Transducer

[1] Transducers are often employed at the boundaries of automation, measurement, and control systems, where electrical signals are converted to and from other physical quantities (energy, force, torque, light, motion, position, etc.).

[2] Transducers can be categorized by the direction information passes through them: Passive transducers require an external power source to operate, which is called an excitation signal.

For example, a thermistor does not generate any electrical signal, but by passing an electric current through it, its resistance can be measured by detecting variations in the current or voltage across the thermistor.

[5][2] Active transducers in contrast, generate electric current in response to an external stimulus which serves as the output signal without the need of an additional energy source.

[5] Some specifications that are used to rate transducers: Electromechanical input feeds meters and sensors, while electromechanical output devices are generically called actuators): Also known as photoelectric:

Mechanical transducer
Transducers are used in electronic communications systems to convert signals of various physical forms to electronic signals , and vice versa . In this example, the first transducer could be a microphone , and the second transducer could be a speaker .