Sensor classification schemes range from very simple to the complex. Depending on the classification purpose, different classification criteria may be selected. Here, we offer several practical ways to look at the sensors.

All sensors may be of two kinds: passive and active. A passive sensor does not need any additional energy source and directly generates an electric signal in response to an external stimulus; that is, the input stimulus energy is converted by the sensor into the output signal.

The examples are a thermocouple, a photodiode, and a piezoelectric sensor. Most of passive sensors are direct sensors as we defined them earlier. The active sensors require external power for their operation, which is called an excitation signal. read more