A sensor is a technological device or biological organ that detects, or senses, a signal or physical condition and chemical compounds.

Artificial sensors

Most sensors are electrical or electronic, although other types exist. A sensor is a type of transducer. Sensors are either direct indicating (e.g. a mercury thermometer or electrical meter) or are paired with an indicator (perhaps indirectly through an analog to digital converter, a computer and a display) so that the value sensed becomes human readable. Aside from other applications, sensors are heavily used in medicine, industry and robotics.

Since a signal is a form of energy, sensors can be classified according to the type of energy they detect. For example:

• light sensors: photocells, photodiodes, phototransistors, photo-electric tubes, CCDs, Nichols radiometer, Image sensor
• sound sensors: microphones, hydrophones, seismic sensors.
• temperature sensors: thermometers, thermocouples, temperature sensitive resistors (thermistors), bi-metal thermometers and thermostats
• heat sensors: bolometer, calorimeter
• radiation sensors: Geiger counter, dosimeter
• subatomic particle sensors: scintillometer, cloud chamber, bubble chamber
• electrical resistance sensors: ohmmeter, multimeter
• electrical current sensors: galvanometer, ammeter
• electrical voltage sensors: leaf electroscope , voltmeter
• electrical power sensors: watt hour meters
• magnetism sensors: magnetic compass, flux gate compass , magnetometer
• pressure sensors: barometer, barograph, pressure gauge, air speed indicator, rate of climb indicator , variometer
• gas and liquid flow sensors: flow sensor, anemometer, flow meter , gas meter, water meter
• chemical sensors: ion-selective electrodes, pH glass electrodes, redox electrodes
• motion sensors: radar gun, speedometer, tachometer, odometer, turn coordinator
• orientation sensors: gyroscope, artificial horizon, ring laser gyroscope
• mechanical sensors: position sensor, selsyn, switch, strain gauge
• proximity sensor- A type of distance sensor but less sophisticated. Only detects a specific proximity. A combination of a photocell and LED or laser. Applications in cell phones, paper detector in photocopiers, auto power standby/shutdown mode in notebooks and other devices.
• distance sensor (noncontacting) Several technolgies can be applied to sense distance:
  • Initialization free sensing
    • scanning laser- A narrow beam of laser light is scaned over the scene by a mirror. A photocell sensor located at an offset responds when the beam is reflected from an object to the sensor, whence the distance is calculated by triangulation.
    • acoustic: uses ultrasound time-of-flight echo return. Used in mid 20th century polaroid cameras and applied also to robotics.
    • focus. A large aperature lens may be focused by a servo system. The distance to an in-focus scene element may be determined by the lens setting.
    • binocular. Two images gathered on a known baseline are brought into coincidence by a system of mirrors and prisms. The adjustment is used to determine distance. Used in some cameras (called range-finder cameras) and on a larger scale in early battleship range-finders.
    • light time-of-flight. Used in modern surveying equipment, a short pulse of light is emitted and returned by a retroreflector. The return time of the pulse is proportional to the distance and is related to atmospheric density in a predictable way.
  • Initialized systems. These require starting from a known distance and accumulate incremental changes in measurements.
    • coherent laser- interference between transmitted and reflective lightwaves are counted and the distance is calculated. Capable of high precision.
• whisker sensor- A type of touch sensor and proximity sensor.

Biological sensors

All living organisms contain biological sensors with functions similar to those of the mechanical devices described. Most of these are specialized cells that are sensitive to:

• light, motion, temperature, magnetic fields, gravity, humidity, vibration, pressure, electrical fields, sound, and other physical aspects of the external environment;
• physical aspects of the internal environment, such as stretch, motion of the organism, and position of appendages (proprioception);
• an enormous array of environmental molecules, including toxins, nutrients, and pheromones;
• many aspects of the internal metabolic milieu, such as glucose level, oxygen level, or osmolality;
• an equally varied range of internal signal molecules, such as hormones, neurotransmitters, and cytokines;
• and even the differences between proteins of the organism itself and of the environment or alien creatures.

The human senses are examples of specialized neuronal sensors.

See also

• List of sensors
• Sensor network

Links

• Federal Standard 1037C, August 7, 1996: transducer
• American National Standard for Telecommunications - Telecom Glossary 2000: sensor
• SensEdu; how sensors work
• Overview of Sensors and Needs for Environmental Monitoring Clifford K. Ho, Alex Robinson, David R. Miller and Mary J. Davis Sensors 2005, 5, 4-37 [1] (open access) article

Disambiguation note for the orthographically challenged

A sensor is not to be confused with:

• censor, a person who decides what can be published
• censor, an ancient Roman political office
• censer, a utensil for incense