A transducer is a device that converts one form of energy to another. Usually a transducer converts a signal in one form of energy to a signal in another.
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.). The process of converting one form of energy to another is known as transduction.
Passive sensors require an external power source to operate, which is called an excitation signal. The signal is modulated by the sensor to produce an output 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 and/or voltage across the thermistor, LVDT.
Active sensors generate electric signals in response to an external stimulus without the need of an additional energy source. Such examples are a photodiode, and a piezoelectric sensor, thermocouple.
A sensor is a device that receives and responds to a signal or stimulus. Transducer is the other term that is sometimes interchangeably used instead of the term sensor, although there are subtle differences. A transducer is a term that can be used for the definition of many devices such as sensors, actuators, or transistors.
An actuator is a device that is responsible for moving or controlling a mechanism or system. It is operated by a source of energy, which can be mechanical force, electrical current, hydraulic fluid pressure, or pneumatic pressure, and converts that energy into motion. An actuator is the mechanism by which a control system acts upon an environment. The control system can be simple (a fixed mechanical or electronic system), software-based (e.g. a printer driver, robot control system), a human, or any other input.
Bidirectional transducers convert physical phenomena to electrical signals and also convert electrical signals into physical phenomena. Examples of inherently bidirectional transducers are antennae, which can convert conducted electrical signals to or from propagating electromagnetic waves, and voice coils, which convert electrical signals into sound (when used in a loudspeaker) or sound into electrical signals (when used in a microphone). Likewise, DC electric motors may be used to generate electrical power if the motor shaft is turned by an external torque.
High dynamic rangeHigh repeatabilityLow noiseLow hysteresisElectromagnetic:Antennae – converts propagating electromagnetic waves to and from conducted electrical signalsmagnetic cartridges – converts relative physical motion to and from electrical signalsTape head, disk read-and-write heads – converts magnetic fields on a magnetic medium to and from electrical signalsHall effect sensors – converts a magnetic field level into an electrical signalElectrochemical:pH probesElectro-galvanic fuel cellsHydrogen sensorsElectromechanical (electromechanical output devices are generically called actuators):AccelerometersAir flow sensorsElectroactive polymersRotary motors, linear motorsGalvonometersLinear variable differential transformers or rotary variably differential transformersLoad cells – converts force to mV/V electrical signal using strain gaugesMicroelectromechanical systemsPotentiometers (when used for measuring position)Pressure sensorsString potentiometersTactile sensorsVibration powered generatorsElectroacoustic:Loudspeakers, earphones – converts electrical signals into sound (amplified signal → magnetic field → motion → air pressure)Microphones – converts sound into an electrical signal (air pressure → motion of conductor/coil → magnetic field → electrical signal)Pickup (music technology) – converts motion of metal strings into an electrical signal (magnetism → electrical signal)Tactile transducers – converts electrical signal into vibration ( electrical signal → vibration)Piezoelectric crystals – converts deformations of solid-state crystals (vibrations) to and from electrical signalsGeophones – converts a ground movement (displacement) into voltage (vibrations → motion of conductor/coil → magnetic field → signal)Gramophone pickups – (air pressure → motion → magnetic field → electrical signal)Hydrophones – converts changes in water pressure into an electrical signalSonar transponders (water pressure → motion of conductor/coil → magnetic field → electrical signal)Ultrasonic transceivers, transmitting ultrasound (transduced from electricity) as well as receiving it after sound reflection from target objects, availing for imaging of those objects.Electro-optical (Photoelectric):Fluorescent lamps – converts electrical power into incoherent lightIncandescent lamps – converts electrical power into incoherent lightLight-emitting diodes – converts electrical power into incoherent lightLaser diodes – converts electrical power into coherent lightPhotodiodes, photoresistors, phototransistors, photomultipliers – converts changing light levels into electrical signalsPhotodetector or photoresistor or light dependent resistor (LDR) – converts changes in light levels into changes in electrical resistanceCathode-ray tubes (CRT) – converts electrical signals into visual signalsElectrostatic:ElectrometersThermoelectric:Resistance temperature detectors (RTD) – converts temperature into an electrical resistance signalThermocouples – converts relative temperatures of metallic junctions to electrical voltageThermistors (includes PTC resistor and NTC resistor)Radioacoustic:Geiger-Müller tubes – converts incident ionizing radiation to an electrical impulse signalRadio receivers converts electromagnetic transmissions to electrical signals.Radio transmitters converts electrical signals to electromagnetic transmissions.