A sensor is a transducer whose purpose is to sense (that is, to detect) some characteristic of its environs. It detects events or changes in quantities and provides a corresponding output, generally as an electrical or optical signal; for example, a thermocouple converts temperature to an output voltage.

But a mercury-in-glass thermometer is also a sensor; it converts the measured temperature into a liquid’s expansion and contraction, which can be read on a calibrated glass tube. Sensors are used in everyday objects, such as touch-sensitive elevator buttons (tactile sensors) and lamps that dim or brighten by touching the base, besides innumerable applications of which most people are unaware. With advances in micromachinery and easy-to-use microcontroller platforms, sensors have expanded beyond the more traditional fields of temperature, pressure, or flow measurement, for example, into MARG sensors.

Moreover, analog sensors such as potentiometers and force-sensing resistors are still widely used. Applications include manufacturing and machinery, airplanes and aerospace, cars, medicine, and robotics. A sensor’s sensitivity indicates how much the sensor’s output changes when the input quantity being measured changes.

For instance, if the mercury in a thermometer moves 1 cm when the temperature changes by one °C, the sensitivity is 1 cm/°C (the slope Dy/Dx assuming a linear characteristic). Some sensors can also impact what they measure; for instance, a room temperature thermometer inserted into a hot cup of liquid cools the liquid while the liquid heats the thermometer.

Sensors need to be designed to have a negligible effect on what is measured; making the sensor smaller often improves this and may introduce other advantages. Technological progress allows more and more sensors to be manufactured on a microscopic scale as microsensors using MEMS technology. A microsensor usually reaches a significantly higher speed and sensitivity than macroscopic approaches.

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