Beginner's Guide to Connecting Outputs with Arduino Opta PLC

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This post is written 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 connect the Arduino Opta Programmable Logic Controller so that it may control real world devices.

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For readers who are new to industrial controls, we should take a moment to recognize the serious nature of our actions. A mistake could cost money, production time, damage to equipment, fire, or even hurt a person.

As a starting point for all work that involves machinery or high voltage systems, there are some critical stipulations:

  • you are qualified to perform the work

  • you understand the risk associated with industrial controls including but not limited to electrical, mechanical, entanglement, chemical, heat, fluid injection, and fire.

  • you have informed your supervisor about non-standard modifications or operations on a piece of equipment

  • you, along with your supervisor and other essential personnel, perform a risk assessment and safety analysis prior to performing the work

  • you have proper Lock Out Tag Out (LOTO) processes in place

  • you follow all applicable employer, state, and federal guidelines

We start with this stern warning because the Arduino Opta is no toy. It is designed to live and work in an industrial environment. It is perfectly capable of controlling large machinery with the potential to hurt people. Likewise, it may be installed in an enclosure next to dangerous high voltage three-phase power to control large motor starter.

With that said, we are going to start slow and safe with a 24 VDC systems. Later, as you gain experience and serve with a mentor, you can move to high voltage systems. In this note we will focus on mitigating the electrical hazards. Perhaps in another post we can explore the concept of machines states and the need to prevent unexpected motions which could lead to bodily harm.

Tech Tip: The coloring of control panel wiring tells a story to the initiated. Recommend you follow a standard where blue wire is used for 24 VDC while white-with-blue-stripe is used for the 24 VDC return. This standard ensures consistency across installations. This “language” of wire has the potential to reduce repair time in the future.

Physical Description of the Opta PLC’s Outputs

The Arduino Opta PLC features a total of four relay-based outputs. According to the data sheet each relay is capable of 10 A at 250 VAC. As a beginner, I advise against getting anywhere near those design maximum specifications. Instead, I recommend a conservative output of no more than 1 A at 24 VDC. If additional current or voltage is required, we should consider using an interposing relay. Again, this is a beginners beginner’s guide. Just because the Opta can doesn’t mean we should. At least not at first, not until we have experience building systems.

Tech Tip: An interposing relay is a control relay that is in the middle. It is installed between the PLC and a larger contactor. For example, suppose the PLC is used to control a large 3-phase motor. Let us further assume that the motor starter has a 120 VAC coil. The PLC will be wired to drive the 24 VDC coil of the interposing relay. The contacts of this interposing relay are then used to power the 120 VAC coil for the motor starter. This configuration increases the capability of the PLC. It also serves as a safety function by segregating the wiring. The interposing relay keeps the 24 VDC control signals away from the 120 VAC wiring.

Safety Schematic Featuring 24 VDC

The good news is that 24 VDC systems are common with many compatible components select from. This includes components such as pneumatic actuators, panel lamps, a host of control relays, and contactors to power large loads. This is good news for the beginner as we will be experimenting with standardized industrial components that are safe and cost effective.

This schematic shows one way to connect the Arduino Opta to a variety of 24 VDC loads. Observe that a measure of protection is granted by a 5 A circuit breaker. While this type of breaker is typically associated with higher voltage AC circuits, it is also rated for low voltage DC operation. Know that this circuit breaker may or may not be required depending on the nature of your chosen DC power supply. Some power supplies feature overcurrent protection while others feature fuse protection. Some have no protection resulting in damage if a wiring mistake is made.

Tech Tip: This wire diagram features a daisy chain connection. This is facilitated with dual-wire ferrules as described in this post.

The next item of interest for that schematic is the daisy-chain connection of the output relays where PLC pins 1, 3, 5, and 7 are provided with 24 VDC. The various loads can then be powered by pins 2, 4, 6, or 8 with the other side of each load connected to the 24 VDC return. The industry calls this a “sourcing configuration.” With this setup the PLC is the source of the positive voltage.

We could go on to explore specific 24 VDC applications with the Arduino Opta PLC. Perhaps we will another day. However, this article should give you a solid foundation with common industrial components.

Parting Thoughts

Please find a mentor who can guide you through the development process as we have only begun to talk about the safety aspects associated with industrial equipment.

If you would like to know more about industrial automation safety you could start by researching key terms such as safety interlocks, emergency shutdown, safety relays, light curtain, and the general topic of SIL (Safety Integrity Levels).

Finally, 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. As an example, you can follow the Industrial Lighting light bulb icon to find Panel indicators. From there, you can down-select components until you find that perfect green panel lamp or that relay that operates on 24 VDC.

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 question about this material on this page or back on DigiKey’s primary TechForum page.

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Best Wishes,



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