An Easy Way to Remember PNP and NPN Sensor Wiring

Here’s a simple way remember how to wire up a 3-wire DC PNP or NPN sensor:

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.

Click here for more information about the Basics of Automation.

2.14.2018_PNP v NPN Graphic


Where Did You Find That Sensor?

I recently visited a customer that has a large amount of assembly lines where they have several machine builders manufacturing assembly process lines to their specification. This assembly plant has three different business units and unfortunately, they do not communicate very well with each other. Digging deeper into their error proofing solutions, we found an enormous amount of sensors and cables that could perform the same function, however they mandated different part numbers. This situation was making it very difficult for maintenance employees and machine operators to select the best sensor for the application at hand due to redundancy with their sensor inventory.

The customer had four different types of M08 Inductive Proximity sensors that all had the same operating specifications with different mechanical specifications. For example, one sensor had a 2mm shorter housing than one of the others in inventory. These 2mm would hardly have an effect when installed into an application 99% of the time. The customer also had other business units using NPN output polarity VS PNP polarity making it even more difficult to select the correct sensor and in some situations adding even more downtime when the employee tried to replace an NPN sensor where a PNP offering was needed. As we all know, the NPN sensor looks identical to the PNP offering just by looking at it. One would have to really understand the part number breakdown when selecting the sensor, and when a machine is down this sometimes can be overlooked. This is why it is so important to standardize on sensor selection when possible. This will result in more organized inventory by reducing part numbers, reducing efforts from purchasing and more importantly offering less confusion for the maintenance personel that keep production running.

Below are five examples of M08 Inductive sensors that all have the same operating specifications. You will notice the difference in housing lengths and connection types. You can see that there can be some confusion when selecting the best one for a broad range of application areas. For example, the housing lengths are just a few millimeters different. You can clearly see that one or two of these offerings could be installed into 99% of the application areas where M08 sensors are needed for machine or part position or simply error proofing a process.

Shawn1Shawn2.pngShawn3Shawn4                                                             Shawn5

For more information on standardizing your sensor selection visit


Imagine the Perfect Photoelectric Sensor

Photoelectric sensors have been around for a long time and have made huge advancements in technology since the 1970’s.  We have gone from incandescent bulbs to modulated LED’s in red light, infrared and laser outputs.  Today we have multiple sensing modes like through-beam, diffuse, background suppression, retroreflective, luminescence, distance measuring and the list goes on and on.  The outputs of the sensors have made leaps from relays to PNP, NPN, PNP/NPN, analog, push/pull, triac, to having timers and counters and now they can communicate on networks.

The ability of the sensor to communicate on a network such as IO-Link is now enabling sensors to be smarter and provide more and more information.  The information provided can tell us the health of the sensor, for example, whether it needs re-alignment to provide us better diagnostics information to make troubleshooting faster thus reducing downtimes.  In addition, we can now distribute I/O over longer distances and configure just the right amount of IO in the required space on the machine reducing installation time.

IO-Link networks enable quick error free replacement of sensors that have failed or have been damaged.  If a sensor fails, the network has the ability to download the operating parameters to the sensor without the need of a programming device.

With all of these advancements in sensor technology why do we still have different sensors for each sensing mode?  Why can’t we have one sensor with one part number that would be completely configurable?


Just think of the possibilities of a single part number that could be configured for any of the basic sensing modes of through-beam, retroreflective, background suppression and diffuse. To be able to go from 30 or more part numbers to one part would save OEM’s end users a tremendous amount of money in spares. To be able to change the sensing mode on the fly and download the required parameters for a changing process or format change.  Even the ability to teach the sensing switch points on the fly, change the hysteresis, have variable counter and time delays.  Just imagine the ability to get more advanced diagnostics like stress level (I would like that myself), lifetime, operating hours, LED power and so much more.

Obviously we could not have one sensor part number with all of the different light sources but to have a sensor with a light source that could be completely configurable would be phenomenal.  Just think of the applications.  Just think outside the box.  Just imagine the possibilities.  Let us know what your thoughts are.

To learn more about photoelectric sensors, visit

Industrial Sensing Fundamentals – Back to the Basics: NPN vs PNP

Written by: Andrew Bollinger

What’s the difference and why should anyone care? If you’re confused by the terms PNP and NPN, then hopefully this post will shed some light on the differences between the two.  In the context of this post, they refer to the construction of a sensor’s transistor and whether it has a p-type or n-type semiconductor.

When it comes to wiring a sensor, you can think of the “N” as standing for “Negative” and the “P” as standing for “Positive”. With respect to sensors, an NPN device is one that can switch the negative side of the circuit while a PNP device switches the positive side.

The next question to ask is, what direction do you want the current to flow?

Continue reading “Industrial Sensing Fundamentals – Back to the Basics: NPN vs PNP”