Industrial Sensor Terminology Reminders

| Scott DeVost

Industrial Sensor

Your automated systems have input devices. One of the most common input devices found in the field is a sensor. That includes photoelectric, process, condition, level and proximity sensors as well as a host of related mechanisms like safety switches and presence sensing devices. This post is going to talk all about sensors and how some sensors are either sinking or sourcing, as well as PNP or NPN. If your automated systems use sensors, this post will help you better understand some terminology as well as provide a few helpful hints for use on your factory floor.

Sinking // Sourcing

45JT Allen-Bradley Rockwell AutomationConsider a simple circuit that consists of one digital input connected to a digital output. The circuit needs a voltage source, a ground, and a load.

  1. A sourcing digital I/O provides the voltage needed for the circuit
  2. A sinking digital I/O provides the ground needed in the circuit

In both cases, the digital input provides the electrical path required for the circuit to work.


In most devices available on the market today there are two specific types of 3-wire sensors available: PNP and NPN.

  1. PNP stands for positive, negative, positive
  2. NPN stands for negative, positive, negative

Sensors are an example of devices that can be either NPN or PNP. The difference is a result of the internal circuit design and the type of transistors used.

Too Much Terminology! Help

Sinking or sourcing, PNP or NPN which one do I need and what is the difference? This is something that I struggled with for a while until someone taught me a trick. A useful mnemonic device. Remember this helpful hint:

  • The word “sinking” (SiNkiNg) has 2 N’s in it and is an NPN circuit
  • That leaves sourcing as the PNP

Takeaway #1 is a sinking circuit = NPN, remember “2 N’s”

Industrial Sensor Applications

45JT Industrial SensorSo what is the difference between the two? In a sourcing/PNP circuit, the sensor is switching the (+) to the load. In the example below if you start at the power supply and see where the switch is in the circuit you can determine if it is a PNP or NPN. If you are switching the (+) it is a PNP (sourcing) while if you are switching the (-) it is an NPN (sinking).

When selecting an output type for your sensor it is important to know which type you need for your application. For example, when connecting the sensor output to an Allen-Bradley I/O module remember that opposites attract, therefore if you have an NPN (sinking) input module you will need to pick a PNP (sourcing) sensor output.

  • If the Input device is NPN (sinking) select a sourcing I/O module.
  • If the Input device is PNP (sourcing) select a sinking I/O module.

Takeaway #2 is: opposites attract when connecting to an I/O module.

Real World Example: Photoelectric Sensors

So if we know the type of digital I/O, we can identify whether we need a PNP or NPN device. From there, we can further identify if we need a sinking or sourcing I/O module. Let’s look at Bulletin 42JS and 42JT VisiSight™ Photoelectric Sensors from Rockwell Automation®. These devices employ a Class 1 “eye-safe” red laser beam, a visible light beam for ease of alignment. They are designed with industry standard mounting holes spacing of 25.4 mm. Each model offers PNP or NPN and light operate or dark operate outputs. Let’s talk about that a little bit more…

42JT devices feature a unique “Auto PNP/NPN” output to continuously monitor how the load is connected and automatically configures the output for proper operation.

That’s a real time saver! I have found not only does it simplify installation, but it also makes maintenance easier and reduces stocking costs. It also helps folks that aren’t as familiar with terminologies like sinking, sourcing, PNP, and NPN. Want to learn more about the specifics on the Auto PNP/NPN functionality? Contact our team today!

Review – Two Helpful Reminders for Industrial Sensors

  • Takeaway #1 is a sinking circuit = NPN, remember 2 N’s
  • Takeaway #2 is opposites attract when connecting to an AB I/O module

Those two helpful hints will save you time and headaches in the field.