Classification Sensors in Automation

In automation, there exists a variety of different sensors. This is only an attempt to have an overview of the different sensor types:

How sensors can be classified

How sensors can be classified


Active and passive sensors

Active sensors generate a voltage themselves and do not require any extra supply voltage to function. Examples: Tachogenerator, photocell, thermocouple

Passive sensors require a supply voltage. Examples: Measuring bridges, etc.


NPN and PNP sensors

NPN sensors have an NPN transistor in their switching stage. Here the load is connected with the switching output and the usually 24V supply voltage.

PNP sensors have a PNP transistor in their switching stage. Here the load is connected to the switching output and the common ground (-). Therefore they are also called positive switching.

NPN amd PNP-Sensors

NPN amd PNP-Sensors


Analog sensors:

Analog sensors produce an analog output signal proportional to the measurand. Examples are: Temperature sensors, pressure sensors, tachometers, etc. They can be classified by their output signals:

Analog sensor with voltage output:

Analog Sensor with Voltage output signal

Analog Sensor with Voltage output signal

  • Sending a signal over long distances produces voltage losses due to wire- and interconnecting resistances.
  • Voltage signals are sensitive to electromagnetic noise pickup. To minimize this drawback, shielded wires can be used, but with long distances they get costly.

Analog sensor with current output:

Analog Sensor with Current output signal

Analog Sensor with Current output signal

The 4...20 mA output is a very robust and reliable signal as the current flows through all components and the current can be maintained due to an internal sensor control. Faults caused by an open circuit or a lost feed can be detected by reading „0 mA“, while reading a signal between 4 and 20 mA would indicate a properly working circuit.