Sensor
A sensor is a device that detects and responds to some type of
input from the physical environment. The specific input could be light, heat,
motion, moisture, pressure, or any one of a great number of other environmental
phenomena. The output is generally a signal that is converted to human-readable
display at the sensor location or transmitted electronically over a network for
reading or further processing.
In a mercury-based glass thermometer, the input is temperature.
The liquid contained expands and contracts in response, causing the level to be
higher or lower on the marked gauge, which is human-readable.
An oxygen sensor in a car's emission control system detects the
gasoline/oxygen ratio, usually through a chemical reaction that generates a
voltage. A computer in the engine reads the voltage and, if the mixture is not
optimal, readjusts the balance.
Motion sensors in various systems including home security lights,
automatic doors and bathroom fixtures typically send out some type of energy,
such as microwaves, ultrasonic waves or
light beams and detect when the flow of energy is interrupted by something
entering its path.
A photosensor detects the presence of
visible light, infrared transmission (IR),
and/or ultraviolet (UV) energy.
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