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A sensor is an equipment which detects and responds to some
characteristic of the sorrounding environment.  A sensor detects changes in the ambient conditions and
generates a corresponding output, typically an electrical or optical signal.
Sensors may be broadly classified into two categories:  passive and active.A passive sensor does not
require an additional energy source and directly produces an electric signal in
response to an external stimulus e.g. temperature changes are converted into
output voltage by  thermo couple. The active sensors require external power for their operation, which
is called as excitation signal. This signal is modified by the sensor to
produce the output signal e.g. thermistor.

Sensors have gained importance in our daily lives. Application and usage
of sensors is expanding quite rapidly. Sensors are increasingly being used in
varied domains like television remotes employ IR
sensors, automatic doors of shopping malls use passive Infrared
sensor and the street system makes use of LDR sensors etc. With advances in technology, the use of
sensors has surpassed the traditional fields of temperature, pressure or flow
measurement to explore the fields of manufacturing and machinery, airplanes and
aerospace, cars, medicine and robotics, artificial intelligence etc.

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Some useful
terms which characterize a sensor are discussed below:

Sensitivity
Sensitivity may be
defined as the output generated by a sensor when it comes into contact of a specific
analyte.

Stability
A sensor must be stable
in its composition. The changes occurring in the sensor while sensing the
analyte must only be temporary and the sensor must return back to its original
parameters once the loses contact with the analyte. Also the sensor must be
neutral to the changing weather conditions. Such changes may turn the sensor unfit
for reuse.

Reproducibility
A sensor must produce
similar results under the presence of the analyte and the results must be reproducible.
It has been observed that continuous usage of sensors at elevated temperatures
over long durations causes such changes in the sensor which hamper their
sensitivity. Such changes are called “drift”. Endeavours must be made
to minimize the drift. For a good sensor, prolonged and repeated use, musn’t diminish
its sensitivity.

Response
time The duration required by
any sensor to achieve ninety percent of its saturation value is known as
response time of that particular sensor.

Recovery
time The time taken by a
sensor to drop down to its original parameters once the analyte has been
removed from the sensor’s ambience.

Selectivity
Selectivity is the ability
of a sensor to respond to a particular analyte amongst a mixture of other things
of similar fashion.

Resolution The smallest modification in the quantum of the analyte
that can be sensed by a sensor is called reolution of that sensor.

The most common types of sensors in use
nowadays are

Temperature Sensor
Displacement
sensor
Ultrasonic
sensor
Humidity
Sensor
Gas
Sensor

All the living organisms possess
biological sensors. These sensors are made of specialized cells and function
excellently. These are sensitive to light, heat, cold, movement, magnetic fields, electrical fields, sound, gravity, humidity, vibration, pressure, and numerous other physical
aspects of the external environment.

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