What is a Sensor? Different Types of Sensors and their Applications
We live in a World of Sensors. You can find different types of
Sensors in our homes, offices, cars etc. working to make our lives
easier by turning on the lights by detecting our presence, adjusting the
room temperature, detect smoke or fire, make us delicious coffee, open
garage doors as soon as our car is near the door and many other tasks.
All these and many other automation tasks are possible because of
Sensors. Before going in to the details of What is a Sensor, What are
the Different Types of Sensors and Applications of these different types
of Sensors, we will first take a look at a simple example of an
automated system, which is possible because of Sensors (and many other
components as well).
Outline
Real Time Application of Sensors
The example we are talking about here is the Autopilot System in
aircrafts. Almost all civilian and military aircrafts have the feature
of Automatic Flight Control system or sometimes called as Autopilot.
An Automatic Flight Control System consists of several sensors for
various tasks like speed control, height monitoring, position tracking,
status of doors, obstacle detection, fuel level, maneuvering and many
more. A Computer takes data from all these sensors and processes them by
comparing them with pre-designed values.
The computer then provides control signals to different parts like
engines, flaps, rudders, motors etc. that help in a smooth flight. The
combination of Sensors, Computers and Mechanics makes it possible to run
the plane in Autopilot Mode.
All the parameters i.e., the Sensors (which give inputs to the
Computers), the Computers (the brains of the system) and the mechanics
(the outputs of the system like engines and motors) are equally
important in building a successful automated system.
This is an extremely simplified version of Flight Control System. In
fact, there are hundreds of individual control systems which preform
unique tasks for a safe and smooth journey.
But in this tutorial, we will be concentrating on the Sensors part of
a system and look at different concepts associated with Sensors (like
types, characteristics, classification etc.).
What is a Sensor?
There are numerous definitions as to what a sensor is but I would
like to define a Sensor as an input device which provides an output
(signal) with respect to a specific physical quantity (input).
The term “input device” in the definition of a Sensor means that it
is part of a bigger system which provides input to a main control system
(like a Processor or a Microcontroller).
Another unique definition of a Sensor is as follows: It is a device
that converts signals from one energy domain to electrical domain. The
definition of the Sensor can be better understood if we take an example
in to consideration.
The simplest example of a sensor is an LDR or a Light Dependent
Resistor. It is a device, whose resistance varies according to intensity
of light it is subjected to. When the light falling on an LDR is more,
its resistance becomes very less and when the light is less, well, the
resistance of the LDR becomes very high.
We can connect this LDR in a voltage divider (along with other
resistor) and check the voltage drop across the LDR. This voltage can be
calibrated to the amount of light falling on the LDR. Hence, a Light
Sensor.
Now that we have seen what a sensor is, we will proceed further with the classification of Sensors.
Classification of Sensors
There are several classifications of sensors made by different
authors and experts. Some are very simple and some are very complex. The
following classification of sensors may already be used by an expert in
the subject but this is a very simple classification of sensors.
In the first classification of the sensors, they are divided in to
Active and Passive. Active Sensors are those which require an external
excitation signal or a power signal.
Passive Sensors, on the other hand, do not require any external power signal and directly generates output response.
The other type of classification is based on the means of detection
used in the sensor. Some of the means of detection are Electric,
Biological, Chemical, Radioactive etc.
The next classification is based on conversion phenomenon i.e., the
input and the output. Some of the common conversion phenomena are
Photoelectric, Thermoelectric, Electrochemical, Electromagnetic,
Thermooptic, etc.
The final classification of the sensors are Analog and Digital
Sensors. Analog Sensors produce an analog output i.e., a continuous
output signal (usually voltage but sometimes other quantities like
Resistance etc.) with respect to the quantity being measured.
Digital Sensors, in contrast to Analog Sensors, work with discrete or
digital data. The data in digital sensors, which is used for conversion
and transmission, is digital in nature.
Different Types of Sensors
The following is a list of different types of sensors that are
commonly used in various applications. All these sensors are used for
measuring one of the physical properties like Temperature, Resistance,
Capacitance, Conduction, Heat Transfer etc.
Temperature Sensor
Proximity Sensor
Accelerometer
IR Sensor (Infrared Sensor)
Pressure Sensor
Light Sensor
Ultrasonic Sensor
Smoke, Gas and Alcohol Sensor
Touch Sensor
Color Sensor
Humidity Sensor
Position Sensor
Magnetic Sensor (Hall Effect Sensor)
Microphone (Sound Sensor)
Tilt Sensor
Flow and Level Sensor
PIR Sensor
Touch Sensor
Strain and Weight Sensor
We will see about few of the above-mentioned sensors in brief. More
information about the sensors will be added subsequently. A list of
projects using the above sensors is given at the end of the page.
Temperature Sensor
One of the most common and most popular sensors is the Temperature
Sensor. A Temperature Sensor, as the name suggests, senses the
temperature i.e., it measures the changes in the temperature.
There are different types of Temperature Sensors like Temperature
Sensor ICs (like LM35, DS18B20), Thermistors, Thermocouples, RTD
(Resistive Temperature Devices), etc.
Temperature Sensors can be analog or digital. In an Analog
Temperature Sensor, the changes in the Temperature correspond to change
in its physical property like resistance or voltage. LM35 is a classic
Analog Temperature Sensor.
Coming to the Digital Temperature Sensor, the output is a discrete
digital value (usually, some numerical data after converting analog
value to digital value). DS18B20 is a simple Digital Temperature Sensor.
Temperature Sensors are used everywhere like computers, mobile phones, automobiles, air conditioning systems, industries etc.
A simple project using LM35 (Celsius Scale Temperature Sensor) is implemented in this project: TEMPERATURE CONTROLLED SYSTEM.
Proximity Sensors
A Proximity Sensor is a non-contact type sensor that detects the
presence of an object. Proximity Sensors can be implemented using
different techniques like Optical (like Infrared or Laser), Sound
(Ultrasonic), Magnetic (Hall Effect), Capacitive, etc.
Some of the applications of Proximity Sensors are Mobile Phones, Cars
(Parking Sensors), industries (object alignment), Ground Proximity in
Aircrafts, etc.
IR Sensors or Infrared Sensor are light based sensor that are used in
various applications like Proximity and Object Detection. IR Sensors
are used as proximity sensors in almost all mobile phones.
There are two types of Infrared or IR Sensors: Transmissive Type and
Reflective Type. In Transmissive Type IR Sensor, the IR Transmitter
(usually an IR LED) and the IR Detector (usually a Photo Diode) are
positioned facing each other so that when an object passes between them,
the sensor detects the object.
The other type of IR Sensor is a Reflective Type IR Sensor. In this,
the transmitter and the detector are positioned adjacent to each other
facing the object. When an object comes in front of the sensor, the
infrared light from the IR Transmitter is reflected from the object and
is detected by the IR Receiver and thus the sensor detects the object.
Different applications where IR Sensor is implemented are Mobile Phones, Robots, Industrial assembly, automobiles etc.
An Ultrasonic Sensor is a non-contact type device that can be used to
measure distance as well as velocity of an object. An Ultrasonic Sensor
works based on the properties of the sound waves with frequency greater
than that of the human audible range.
Using the time of flight of the sound wave, an Ultrasonic Sensor can
measure the distance of the object (similar to SONAR). The Doppler Shift
property of the sound wave is used to measure the velocity of an
object.
Sometimes also known as Photo Sensors, Light Sensors are one of the
important sensors. A simple Light Sensor available today is the Light
Dependent Resistor or LDR. The property of LDR is that its resistance is
inversely proportional to the intensity of the ambient light i.e., when
the intensity of light increases, its resistance decreases and
vise-versa.
By using LDR is a circuit, we can calibrate the changes in its
resistance to measure the intensity of Light. There are two other Light
Sensors (or Photo Sensors) which are often used in complex electronic
system design. They are Photo Diode and Photo Transistor. All these are Analog Sensors.
There are also Digital Light Sensors like BH1750, TSL2561, etc.,
which can calculate intensity of light and provide a digital equivalent
value.
One of the very useful sensors in safety related applications are
Smoke and Gas Sensors. Almost all offices and industries are equipped
with several smoke detectors, which detect any smoke (due to fire) and
sound an alarm.
Gas Sensors are more common in laboratories, large scale kitchens and
industries. They can detect different gases like LPG, Propane, Butane,
Methane (CH4), etc.
Now-a-days, smoke sensors (which often can detect smoke as well gas) are also installed in most homes as a safety measure.
The “MQ” series of sensors are a bunch of cheap sensors for detecting
CO, CO2, CH4, Alcohol, Propane, Butane, LPG etc. You can use these
sensors to build your own Smoke Sensor Application.
As the name suggests, an Alcohol Sensor detects alcohol. Usually,
alcohol sensors are used in breathalyzer devices, which determine
whether a person is drunk or not. Law enforcement personnel uses
breathalyzers to catch drunk-and-drive culprits.
We do not give much importance to touch sensors but they became an
integral part of our life. Whether you know or not, all touch screen
devices (Mobile Phones, Tablets, Laptops, etc.) have touch sensors in
them. Another common application of touch sensor is trackpads in our
laptops.
Touch Sensors, as the name suggests, detect touch of a finger or a
stylus. Often touch sensors are classified into Resistive and Capacitive
type. Almost all modern touch sensors are of Capacitive Types as they
are more accurate and have better signal to noise ratio.
If you want to build an application with Touch Sensor, then there are
low-cost modules available and using those touch sensors, you can
build TOUCH DIMMER SWITCH CIRCUIT USING ARDUINO.
Color Sensor
A Color Sensor is an useful device in building color sensing
applications in the field of image processing, color identification,
industrial object tracking etc. The TCS3200 is a simple Color Sensor,
which can detect any color and output a square wave proportional to the
wavelength of the detected color.
If you are interested in building a Color Sensor Application, checkout this ARDUINO BASED COLOR DETECTOR project.
Humidity Sensor
If you see Weather Monitoring Systems, they often provide temperature
as well as humidity data. So, measuring humidity is an important task
in many applications and Humidity Sensors help us in achieving this.
Often all humidity sensors measure relative humidity (a ratio of
water content in air to maximum potential of air to hold water). Since
relative humidity is dependent on temperature of air, almost all
Humidity Sensors can also measure Temperature.
Humidity Sensors are classified into Capacitive Type, Resistive Type
and Thermal Conductive Type. DHT11 and DHT22 are two of the frequently
used Humidity Sensors in DIY Community (the former is a resistive type
while the latter is capacitive type).
Often used to detect inclination or orientation, Tilt Sensors are one
of the simplest and inexpensive sensors out there. Previously, tilt
sensors are made up of Mercury (and hence they are sometimes called as
Mercury Switches) but most modern tilt sensors contain a roller ball.
In this article, we have seen about What is a Sensor, what are the
classification of sensors and Different Types of Sensors along with
their practical applications. In the future, I will update this article
with more sensors and their applications.