Introduction to the Lighted Pushbutton for Industrial Control Panels

What is a lighted pushbutton?

A lighted pushbutton is a common user interface found in industrial control panels. It allows single button on and off control of a mechanism or process. The Schneider Electric Harmony XB5 Switch assembly as shown in Figure 1 is a representative example. This switch is shown in a disassembled state so that the individual components may be seen including:

  • 22 mm pushbutton panel mount with blue lens ZB5AW363
  • switch body ZB5AZ009
  • Light Emitting Diode (LED) assembly ZBVB1
  • normally open contact (green tab) ZBE-101
  • normally closed contact (red tab) VBE-101

Tech Tip: Industrial components such as this Harmony XB5 switch are sold as complete modules or you can purchase the individual pieces as listed above. Availability of the preassembled modules is dependent on the popularity of a given configuration as there are often thousands of combinations and permutations for any given family of parts. Do look for the preconfigured modules as they have the potential to save you time and money.

Figure 1: Expanded view of the Schneider XB5AW36B5 Switch showing the individual components.

Tech Tip: The featured Schneider Harmony XB5 switch is but one of many configurations. For example, the switch blocks are available with quick disconnect spade terminals, spring loaded terminal, and the pictured screw terminals. Careful attention is required to select the proper components especially when we consider that the Harmony XB5 family includes over 2000 products ranging from the featured pushbutton, buzzers, and even potentiometers.

Why is the single pushbutton control preferred over a selector switch?

When coupled with a Programmable Logic Controller (PLC) the lighted pushbutton allows the software to be in control of the process. Normally, the pushbutton will respond to a user press; one press to activate and another press to deactivate the process. The integrated lamp provides immediate user feedback indicating the on off state of the process.

In an abnormal situation, the lighted pushbutton responds differently. For example, if the process is in a lockout condition (not ready), the pushbutton will not respond. The lack of illumination will signal that something is wrong. In another situation a running process may be halted. In the background the PLC performs the necessary actions to halt the process and then extinguishes the pushbutton’s LED.

The fundamental understanding is that the PLC is in control of the pushbutton’s LED. Consequently, the LED always indicates the state of the process. To better understand this statement, consider a control panel that uses a toggle switch. It is possible for the switch to physically be in the on state while the PLC has placed the process into an off state. This is highly undesirable as the state of the physical switch is not the same as the state of the PLC controlled process.

What are the attributes of an industrial switch?

To answer this question, we need to understand the nature of manufacturing and the extraordinary high cost of down time. Here the term extraordinary carries a cost of hundreds to thousands of dollars for every minute our production line is broken. This includes the cost of an idle workforce, time to restart the line, wasted product, lost opportunity, and overtime to get back on target.

Quality industrial components are therefore built to minimize down time. With regards to the switch, this includes modular construction with quick release tabs to disassemble the switch into its individual components. For example, consider Video 1 where Schneider Harmony switch body is disassembled in less than 10 seconds.

This modular construction allows the repair technician to quickly repair the system by replacing only the damaged subassembly. For example, a technician can replace a burned out normally open contact block very quickly. With two wires moved and a few clicks the switch is replaced. In practice it will take longer for the technician to arrive on scene and perform the Lock Out Tag Out (LOTO) procedure.

Returning to the topic of money, we see that the cost of an industrial switch is insignificant when compared to the cost of downtime. Reducing the technicians time and effort will impact the bottom line. For example, the switch will pay for itself multiple times over when compared to the time a technician takes to walk to the parts crib and back. Far better to have a factory floor with a limited set of components and technicians with spare parts at the ready in their toolkit.

Video 1: Fast disassembly of the Schneider Harmony XB5 switch.

Tech Tip: Tightening 22 mm industrial switches and panel indicators is facilitated using proper tools. For example, the Schneider Harmony XB5 tightening tool makes it easy to tighten the switch body while keeping the orientation square with the control panel resulting in a professional appearance.

How is the lighted pushbutton integrated into the PLC code?

The lighted pushbutton is associated with a Finite State Machine (FSM) in the PLC. Here, the PLC must detect a user’s button press and then toggle a binary FSM. The PLC code isn’t overly complex; however, it does require some familiarity with the subtle operations associated with the PLC’s program scan.

This tech forum post shows how to implement single pushbutton control using ladder logic on a Schneider Modicon TM221CE24T PLC. This is a good review of ladder logic that covers topics such as jumping to a label and the use of lockout variables.

Conclusion

The lighted pushbutton is at the heart of industrial control and automation. It’s a simple hardware device that serves as a gateway to greater PLC programming concepts. We find ourselves at the intersection of user interface design and programming based on state changes.

Your comments and suggestions are welcomed. Stories about well-built or not so well built control panels are especially welcomed.

Best Wishes,

APDahlen

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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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