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this is more of a theoretical question, and I do not know if this is suited for this QA site at all, but at school I was asked to provide 3 examples of sensors (in the narrow sense) that are also actuators. I cannot find any information for the life of me. The only example I can find are piezoelectric sensors, but I am not sure if this is correct.

A sensor in the narrow sense is a device which reacts to a signal/stimulus from a electrical signal.

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    \$\begingroup\$ Due to the observer effect all sensors are actuators ;-) \$\endgroup\$ – Stefan Paul Noack Apr 24 '12 at 10:33
  • \$\begingroup\$ The last sentence is not clear: the sensor in that sense creates the signal from the stimulus, not vice versa \$\endgroup\$ – clabacchio Apr 24 '12 at 11:27
  • \$\begingroup\$ @noah1989 not always: potentiometric sensors are an example of not perturbing sensors, as they don't alter the position of the cursor \$\endgroup\$ – clabacchio Apr 24 '12 at 11:33
  • \$\begingroup\$ @clabacchio the position of the cursor is slightly altered by the electrons flowing through the pot ;-) but also the temperature increases (significantly sometimes!) \$\endgroup\$ – Stefan Paul Noack Apr 24 '12 at 11:49
  • \$\begingroup\$ @noah1989 mh I'd say as more plausible that the presence of the pot itself may alter the positioning accuracy of the object, but it may also be true...uhm... \$\endgroup\$ – clabacchio Apr 24 '12 at 11:57
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Examples:

  • (As you said) a piezoelectric transducer. It can produce sounds, or work as a microphone.
  • An electromechanical speaker. The same device can act as a speaker, or as a microphone (old walkie-talkie toys use that fact).
  • Even an LED. If you apply forward current through an LED, it produces light, and it is an actuator. If you bias it with a reverse voltage, and measure the reverse current (very low), you can measure light with it, and it will be a sensor.
  • An antenna. It can transmit and receive (even simultaneously, at different frequencies, or using other discrimination schemes).
  • An electric motor. You can use it to produce and to detect (and measure) movement.
  • Even an NTC/PTC resistor (which is supposed to be "only" a sensor), could also work as an actuator, if I inject enough current so as to modify its temperature, and that of its surroundings. I could use it to keep a (very) small mass at a constant temperature, in a low ambient temperature environment, and a single device would suffice as a sensor and actuator.
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  • \$\begingroup\$ About the speaker: you can also use your headphones as microphones, I did it in some cases \$\endgroup\$ – clabacchio Apr 24 '12 at 11:26
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To add some other cases:

  • To some extent, a resistor is also a temperature sensor/actuator

  • Peltier cells are also temperature/electricity converters

  • Inductors convert magnetic field into electric current and vice versa

  • Batteries are also a sort of chemical/electrical transducer, and they can work in both senses

  • A resistor can be considered also as current/voltage transducer, and it can be considered a current sensor, since usually the output (for ADC conversion) is a voltage

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  • \$\begingroup\$ Sorry, we overlapped the comment about resistors :-) \$\endgroup\$ – Telaclavo Apr 24 '12 at 11:20
  • \$\begingroup\$ @Telaclavo ouch :) no problem, I would remove it because yours is more complete but it's also a bit more specific (all resistors are temperature sensitive) \$\endgroup\$ – clabacchio Apr 24 '12 at 11:23

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