There are many types of sensors. They are used to measure and/or detect a huge variety of conditions including: temperature, pressure, level, humidity, speed, motion, distance, light or the presence/absence of an object and many other types. There are many versions of each type which may use a different sensing principle or may be designed to operate within different ranges. Sensors in some cases react to the environment in which they are placed and this reaction is used to measure the property being sensed. For example, a common temperature detector is known as an RTD (resistance temperature detector) and this contains a platinum wire. The electrical resistance of the wire changes with temperature so how the resistance changes can be used to measure the temperature. Many sensors use this type of principle where the variation of an electrical property of a sensing element is a measure of a property being sensed. Other types of sensors emit a signal and either measure how the area reacts to the emission or measure the reflection of the signal bouncing off an object in front. Inductive proximity sensors are one of the commonest sensors in use today. They emit a small electromagnetic field and use this to sense the properties of the area in front of the sensor. So they can detect a metal object in front and can be used to check if a machine is in the required part of a cycle or if a door is closed. Magnetic, capacitive and many other sensor use this type of principle. Some sensors send out a light signal and measure if it is reflected back. These are called photoelectric sensors and come in a multitude of varieties. Some directly detect a reflected signal (Direct detection mode), some check if a beam being reflected from a reflector is interrupted (retroflective mode) and others send a beam to another sensor receiver and detect an interruption of the beam (Opposed mode sensor). Other sensor such as radar and ultrasonic sensors operate also by detecting the reflected signal from the object being detected. Everyone is familiar with the police radar guns that detect the speed of a vehicle, the way a bat can "see" using reflected signals and the wonder of the pictures of unborn babies that can be produced from reflected ultrasonic signals.
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