklyIssues/2004-07-16/feature1/index.html

16. Pros
 Less Prone to false alarms
 Less sensitive to steam and cooking fumes
 Better at detecting smoke from smoldering fires

17. Cons
 Slower to detect smoke than ionization method
Image courtesy of Wikimedia Commons
LED Photodetector

18. Ionization
 A radioactive source ionizes the air between two
electrodes.
 An ionization smoke detector uses a radioisotope
such as americium-241 to produce ionization in air; a
difference due to smoke is detected and an alarm is
generated. Ionization detectors are more sensitive to
the flaming stage of fires than optical detectors,
while optical detectors are more sensitive to fires in
the early smouldering stage.
 Changes in the current, due to particulate matter,
trigger an alarm
http://www.smokealarmdetectors.com/images/ionization%20smoke%20detector.jpg

19. Pros
 Detects particles too small to be visible
 Cheaper than optical detector
 Better at detecting smoke from hot fires

20. Cons
 Most prone to false alarms
Ionization Chamber
Image courtesy of Wikimedia Commons

21. Gas detector
 A gas detector is a device that
detects the presence of gases in an
area, often as part of a safety
system.
 Gas detectors can be used to detect
combustible, flammable and toxic
gases, and oxygen depletion.
 Gas leak detection is the process of
identifying potentially hazardous
gas leaks by sensors.
 Common sensors include infrared
point sensors, ultrasonic sensors,
electrochemical gas sensors, and
semiconductor sensors.

22.  Applications of Gas
Sensor:
 Process control industries
 Environmental monitoring
 Boiler control
 Fire detection
 Alcohol breath tests
 Detection of harmful gases
in mines
 Home safety
 Grading of agro-products
like coffee and spices

23. Rain sensor
 A rain sensor or rain switch is a
switching device activated by rainfall.
 There are two main applications for
rain sensors.
1. The first is a water
conservation device connected to an
automatic irrigation system that causes
the system to shut down in the event of
rainfall.
2. The second is a
device used to protect the interior of an
automobile from rain and to
support the automatic mode of
windscreen wipers
The sensor projects infrared light into the
windshield at a 45degree
angle. If the glass is dry, most of this light
is
reflected back into the sensor by the front
of the windshield. If water droplets are on
the glass, they reflect the light in
different directions the
wetter the glass, the less light makes it
back into the sensor.

24. Physics of Optical Sensor
 Optical sensor works on concept of reflection and
refraction.
 At critical angle of incidence only reflection occurs.
Air
Glass
Air
Glass
(a) Reflection and Refraction (b) Total Internal Reflection
θr
θi θi
θi>θcrit
Reflection
Refraction
Total Internal
Reflection

25. Physics of Optical Sensor
 Critical angle of incidence for some material
combinations
Material Critical angle
Windshield Glass / Air 41.15 degree
Windshield Glass / Water 61.28 degree
Windshield Glass / Ice 59.52 degree
Windshield Glass / PVC 76.83 degree
PVC / Air 42.52 degree

26. Physics of Optical Sensor
 Angle of incidence is kept slightly more than critical
angle for Windshield material/air combination.
 Presence of rain changes critical angle value.
1
2
3
Prisms
Air
IR LED IR Phototransistor
Measure
area
Plastic

27. Ceilometer
 The Cloud Height Sensor (the
ceilometer), determines cloud height
and levels in the atmosphere. The
ceilometer uses invisible laser
radiation to detect cloud levels.
 The ceilometer works by transmitting a
pulse of laser light into the atmosphere
and sensing the light return as it is
reflected back toward the ceilometer
by objects in its path. By timing the
interval between the transmission and
reception, the height of particles (such
as water droplets or ice crystals in
clouds) above the ceilometer is
calculated and reported to the Data
Collection Package.

28. Ceilometer
 One important use of the
ceilometer is to determine
cloud ceilings at airports.
 Height =h = ct/2 where:
h = Height of Cloud
c = Speed of Light (3 x 10^8
m/second)
t = Time from Transmission to
Reception

29. Radar gun
 A radar speed gun (also radar gun and
speed gun) is a device used to measure
the speed of moving objects.
 For things such as the measurement of
bowling speeds in cricket, speed of
pitched baseballs, athletes and tennis
serves.

30. Radar gun
 Mode of operation
Radar guns that operate using the X band (8 to 12 GHz)
frequency range are becoming less common because they
produce a strong and easily detectable beam.
V=(∆f*c)/(f*2)
The velocity of moving object is given by,

31. 31
Doppler effect
It is most common means of measuring remote moving objects.
The police radar trap is a well known example of this technique.
The Doppler effect is a very effective and accurate means of
measuring velocity. If a narrow radio beam or ultrasonic beam
is aimed at an object the beam will be reflected back to the
source. However, if the object is moving the frequency of the
received signal differs from that of the transmitted signal.
The difference between the two frequencies being a measure of
the velocity of the moving object. The received frequency will
be higher than the transmitted frequency if the moving object is
travelling towards the receiver and lower if the object is
travelling away.

32. Metal detector
 A Metal Detector is a
device which is used to
detect a metal object that
is not visible to our naked
eye .
 It consists of an oscillator
which produces an
alternating current that
passes through coil
producing alternating
magnetic field

33. COMPONENTS OF TYPICAL
METAL DETECTOR
 Stabilizer (optional) - used
to keep the unit steady as
you sweep it back and forth
 Control box - contains the
circuitry, controls ,
speaker, batteries and
the microprocessor
 Shaft - connects the control
box and the coil; often
adjustable so you can set it at
a comfortable level for your
height
 Search coil - the part that
actually senses the metal;
also known as the "search
head," "loop" or "antenna"

34. OPERATING PRINCIPLE
1. Transmit signal from the electronics causes transmit electrical current in transmit
coil.
2. Electrical current in the transmit coil causes a transmitted magnetic field.
3. Transmitted magnetic field causes electrical currents to flow in metal targets
(called eddy currents.)
4. Eddy currents generate a magnetic field. This field is altered compared to the
transmitted field.
5. Receive coil detects the magnetic field generated by eddy currents as a very small
voltage.
6. Signal from receive coil is amplified by receive electronics, then processed to
extract signal from the target, rather than
signals from other environment magnetic sources such as earth’s magnetic field.

35. • Principle of operation of metal detector:
 The operation of a metal detector is based on the
principle of electromagnetic induction.
 Metal detectors contain one or more inductor coils.
When metal is placed in a close proximity to a
varying magnetic field (generated by the coil or
coils), currents are induced in the metallic part .

36.  These current are called eddy Currents. The eddy Currents,
in turn, induce their own magnetic field (called eddy fields)
.
 These fields act in such a direction as to oppose that
generated by the coils. The resultant field (
H applied – H eddy ) and using a specially designed
electronic circuit can indicate the type of material being
magnetized.

37. APPILICATIONS
The most common applications for metal
detector include the following :
 Helpful for soldiers to detect land mines.
 Security in the places like airports , metro railways
etc.,
 Archaeology department like treasure hunting.
 Construction industry.

38.  1. A battery in the top of the metal detector activates the transmitter circuit (red)
that passes
 electricity down through a cable in the handle to the transmitter coil (red) at the
bottom.
 2. When electricity flows through the transmitter coil, it creates a magnetic field all
around it.
 3. If you sweep the detector above a metal object (such as this old gray spanner), the
magnetic field
 penetrates right through it.
 4. The magnetic field makes an electric current flow inside the metal object.
 5. This flowing electric current creates another magnetic field all around the object.
The magnetic
 field cuts through the receiver coil (blue) moving about up above it. The magnetic
field makes
 electricity flow around the receiver coil and up into the receiver circuit (blue) at the
top, making a
 loudspeaker buzz and alerting you you've found something
 Generally speaking, metal detectors work at a maximum depth of about 20–50cm
(8–20in).

39. Anemometer
An anemometer or windmeter is a
device used for measuring wind
speed, and is a common weather
station instrument.
1) Windmill anemometers
2) Hot Wire / Film Anemometer
3) Laser Doppler anemometers
4) Sonic anemometers

40. Anemometer

41. Anemometer
Uses of Anemometers
 Weather stations
 Wind speed helps indicate a change in weather patterns,
such as an approaching storm, which is important for
pilots, engineers, and climatologists.
 Aerospace engineers and physicists often use
anemometers. This type of anemometer is used in velocity
experiments. Velocity is the measurement of the rate and
direction of change in the position of an object. Laser
anemometers calculate the wind speed around cars,
airplanes, and spacecraft, for instance. Anemometers help
engineers make these vehicles more aerodynamic.

42. THE ELECTROMAGNETIC SPECTRUM

43. Hygrometer
 A hygrometer is an instrument used for measuring
the moisture content in the atmosphere.
1. WORKING OF Hair tension hygrometers

44. Hygrometer
 These devices use a human or animal hair under
tension. The length of the hair changes with
humidity and the length change may be magnified by
a mechanism and/or indicated on a dial or scale.
Whale bone may be used in place of hair.
 The instrument can be made more sensitive by
removing oils from the hair, such as by first soaking
the hair in diethyl ether.

45. Hygrometer
APPLICATIONS:
 Besides greenhouses and industrial spaces, hygrometers
are also used in some incubators (egg), and museums.
They are also used in the care of wooden musical
instruments such as guitars and violins
 Hygrometers are also used in the coating industry
because the application of paint and other coatings may
be very sensitive to humidity and dew point.
 EXAMPLE:
1kg of wet wood, which contains 500g of water has a
moisture content u of 50%

46. Reference:
http://electronics.howstuffworks.com/gadgets/other-
gadgets/metal-detector1.htm
http://education.nationalgeographic.com/education/ency
clopedia/anemometer/?ara=1
http://en.wikipedia.org/wiki/Anemometer
http://en.wikipedia.org/wiki/Smoke_detector