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An easy way to remember PNP and NPN sensor wiring

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There are 2 different technologies for the use of sensors in automation technology. 3-wire sensors are very often used in automation. There are 2 different types of inductive proximity switches, optical sensors and capacitive switching sensors: PNP sensors and NPN sensors. Here is an easy way to memorize how to wire a 3-wire PNP or NPN sensor:

Difference between PNP and NPN

  • PNP = Switched Positive

  • NPN = Switched Negative

“Switched” refers to which side of the controlled load (relay, small indicator, PLC input) is being switched electrically. Either the load is connected to Negative and the Positive is switched (PNP), or the load is connected to Positive and the Negative is switched (NPN). These diagrams illustrate the differences between the two connections.

These diagrams illustrate the differences:

Distribution of PNP and NPN technology

PNP technology is a standard in automation technology in Europe. Common PLC input modules such as the Siemens ET200 require a PNP sensor as a signal transmitter. The digital inputs of this PLC module expect +24V as an input signal. (Attention, however, there are also NPN input cards)

NPN sensors are mainly used by control systems in Asia, especially in Japan. However, their importance and prevalence continues to decline.


  • Basics of Automation
  • Sensors
  • IO-Link
  • RFID

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Manfred Münzl

Manfred Münzl

Active in industrial sensor & automation technology for over 30 years.

1 Contributions


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