PMI Design Guide for Engineers

Status indicators are all around us in everyday life. So much that the average person — or even a seasoned engineer — may not appreciate how much we rely on status indicators every day. The operation status of a device can be communicated using light pipes, panel mount indicators (PMIs), capacitive touch buttons, or some other form of light. And in many cases, the light is combined with icons or text to enhance the user experience.

It’s not all about looks and cool, sleek interfaces, either. The European Union’s equivalent of OSHA cited HMI design as an emerging risk due to the increased dependence on technology in the workplace. Translation: HMI design has the power to influence safety and outcomes. Effective status indication can:

Enhance the end-user experience
Prevent workplace accidents
Improve operator reaction times
Provide reliable indication for years

Fortunately, human-machine interfaces and control panels can be designed for optimal indication, whether they use PMIs, light pipes, or other illuminated communication signals.

This guide covers the key decisions designers and engineers must make to ensure their interfaces use the appropriate PMIs for the job. Whether you’ve been working with panel mount indicators for one year or 20, here’s a handy resource to help ensure you make informed indicator decisions early on in the design process. The sooner the right PMI is specified, the fewer compromises will need to be made later when the design is set and tooling is completed.

PMI FAQs

Common questions about panel mount indicator

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What’s a Panel Mount Indicator (PMI)?

A panel mount indicator is a type of status indicator attached directly to a device’s operator panel rather than the machine’s inner workings. A panel mount indicator may also be known as a:

Pilot light
Indicator light
Status indicator

Panel mount indicators go above and beyond communicating on or off status. These indicators are used in many industries to communicate operating and warning status to end-users. They can be powered with a range of lighting technologies — from LED panel mount indicators to neon or even incandescent PMIs.

Which industries use PMIs?

The short answer is, almost all of them. While you may think about the pilot light on your coffee maker or a power button on a conveyor belt at a warehouse, PMIs are used in devices of all sizes and industries, including:

Aerospace

Automotive

Medical

Industrial

Electronics

Entertainment

Power

Food and Beverage

Transportation

Technology

What do PMIs look like?

While PMIs can be customized in many ways to suit the application and the industry, they are all typically made up of:

Power source

PMIs can range from 2V up to 250VAC, depending on the application and design requirements.

Terminal

how is the PMI connected to the power source? Wire leads, screw terminals, solder terminals, or tabs that conduct electricity are used, depending on the design.

Housing

the plastic or metal piece of the indicator that encases the lamp.

Light source

the bulb used in the PMI, whether LED, incandescent, or neon.

IP67 Sealed Waterproof LED FlexVolt Panel Mount Indicator

Color(s)

the color of the lamp itself. Standard colors are yellow, red, blue, green, white and orange, with amber and violet options available in some indicators. Panel mount indicators can also be bi-color or tri-color.

Lens

the lens can come in various shapes and colors to provide the desired aesthetics and viewing angle for proper operation.

Dome – lens is rounded at the top and provides the widest viewing angle
High-dome – lens extends prominently above the control panel and is rounded at the top
Semi-dome – mid-range viewing angle compared to dome or flat
Flat – flush with the panel for more targeted light
Hi-hat – lens extends well beyond the surface of the HMI while maintaining a flat shape

Bezel

a bezel is an optional ring or sleeve that encases the lamp. Using a bezel can be purely aesthetic, or increase contrast between the bezel and LED to improve visibility. Bezels can also add a layer of protection from dust and moisture. Bezels are typically made of plastic or stainless steel and available in multiple finishes, styles, and colors.

Bezel color will be dictated by the type of material used. Common colors are:

White – plastic or metal PMIs
Black – plastic or metal PMIs
Silver – stainless / aluminum PMIs only

Mounting Types

PMIs are affixed to the control panel in many ways, each requiring a mounting hole where the indicator can be inserted and attached. Mounting hole sizes vary depending on the device. Typical standard sizes for VCC’s indicators range from 0.16” (3.98mm) to 0.88” (22mm), with the ability to customize the size. There are several ways a PMI can be attached to the control panel:

Snap-fit

like the name implies, a movable “snap” allows the PMI to be easily assembled to the panel without any additional tools or hardware.

Snap-fit with D mounting

a D-shaped mounting hole on the panel helps secure the snap-fit PMI to prevent rotation that could impact performance due to vibration. This option is easy to install without additional hardware.

Press-fit

the PMI is secured with the help of friction as the LED housing and panel are pressed together. The interference allowance will determine the tightness of the seal.

Threaded housing

this mounting helps reduce shock and vibration due to a threaded housing that screws securely into the panel. A hex nut can add additional security.

Speed nut

a speed nut is an additional accessory used to secure a PMI to the panel and provide extra protection from vibration and dust or moisture. A benefit of the speed nut is that it holds the PMI in place while allowing it to be easily removed or loosened if needed for maintenance of the device.

What’s the difference between a panel mount indicator and a light pipe?

Status indication is usually provided by using either PMIs or light pipes. They both do essentially the same thing — transmit light from A to B to communicate with the operator. However, there are some key differences, including:

Design flexibility

PMIs are mounted directly to the HMI or panel, so the light doesn’t have to travel very far. This means PMIs have less design flexibility than light pipes, which can be attached to the control panel or the circuit board. However, there are several ways to customize a panel mount indicator.

Space/footprint

depending on the visual communication within the device, space could be an issue. Are there three separate indicators, or a single indicator with a tri-color LED? Both PMIs and light pipes can communicate multiple functions within a single spot on the HMI. PMIs are very popular in industrial applications where individual indicators per function are common, and a heavy-duty panel may require a larger, more robust indicator.

Cost

PMIs typically cost more than light pipes, so if a device has more than two or three indicators, light pipes may be the preferred indicators.

How much does a PMI typically cost?

A panel mount indicator’s cost will vary greatly depending on a range of inputs, including:

Materials used
Size
Working environment / moisture protection
Device design
Number of indicators used
Lens type
Power needed
Brightness/intensity
Terminal type
Additional hardware

On average, engineers can expect to pay $1 to $6 per unit for a very basic PMI. On the other hand, those with additional features or customization can run $20 each or more. Consulting with an experienced team of experts can help you compare several options to find the ideal solution for your needs and budget.

Benefits of Using Panel Mount Indicators

There are many pros to using panel mount indicators instead of light pipes to communicate device status, including:

Several standard options available to meet most needs

for engineers looking to hit the ground running with a standard product, VCC offers thousands of standard PMI to choose from.

Easy to customize

if you need to make a slight modification to an existing product, you can change just about anything — from the wire length to the color of the LED.

Cost-effective installation + assembly

most panel mount indicators are designed to be assembled in place without the use of extra hardware or accessories, saving time in production.

Durability

PMIs are made to withstand harsh working environments. Depending on the housing used, terminals, and other features, they can be moisture- and dust-resistant.

Reliable, uniform illumination

when combined with LEDs, PMIs deliver reliable indication while eliminating the need for downtime due to lamp replacement.

Lighting Technology Used in PMIs

There are many pros to using panel mount indicators instead of light pipes to communicate device status, including:

Incandescent

similar to the first light bulb, today’s incandescent bulbs are an inexpensive PMI technology. However, they require more energy to operate and only last a fraction of the active life of LEDs. Therefore, burnt-out bulbs must be replaced often, which drives up maintenance costs and downtime. The average operating life of an incandescent bulb is 1,000 to 1,500 hours.

Neon

this lighting technology lasts much longer than incandescents while still using a minimal amount of power. Plus, high-voltage devices are ideal for neon technology. The average operating life of a neon lamp is 25,000 hours.

LEDs

light-emitting diodes (LEDs) use the latest technology to provide a host of benefits for manufacturers and end-users alike. With low power requirements, an average lifespan of an LED lamp is 100,000 hours or more.

Average Operating Life

Incandescent – 1,500 Hours 3% 3%
Neon – 25,000 Hours 25% 25%
LED – 100,000 Hours 100% 100%

While the LED panel mount indicator won’t always be the right fit for every device, it’s the technology we recommend to our clients whenever possible to provide efficiencies and greater value. In many cases, VCC helps customers upgrade their legacy products with LEDs that replace incandescents or neon. This option is more cost-effective than a redesign while eliminating the need for maintenance or bulb replacement. Learn more about LED retrofits in this example….

LED PMI Retrofits Deliver a Win-Win

As new products become more complex and expensive to produce, one cost-efficient alternative is to upgrade legacy products with LED PMIs. By converting from incandescent or neon lamps, OEMs can provide a more energy-efficient product that lasts longer, requires no maintenance, and uses the latest lighting technology. Here are a few examples:

Manual electric motor starter

CC helped one industrial electric motor starter OEM upgrade from neon to energy-efficient LED PMIs with the exact specifications required, plus the benefits of:

Up to 50% savings in energy consumption
An LED lamp that lasts 4x longer than its neon predecessor
Streamlined production with snap-fit design
High visibility for enhanced operator safety
Upgrading an existing motor starter unit with a custom PMI instead of a new product design

Industrial dishwasher

unlike consumer dishwashers, industrial dishwashers must withstand constant use and abuse from high-temp operation designed to eliminate the need for cleaning chemicals. Our design team helped one industrial dishwasher OEM upgrade its indicator from neon to LED technology and enjoy:

Significant energy savings
Switching to wire leads instead of tabbed terminals that kept breaking during high-vibration environment
Elimination of maintenance and warranty claims from tabbed terminal
Fast, tool-free assembly

What to Consider When Choosing a PMI

Of course, the application and budget may influence the decision. If not, here are some key design considerations:

Working Environment

The conditions where the device will be used must be taken into consideration, as some PMIs are better suited for harsh working conditions than others. Working environment is a collection of the following factors:

Moisture and Dust Protection

While the typical distinction will be outdoor vs. indoor, there are several indoor applications that could expose a device to moisture and dust. Ask yourself: Will the device be used indoors or outdoors? Will it be exposed to dust or moisture, whether from rain or harsh cleaning chemicals? If so, at what pressure and level? Then, use the IP rating system to help specify the appropriate component. The Ingress Protection, or IP rating system, helps engineers determine how protected a PMI may be from dust or moisture. Also known as International Protection ratings, these numeric scores reflect the sealing effectiveness of electrical enclosures when exposed to unwanted materials like dirt and moisture. See the chart below for more information on how the IP rating system works:

Solids are measured against the size of the particles, while liquids are mature in pressure and type — from drips and sprays to total immersion. IP ratings will vary by product, but VCC offers many solutions that fall within suitable ranges for a host of applications. For example: IP65 – this common rating is dust-tight and protects devices against water projected from a jet at any angle. IP67 – for additional protection, this rating is dust-tight and watertight when facing complete submersion in water for 30 minutes, strong gets, and waves. And if you need even more water protection, additional hardware such as an O-ring gasket can help you achieve your IP requirements. Be sure to check out each product’s datasheet for the specific IP ratings and optional accessories available.

Operating and Storage Temperatures

The operating and storage temperatures are other environmental factors that impact the overall life of a device. Therefore, working and storage temps should be a key design consideration for the PMI. Average operating range for VCC’s PMIs: -13℉ (-25℃) to 176℉ (80℃) but can be altered by the device design and work environment. Average storage temperature range for VCC’s PMIs: -22℉ (-30℃) to 212℉ (100℃) Several factors influence the operating and storage temperatures of a PMI, including the LEDs or lamps themselves. Some outdoor applications may experience huge ranges in temperatures, especially in construction environments. Safe operating and storage temperatures will always be listed on the datasheet for VCC’s PMIs.

Vibration

Many industrial applications — and some non-industrial ones, too — put extra stress on the device due to vibrations. There are several ways to address this and help prolong the operating life of the device, including:

Hexnuts or additional hardware to help secure the device in place
Threaded housing
D mounting hole prevents rotation

Viewing Angle

In order to determine the ideal viewing angle, engineers should consider the many ways a device will be used in the real world. How will the device be used? Are the operators standing, sitting, bending down? How far away will the operator be from the device? Will daytime visibility be a factor? Constant communication is the key, so the right viewing angle can help ensure safe operation in a variety of distances and environmental lighting conditions (daytime vs. night). Viewing angles can be impacted by the following parts of the PMI:

Lens shape
Lens texture
Lamp intensity
Housing/Bezel
Finish

Power Requirements

Of course, the application and budget may influence the decision. If not, here are some key design considerations:

Low voltage

most devices that use LED PMIs can operate at a lower voltage.

High voltage

more power is required for industrial machines, such as a commercial grill or dishwasher. VCC has a range of high-voltage PMIs available in both LED and neon. The LED option is great for updating high-voltage neon devices with longer-lasting lamps.

Flexible voltage

instead of a fixed power setting, indicators with flexible voltage can accommodate a range of needs. Flexible, or flex voltage PMIs are an excellent way for OEMs to save money by specifying the same PMI for multiple devices, each with varying power requirements.

Consider this example: Our PLM50 Series PMIs can operate in voltages from 5 to 28 VDC, offering greater savings and efficiencies to manufacturers with multiple product designs.

Application

Think only industrial applications use PMIs? Think again. Panel mount indicators can be found in a range of industries, including:

Foodservice

Ventless grill

we modified our 1092 Series to create a high-voltage PMI (250V) to help one commercial grill manufacturer save on production costs by eliminating the need for an additional 12V power converter.

Pizza cabinet

this innovative pizza warmer utilizes neon and LED PMIs to ensure intuitive status indication for self-serve customers.