This post is for individuals familiar with the Arduino family of microcontrollers who wish to delve into the world of industrial control systems. In this short post we will explore how to select and then connect digital devices to the input of the Arduino Opta Programmable Logic Controller.
The Opta seamlessly integrates into the 24 VDC ecosystem associated with many industrial control systems. Consequently, we will focus exclusively on the 24 VDC compatibility feature and exclude the 12 VDC compliant variants.
Exploring the Opta PLC’s digital inputs
The Arduino Opta is designed for two general types of digital input devices including contact (switch / relay) and PNP (semiconductor) types. There are some important distinctions that must be understood to select compatible devices.
Contact type inputs
A contact can be a switch, sensor, or relay that features a contact closure that provides continuity to a PLC input. For example, in this sketch you can see a variety of inputs including contacts from the Master Control Relay (MCR) and a variety of normally open and normally closed switches to control the operation of the PLC. Collectively these are known as field device inputs.
This article is part of the DigiKey Field Guide for Industrial Automation
Location: Program It → PLC → Arduino Opta
Difficulty: 
Student — difficulty levels explained
Author: Aaron Dahlen | MSEE | Senior Applications Engineer, DigiKey
Last update: 13 Mar 2026
Observe that one side of each field device is tied to the 24 VDC rail while the other is connected to the associated PLC inputs. Depending on the state of the field device, 24 VDC may or may not be sent to the Opta’s input. There is no need for external pull-up or pull-down resistors. The Opta has the equivalent of a 9 kΩ input resistor.
Tech Tip: A natural question concerns selection of normally open vs normally closed contacts. As a rule, select the contact type so that field devices that start the machine are normally open while things that stop the machine are normally closed. This will provide a measure of failsafe operation against a broken wire or loose connection. The PLC program is modified to accommodate the switch polarity.
Semiconductor PNP type inputs
Many industrial sensors feature semiconductor outputs. An example is the proximity (prox) sensor shown in the schematic above. We must be careful when selecting semiconductor sensors as they come in two different configurations. In one version, the sensor will “pull” low while the other version will “pull” high. In the industry, these are known as NPN and PNP respectively. The name is a description of the output transistor used to control the sensor’s output. The NPN transistor will pull low like the open collector device you may have encountered in a digital logic class. The PNP transistor will pull high.
Knowing the transistor configuration is essential as it determines how sensor will interface with the PLC. Choose the PNP configuration for direct connection with the Opta!
Tech Tip: An NPN sensor may be used with the Opta. However, an external pull-up resistor must be included. This post describes how to configure the NPN sensor. You will find that the NPN sensors “pulls” down while the PNP pulls up. Observe that the PNP as describe in in this article is pulling up against the PLC’s internal resistance. No such resistor is available for the NPN device necessitation an external resistor.
When selecting a sensor from DigiKey’s parametric search engine, you can filter for the desired sensor configuration by selecting PNP from the “Output Type” field as shown here.
There are thousands of available field devices allowing you to optimize your process and control systems. As you build your industrial control system, please consider DigiKey’s offering by first visiting the Automation & Control page. The page contains a series of pictures showcasing the products to help you quickly locate the desired components.
We look forward to hearing from you as you design, build, troubleshoot, and repair your industrial control systems. Your feedback is invaluable to us and our readers. Please share your comments and questions about this material on this page or back on DigiKey’s primary TechForum page.
Best Wishes,
APDahlen
Continue Exploring Industrial Control Systems
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About This Author
Aaron Dahlen, LCDR USCG (Ret.), is a Senior Applications Engineer at DigiKey in Thief River Falls. His background in electronics and industrial automation was shaped by a 27-year military career as both technician and engineer, followed by over a decade of teaching.
Dahlen holds an MSEE from Minnesota State University, Mankato. He has taught in an ABET-accredited electrical engineering program, served as coordinator of an electronic engineering technology program, and instructed military technicians in component-level repair.
Today, he has returned to his home in northern Minnesota, completing a decades-long journey that began with a search for capacitors. Read his story here.