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There are many different technologies used in sensors. To understand them you need to understand the individual technologies. You should select a group of sensors and learn that technology.
A collection of Sensors
Gyroscope
Linear Encoder
GPS
Camera
Laser Rangefinder
PIR
Sonar Ranging
Accelerometer
Lever Switch
Rotary Encoder
Piezo Bend
Metal Detector
Pendulum Resistive Tilt
Infrared Ranging
Pyroelectric Detector
Pressure
Resistive Bend
UV Detector
IR Modulator Receiver
Magnetometer
Microphone
Radiation
Compass
CDS Cell
Infrared Ranging
Gas
A collection of Sensors
Gyroscope
Linear Encoder
GPS
Camera
Laser Rangefinder
PIR
Sonar Ranging
Accelerometer
Lever Switch
Rotary Encoder
Piezo Bend
Metal Detector
Pendulum Resistive Tilt
Infrared Ranging
Pyroelectric Detector
Pressure
Resistive Bend
UV Detector
IR Modulator Receiver
Magnetometer
Microphone
Radiation
Compass
CDS Cell
Infrared Ranging
Gas