An Improved Trainer for Relays and Small PLCs

The best way to start your journey into the world of industrial control and automation is to build circuits. The trainer described here provides an ideal platform to explore relay-based circuits and small Programmable Logic Controllers (PLC). This exploration is done using the actual components as found in an industrial control panel. This includes DIN rail mounted components as well as standard 22 mm switches and panel indicators as shown in Figure 1 featuring an Arduino Opta PLC.

This article serves as a brief introduction to the trainer. It showcases the attributes and a pedagogical philosophy that make this product desirable for classrooms and hobbyists interested in industrial automation.

Note: A recommended list of components (sold individually) is listed here.

Figure 1: Picture of the PLC trainer featuring the Phase Dock 1010 base and the Arduino Opta PLC.


Industrial control and automation components are available with many different operating voltages; with 24 VDC and 120 VAC devices being the most common. This trainer is designed for use with the low voltage 24 VDC components.

Do not use 120 VAC on this system.

The 24 VDC power is supplied by a user-supplied external bench power supply (not shown). This is ideal as most external power supplies contain current limiting circuitry. This will mitigate the novice mistakes that accompany the learning.

The circuit breaker shown in Figure 3 may be added to provide an additional layer of over current protection. It is also a useful place to secure power as the circuit is modified.

Tech Tip Do not use 120 VAC on this trainer. It lacks proper grounds to components such as the DIN rail. Also, line powered devices are unforgiving, resulting in burned or otherwise damaged components unless proper precautions are taken. Our recommendation is to learn using the relatively safe 24 VDC system and then transfer to line powered metal-based trainers or real-world production under the watchful eye of a mentor. Know that 24 VDC based control systems are common in industry to reduce the chance of electric shock.

Learning philosophy

Hands-on activities conducted with real-world components are one of the best ways to learn industrial automation and control circuits. Over time, students will become familiar with the industrial components. To be sure, they will struggle with the first lessons associated with a relay. With precut wires and the ability to try new ideas, they will quickly learn. In many respects, the trainer is like a solderless breadboard, allowing students to quickly fail upward.

With that consideration in mind, there is no attempt to make the trainer pretty. Wire duct could certainly be added. However, given rapid assembly and disassembly, such measures are not considered necessary. Also, the ability to follow a short wire from start to finish can be advantageous to the learner. At the same time, we encourage later lessons in wire management and labeling. After all, clean professional layouts with matching wire diagrams and wire labels are an essential skill for any panel construction shop.

Removable switch plate

The 4-hole plate may be removed, allowing access to the switches and panel indicators as shown in Figure 2. This feature continues the hands-on construction philosophy. It is especially useful for students to implement the many different switch types. For example, a forward reverse motor control circuit may feature pushbuttons with both normally closed and normally open contacts. Students learn valuable logistics lessons on selecting the proper configurable switch components along with the lesson in wiring the circuits.

Figure 2: The switch and indicator plate is released by removing the 6 knurled thumbscrews allowing access to the electrical contacts.

Use of precut wires with ferrule ends to reduce construction time

Precut wires with ferruled ends are not specifically part of the trainer but are certainly part of the design philosophy. In some respects, this technique is like the banana-plug equipment that has been used in schools for many decades. However, with this trainer, connections are made to real-world components. For example, instead of an abstract (plug board) concept of a normally closed or normally open relay contact, the student must locate the actual terminal connections on the actual device. This concept is especially useful when expanded to support a variety of components from a variety of manufactures.

A few precut wires are presented on the lower right-hand side of Figure 3. Here are several links that describe the selection and handling of wire with ferrule terminations:

Figure 3: Picture showing a variety of experiment-ready components that could be installed on the Phase Dock base.

Learn using a diversity of common industrial and automation components

Perhaps the trainer’s greatest strength is the ability to select a wide range of components. This diversity will prepare them for the variety of components found in the field. It’s like archery target practice. While it is true that shooting the same distance every day will improve performance, changing the distance with every shot will yield better overall performance. To be sure, this comes at the expense of short-term learning, but yields the best long-term results.

Part of this short-term learning process requires students to research component data sheets. Over time, students will learn to generalize the connections for a variety of components. For example, with a clear picture of a 4PDT relay, students will be able to intuit the connections without reference to the datasheet. This is a skill best developed only exploring the operation of a variety of relays.

Figure 4: A collection of control relays that may be encountered in the field including DPDT and 4PDT types, along with both spring and screw terminal connections.


The industrial control and automation trainer introduced in this note provides a solid learning platform students and hobbyists. The greatest strength lies in the diversity of supported components. This includes thousands of switch / pushbutton combinations, panel indicators, relays, programmable relays, and PLC options.

The trainer provides a relatively low-cost introduction to industrial control. Perhaps the greatest advantage is the investment in versatile industrial components. These components may be used and reused across the curriculum from a first introduction to circuits all the way to capstone projects. Again, think of this as a solderless breadboard for industrial components. Your students may try new ideas and then transfer their working concepts into a more permanent form.

Please share your ideas for future labs. Better yet, add design challenges that can be accomplished using a variety of relays.

Please bookmark this page. It is under development, and we plan to add representative labs soon.

Best Wishes,


About the author

Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (interwoven). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing articles such as this. LinkedIn | Aaron Dahlen - Application Engineer - DigiKey

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