At its most basic, a
transducer is a device that converts one type of energy into another. There are two kinds of transducers; sensors and actuators.
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Sensors detect energy forms, such as light or force, and convert that energy into digital information that can be output.
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Actuators also receive an input and release an output; however, they do it in the opposite way that a sensor functions. An actuator is a transducer that takes in information and outputs some form of physical energy. For example, your smartphone can vibrate in your pocket through the use of actuators.
Sensors are transducers changing a physical impact into an output signal. This may be a thermocouple giving out a measurable change in voltage if heated up or down. Charge occurs if a force is introduced into a piezoelectric force washer.