A sensor is a device, which receives and responds to a signal or stimulus. The "stimulus" means a property or a quantity that needs to be converted into electrical form. Hence, sensor can be defined as a device, which receives a signal and converts it into electrical form, which can be further used for electronic devices. A sensor differs from a transducer in the way that a transducer converts one form of energy into other form whereas a sensor converts the received signal into electrical form only.
A photoelectric sensor, or photo eye, is a device used to detect the distance, absence, or presence of an object by using a light transmitter, often infrared, and a photoelectric receiver. They are used extensively in industrial manufacturing.
Ultrasonic sensors generate high frequency sound waves and evaluate the echo which is received back by the sensor. These sensors are used to detect the presence of targets and to measure the distance to targets in many automated factories and process plants.
Automotive oxygen sensors, colloquially known as O2 sensors, make modern electronic fuel injection and emission control possible. The sensor does not actually measure oxygen concentration, but rather the amount of oxygen needed to completely oxidize any remaining combustibles in the exhaust gas.
A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact. A proximity sensor often emits an electromagnetic or electrostatic field, or a beam of electromagnetic radiation (infrared, for instance), and looks for changes in the field or return signal.
A humidity sensor also called a hygrometer, measures and regularly reports the relative humidity in the air. They may be used in homes for people with illnesses affected by humidity; as part of home heating, ventilating, air conditioning, and HVAC systems, and in humidors or wine cellars.
The resolution of a sensor is the smallest change it can detect in the quantity that it is measuring. Often in a digital display, the least significant digit will fluctuate, indicating that changes of that magnitude are only just resolved. The resolution is related to the precision with which the measurement is made.
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