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A transducer is a device, usually electrical, electronic, electro-mechanical, electromagnetic, photonic, or photovoltaic that converts one type of energy to another for various purposes including measurement or information transfer (for example, pressure sensors). In a broader sense (for example in the Viable System Model) a transducer is sometimes defined as any device that converts a signal from one form to another. A very common device is an audio speaker, which converts electrical voltage variations representing music or speech, to mechanical cone vibration. The speaker cone in turn vibrates air molecules creating acoustical energy.

This list is confined to the narrower definition of the term.

• Electromagnetic:
  • Antenna - converts electromagnetic waves into electric current and vice versa.
  • Cathode ray tube (CRT) - converts electrical signals into visual form
  • Fluorescent lamp, light bulb - converts electrical power into visible light
  • Magnetic cartridge - converts motion into electrical form
  • Photocell or light-dependent resistor (LDR) - converts changes in light levels into resistance changes
  • Tape head - converts changing magnetic fields into electrical form
  • Hall effect sensor - converts a magnetic field level into electrical form
• Electrochemical:
  • pH probe
  • Electro-galvanic fuel cell
• Electromechanical (electromechanical output devices are generically called actuators):
  • Electroactive polymers
  • Galvanometer
  • MEMS
  • Rotary motor, linear motor
  • Vibration powered generator
  • Potentiometer when used for measuring position
  • Load cell converts force to mV/V electrical signal using strain gauges
  • Accelerometer
  • Strain gauge
  • String Potentiometer
  • Air flow sensor
• Electroacoustic:
  • Geophone - converts ground movement (displacement) into voltage
  • Gramophone pick-up
  • Hydrophone - converts changes in water pressure into an electrical form
  • Loudspeaker, earphone - converts changes in electrical signals into acoustic form
  • Microphone - converts changes in air pressure into an electrical signal
  • Piezoelectric crystal - converts pressure changes into electrical form
  • Tactile transducer
• Photoelectric:
  • Laser diode, light-emitting diode - convert electrical power into forms of light
  • Photodiode, photoresistor, phototransistor, photomultiplier tube - converts changing light levels into electrical form
• Electrostatic:
  • Electrometer
• Thermoelectric:
  • RTD Resistance Temperature Detector
  • Thermocouple
  • Peltier cooler
  • Thermistor (includes PTC resistor and NTC resistor)
• Radioacoustic:
  • Geiger-Müller tube used for measuring radioactivity.
  • Receiver (radio)

Info-acoustic transducers are transducers which convert information to sound, or sound to information. Typically, but not necessarily, the information is represented as an electrical signal.

There are four broad categories of info-acoustic transducers which may be categorized according to the state of matter in which they operate:

Since plasma is a gas or gaslike medium of charged particles, it has an acoustic impedance similar to that of gas such as air, thus ionophones can be used as microphones and loudspeakers.

Water is much more dense than air (about 1000 times more dense) so hydrophones do not make very good microphones, and likewise microphones (even if made waterproof) do not work well in water. However, geophones and hydrophones can sometimes produce acceptable results when interchanged, notwithstanding the fact that hydrophones respond to pressure but geophones respond to the derivative (i.e. can't respond all the way down to DC).

• Finite state transducer
• Ultrasonic transducer
• Signal transduction

• J. Allocca and A. Stuart, Transducers: Theory and Application, Reston 1984.

Wikimedia Commons has media related to:

Transducers

• What are Transducers?
• whatis: transducer
• Federal Standard 1037C, August 7, 1996: transducer
• American National Standard for Telecommunications - Telecom Glossary 2000: transducer