1. Sensors and Transducers
Characteristic
Transducers converts the form of energy
A microphone coverts sound energy into electrical energy
A speaker converts electrical energy into sound
Sensors are transducers that used to detect and/or measure something
Used to convert mechanical, thermal, magnetic, chemical, or etc variations into electrical voltages and currents

2. Sensors and Transducers
This course only focuses on a limited number of sensor types. Specifically:
Some Temperature sensors
Some light sensors
Some strain gages
Many other types exist
Smart Sensors
Infrared
Switches
RFID
etc

3. Sensors and Transducers
Temperature Sensors
Types of Temperature Sensors
Thermocouple
Resistance temperature device(RTD)
Thermistor
Monolithic IC Sensors
Characteristics
Most common sensor
A pair of dissimilar wires welded together at the sensing location
A temperature difference from the welded end and the other end causes a DC voltage at the non welded end
Can be used under extreme conditions
Ovens, Furnaces, Nuclear tests

4. Sensors and Transducers
Temperature Sensors

5. Sensors and Transducers
Temperature Sensors
Thermocouple
Operation
When wires made of dissimilar metals are
Welded together at both ends
With different temperatures at both ends
Current flows

6. Sensors and Transducers
Temperature Sensors
Thermocouple
Operation
Open the pair of wires in
between the two ends a
voltage develops
Called Seebeck Voltage
Proportional to
temperature difference

7. Sensors and Transducers
Temperature Sensors
Thermocouple
Operation
Equation is linear over only a small range of temperatures
Tables of corrected voltages in 10 increments is available from the NBS for each type
Reference Junction
Voltage developed is dependent upon the temperature difference between ends – NOT Absolute Temperature of the welded end
Where’s the
cold junction
It’s at room temp
Voltage will be wrong
Need a 00C ref

8. Sensors and Transducers
Temperature Sensors
Thermocouple
Reference Junction
Lab Set up
Not practical for most situations
Practical Reference Junction
solutions
Electronic Ice points
Available for All types
of thermocouples
Encased electronic device
that balances an internal bridge
circuit which generates a voltage to cancel out effect that
the measurement end isn’t at OOC

9. Sensors and Transducers
Temperature Sensors
Thermocouple
Practical Reference Junction
solutions
Isothermal block
Usually used with
computerized (also microcontrollers) data collection systems
The isothermal block is a good conductor of heat not electrical current
However it’s resistance is effected measurably by changes in temperature
Block is always near the point were the voltages are measured
Computerized measuring system calculates cool end temperature based on the block resistance and corrects the voltage reading

10. Sensors and Transducers
Temperature Sensors
Thermocouple
Typical Problems
A short of the two wires
Junction then will be at the point of the short
Temperatures readings will be incorrect
No Reference Junction Compensation
Test – Short the inputs to the compensator and room temperature should be the new reading
If extensions of the thermal couple wires are used they should be of a larger size and material
Different materials create Incorrect readings since the connection of dissimilar materials creates a new junction
Larger size is needed for IR drops

11. Sensors and Transducers
Temperature Sensors
Thermocouple
Typical Problems
Noise pick-up
Long leads form an antenna – uses shielding. e.g., grounded over braiding of copper
Extreme Temperature Gradient
Can damage the thermocouple should have protection
Environment can change the metal and it’s thermal characteristics
Chenicals
molten metals
– new alloys
Sample Commercial thermocouple assemblies

12. Sensors and Transducers
Temperature Sensors
Resistance Temperature Device
Key principle
As the temperature of a resistor increases so does its resistance
Measure the change in the resistance of a known resistor – calculate the temperature change
Linear relation ship for smaller changes – more linear than thermocouples – NBS has correction tables for the typical types of measurement resistors
Typical construction
Wire wound resistor in on a ceramic core using platinum wire
Stable (linear) over a wide range of temperatures
Temperature coefficient = /0C
Typical Values: 10 – several kilo-ohms
Most common value 100Ω

13. Sensors and Transducers
Temperature Sensors
Resistance Temperature Device
Measuring Circuit Types
RTD Bridge circuit
Constant Current Source
Platinum resistor is remote from the bridge circuit which is isolated from the sensing point
Bridge is balanced at a known temperature
Eliminates consideration of the connecting leads
Voltage developed is proportional to the temperature change

14. Sensors and Transducers
Temperature Sensors
Resistance Temperature Device
Constant Current through RTD
Voltage across the RTD rises and the resistance increase with the rise in temperature
The constant current also increases the temperature of the resistance and effects the temperature reading
The correction factor for common platinum RTDs has been determined

15. Sensors and Transducers
Temperature Sensors
Thermistor
Resistors with high negative temperature coefficients
Resistance decreases with an increase in temperature
High temperature coefficients means that there is a significant change in resistance for a small temperature change
Construction
Semiconductor material
In either tube or bead shapes
Can be used as a plain resistor in circuits such as a bridge or voltage divider
Come in a Wide range of values
Also come with manufacturer provided resistance vs temperature curves

16. Sensors and Transducers
Temperature Sensors
Thermistor
Construction
Also come manufacturer provided resistance vs temperature curves
Sample for thermistor with nominal value of 5kΩ at 00C

17. Sensors and Transducers
Temperature Sensors
Monolithic IC Sensors
Current or voltage types are available
They have linear output voltages or currents with temperature changes
Typical values: 1µA/0K; 10mV/0K
1 0K = 1 0C

18. Sensors and Transducers
Light Sensors
Typical uses of the sensors
Measure intensity of the light
Detect the presence or absence of light
Types of Light Sensors
Photovoltaic Cells
Photoconductive Cells
Photo Diodes
Phototransistors
aka, Solar Cells
Semiconductor material that generates a voltage when light shines on it
2.5 by 5 cm cell can produce 0.4 V with 180mA of current

19. Sensors and Transducers
Light Sensors
Photovoltaic Cells
Sometimes used to detect the presence of light
Photoconductive Cells
aka, photoresistors
Characteristics of Photoresisters
Uses bulk resistivity which decreases with increasing illumination, allowing more photocurrent to flow.
Signal current from the detector can be varied over a wide range by adjusting the applied voltage.
Thin film devices made by depositing a
layer of a photoconductive material
on a ceramic substrate.

20. Sensors and Transducers
Light Sensors
Photoconductive Cells
Characteristics of Photoresisters
Metal contacts with external connection. These thin films have a high sheet resistance. Therefore, the space between the two contacts is made narrow and long for low cell resistance at moderate light levels.

21. Sensors and Transducers
Light Sensors
Photoconductive Cells
Light Intensity Application
With little or no light the
voltage at point X is low
As the intensity of the
light on the sensor increases
the voltage at X will increase
By adjusting Rf,a usable output range of voltages that the is proportional to the light intensity can be obtained
Presence or Absence of
Light application
Activates a electromechanical
counter when the light is blocked

22. Sensors and Transducers
Light Sensors
Photoconductive Cells With a Microcontroller
Critical aspect of this application a BASIC command for measuring the RC decay time on a connected circuit
RCTIME command is designed to measure RC decay time on a circuit like the one below. The lower the count recorded the brighter the light measured
RCTIME Pin, State, Duration
Pin argument is the number of
the I/O pin that you want to measure
State argument - 1 if the voltage across the capacitor starts above 1.4 V and decays downward. 0 if the voltage across the capacitor starts below 1.4 V and grows upward
Duration argument has to be a variable that stores the time measurement, which is in 2 μs units
Very simple circuit – range of measured light is limited only by the size of the variable used to store the count.

23. Sensors and Transducers
Light Sensors
Photodiodes
A diode that is forward biased by light
Very fast reactions to changing light levels
Same physical size as LEDs
Have small windows through which light is
sensed
Testing is simple
When the window is blocked
High resistance is read
Shine a bright light (several footcandles) on it while still connected to an ohmmeter
The resistance will drop significantly
Phototransistors
Usually used instead of photodiodes when low light levels are measured

24. Sensors and Transducers
Light Sensors
Phototransistors
Usually used instead of photo resistors when low light levels are broken at high rates
Typical ratings
Like low power transistors
30-50V maximum collector to emitter voltages
Max collector currents of 25mA

25. Sensors and Transducers
Light Sensors
Replacement Considerations
Best option is an exact replacement
If not possible match the following characteristics :
Voltage, current, & power ratings; physical size
Light sensitivity
Can be specified nm (human sight ) nm (700nm – red light)
Called spectral response
Can also be specified in angstroms Å. 10 Å = 1nm
Light Insensitivity
For photoresistors – X-kΩ at Y-footcandles
1 Foot candle = light falling on 1 square foot – one foot from a standardcandle
For phototransistors: Collector current at a specified light level

26. Sensors and Transducers
Light Sensors
Other Problems with light sensing systems
Burned out, weak, or obstructed light sources
Can be a simple problem of dirty light filters or lens
Light shields may have been misaligned by a bump
Mechanical Sensors
Characteristics
Used to measure:
Force
motion
position
The chapter covers Strain gages
They measure Forces
Weight is a common force

27. Sensors and Transducers
Strain gages
Characteristics
Sensors used to measure change in the dimensions of solid objects caused by forces
Information is critical to designs of mechanical systems
Used in load cells which are used to measure weights of objects
Measurements can range from a few pounds to the weight of a fully loaded tractor trailer rig
Strain and Stress
Strain = ΔL/L0 , where ΔL = change in length due to a force
and L0 = the original length before the force was applied
Can be caused by tension or compression forces

28. Sensors and Transducers
Strain gages
Strain and Stress
Strain = ΔL/L0 , where ΔL = change in length due to a force
and = the original length before the force was
applied
Can be caused by tension or compression forces
Stress is a measure of the force applied that has been normalized to a unit area
Stress = F/A , where F= the total force applied and A= cross-
sectional area
The ratio of Stress/Strain is a constant value for each material
Called Young’s Modulus and has been tabulated for many material
Most metals won’t stretch beyond 0.5% without deforming

29. Sensors and Transducers
Strain gages
Strain and Stress
Resistor conductance can be determined from: R=ρL/A
Where R= resistance in ohms, ρ (rho) is the resistivity of the material, L= length of the material, & A is the cross-sectional area of the material
If the gage material under stress increases it length by0.4% - it’s resistance will increase by 0.4%
Some commercial gages have been designed to yield multiples of the change in length in change of resistance – A Gage Factor
Construction
Metal or semiconductor foil woven back and forth to increase the length
Range of common values Ω
Most common sizes 120 Ω and 350 Ω

30. Sensors and Transducers
Strain gages
Strain and Stress
Calculations
Where R =resistance of the gage under stress, R0 = Original resistance of the gage, ΔL = change in length of the gage, L = original length of the gage, GF = gage factor
Typical Bridge configurations

31. Sensors and Transducers
Strain gages
Typical Bridge configurations
The 1/4 bridge has a gain factor of 1
Change of resistance causes the bridge to unbalance
The ½ bridge has two strain gages
One in tension mode and one in compression mode, like in the metal beam drawing –bottom right of previous slide
rg1 is stretched and rg2 is compressed
Changes double the resistance change
GF = 2
The full bridge has four gages and a GF of four
Problems with Strain Gages
Temperature changes
If outside the circuitry must have temperature compensation
e.g., Las Vegas temperatures range from the 20’s – 115+

32. Sensors and Transducers
Strain gages
Typical

33. Some Smart Sensors Smart Sensor Characteristics
Inside every smart sensor
One or more primitive sensors
Primitive sensors are devices or materials that have some electrical property that changes with some physical phenomenon
Additional, built-in electronics makes a smart sensor "smart
Smart sensors able to do one or more of the following:
Pre-process their measured values into meaningful quantities
Communicate their measurements Orchestrate the actions of primitive circuits and sensors to "take" measurements
Make decisions and initiate action based on sensed conditions, independent of a microcontroller
Remember calibration or configuration settings
Three samples of smart sensors follow

34. Ultrasonic Distance Detector
Example – Parallax Ping)))
The Ping))) sensor interfaced
with a Controller measures
distances to objects
Range of 3 centimeters to 3.3 meters
Accurate to the centimeter
How it works
The controller starts by sending the Ping))) sensor a pulse to start the measurement.
Then, the Ping))) sensor waits a short period of time, enough for the controller to start a elapsed time counter.
Then, at the same time the Ping))) sensor chirps its 40 kHz tone, it sends a high signal to the controller.

35. Ultrasonic Distance Detector
Characteristics of the Parallax part
How it works
When the Ping))) sensor detects the echo with its ultrasonic microphone, it changes that high signal back to low.
The controller uses a variable to store how long the high signal from the Ping))) sensor lasted.

36. Ultrasonic Distance Detector
Characteristics of the Parallax part
How it works
This time measurement is how long it took sound to travel to the object and back.
Using this measurement and the speed of sound in air, you can make your program calculate the object's distance in centimeters, inches, feet, etc.
The Ping))) sensor's chirps are not audible because 40 kHz is ultrasonic.
What we consider sound is our inner ear's ability to detect the variations in air pressure
caused by vibration. The rate of these variations determines the pitch of the tone. Higher
frequency tones result in higher pitch sounds and lower frequency tones result in lower pitch
tones.
Most very young people can hear tones that range from 20 Hz, which is very low pitch, to 20 kHz, which is very high pitch. Subsonic is sound with frequencies below 20 Hz, and ultrasonic is sound
with frequencies above 20 kHz. Since the Ping))) sensor's chirps are at 40 kHz, they are
definitely ultrasonic, and not audible to people.

37. Accelerometer Acceleration
A measure of how quickly speed changes.
Just as a speedometer is a meter that measures speed
An accelerometer is a meter that measures acceleration.
Some of the measurements that an accelerometer can take:
Tilt and tilt angle
Incline
Rotation
Vibration
Collision
Gravity

38. Accelerometer
Accelerometers are already used in many different devices, including personal electronics, specialized equipment and machines. Here are just a few examples:
Self-balancing robots
Tilt-mode game controllers
Model airplane autopilots
Car alarm systems
Crash detection/airbag deployment systems
Human motion monitoring systems
Leveling tools

39. RFID
Definition
Radio-frequency identification (RFID) is an automatic identification method, relying on storing and remotely retrieving data using devices called RFID tags or transponders.
An RFID tag is an object that can be applied to or incorporated into a product, animal, or person for the purpose of identification using radiowaves. Some tags can be read from several meters away and beyond the line of sight of the reader.
Most RFID tags contain at least two parts. One is an integrated circuit for storing and processing information, modulating and demodulating a (RF) signal, and other specialized functions. The second is an antenna for receiving and transmitting the signal.

40. RFID A significant thrust in RFID use is: Types of Tags
In enterprise supply chain management
improving the efficiency of inventory
tracking and management.
Types of Tags
RFID tags come in three general
varieties:- passive, active, or semi-passive (also known as battery-assisted). Passive tags require no internal power source, thus being pure passive devices (they are only active when a reader is nearby to power them), whereas semi-passive and active tags require a power source, usually a small battery.
Walmart RFID Tag

41. RFID Simple Security System based on RFID
The system enables the reader
Waits for a Tag to be read
Compares the read identification to a list of known IDs
It there is a match the an electronic door lock is opened
We will use a Red LED
In addition the name associated with that Tag will be printed in the debug window
And the Piezo electric speaker beeps
If there isn’t a match – the speaker groans

42. Contact and Touch Sensors
Characteristics
As complex as a touch screen on a CRT or LCD monitor
Use in such varied applications as Customer service Kiosks in stores to video poker EGMs in casinos
To a tactile sensors
Used as input devices to video games, to the joy stick in the cockpit of a F-16 fighter …….
Contact sensors can be as simple as a switch
Used in numerous systems to detect a changed condition

43. Touchscreens Overview
Touchscreens systems come in three technologies each having three components:  The technologies are:
Surface Acoustic Wave (SAW)
Resistive and,
Capacitive
Information provided to the system
X, Y coordinates of the point touched
Used as substitute for button activation
It is based on sending acoustic waves across a clear glass panel with a series of transducers and reflectors.
When a finger touches the screen, the waves are absorbed, causing a touch event to be detected at that point (X, Y coordinates)

44. Tactile Sensors Can be used to detect a wide range of stimuli:
Presence or absence of a grasped object
Successfully pick up object, or failed to grasp object
To discovering a complete tactile image.
Texture, impact, slip and other contact conditions generate specific force and position patterns
This information again can be used to identify the state of manipulation.
Objects size, shape and mass can be used to test whether the object has been rotated ninety degrees and flipped , etc
Measure contact forces
Critical for Joy Stick implementations

45. Tactile Sensors Can be used to detect a wide range of stimuli:
Measure contact forces
Many of these sensors are on input devices that don’t move
i.e., a joy stick
The greater the force in a direction the greater of the output
The output verses input levels can be either linear or exponential
Control stick on an F-16
For easy and accurate
control of the aircraft during
high G-force combat
maneuvers, a side stick
controller is used