Multi-functional backlighting technology illuminates Carrier Programmable Thermostat

The Edge® Programmable Thermostat is a stylish, ultra-slim “designer” thermostat for precise temperature and humidity control that borrows stylistically from high-end photo frames.

From the time its founder invented the basics of modern air conditioning in 1902, Carrier Corp. has been a leading innovator in air conditioning, heating and refrigeration systems. The company offers a wide variety of highly engineered thermostats for precise control over temperature, humidity, air treatment and scheduling, including the Edge® Programmable Thermostat.

The Edge® Programmable Thermostat is a stylish, ultra-slim “designer” thermostat for precise temperature and humidity control that borrows stylistically from high-end photo frames, according to Carrier. It provides a clean, elegant, simplified appearance on the outside with a large illuminated display highlighting inside temperature, outside temperature and time, with a choice of seven different stainless steel designer face plates. (Fig. 1)

When opened, it offers a wide range of push-button programmable features, including personalized humidity settings, customized settings for all seven days of the week, four daily time periods for more temperature options, and vacation programming for optimum energy efficiency.

“Our goal was to balance our feature-rich thermostat offerings with a clean simplified look that communicated essential information to the end user without appearing overwhelming and overly complicated,” says Mike Roher, senior product manager with Huntington, IN-based UT Electronic Controls, a subsidiary of the Carrier Electronics division of Carrier Corp. “Additionally, we met with our target consumer base and quickly noted trends in home decor that were not being met with traditional thermostat designs.”

Multiple Programmable Functions

Carrier decided to employ dedicated buttons for the thermostat’s programmable functions, determining through focus group testing that the buttons were more user-intuitive and removed some of the ambiguity associated with other technologies such as touch screen menus.

These multi-functional thermostats presented special backlighting requirements, such as discrete area backlighting for different parts of the display, including backlighting of the LCD, overlays (characters, logos), buttons, keypads and any other areas that needed illumination. The backlighting had to be bright, attractive and uniform, with no hot spots or dark areas.

Another design goal was to minimize thermostat thickness – Edge® Programmable thermostats are only 8/10ths of an inch thin – and to reduce the depth required to support backlighting.

Carrier chose LEDs as the light source for the backlight, as opposed to the EL (electroluminescent) technology used by some competitors. “We went with LEDs because it would enable us to vary the brightness intensity of the display and allow us to incorporate a variable backlight adjustment,” explains Roher.

Also, the number of LEDs used in the backlight needed to be kept to a minimum to make the most efficient use of the available LED light sources and reduce power consumption.

Backlighting Multiple Discrete Areas

Carrier turned to Global Lighting Technologies, Brecksville, OH, to help them design and manufacture the backlight unit (BLU). “We were introduced to GLT a couple of years ago and have found them to be the leading provider of backlighting solutions,” Roher says. “Their ability to provide consistent, even lighting over a large area at low power levels was critical to our new design.”

“Carrier approached us with what they had in mind,” says Tyra Jackson, the application engineer at GLT who worked with Carrier on much of the engineering design. “They presented us with parameters such as the room we would have to work with – including the space between the LCD and the printed circuit board (PCB) – as well as the preferred number and placement of the LEDs and the power requirements.”

Illumination had to range from straight backlighting of the LCD, characters and symbols to a "halo" effect around the keypad buttons that was both aesthetically appealing and functional.

Getting the light to the multiple locations behind the display and around the buttons with optimal brightness and uniformity was the main challenge, according to Tim Bixler, the GLT Sales Manager who worked with Carrier on the project. These areas included the main 3.5” x 3” LCD, the characters (lettering) and directional symbols (such as up/down arrows) at the top and sides of the unit, and the buttons at the top and bottom – nine discrete areas in all totaling approximately 22 square inches. Illumination had to range from straight backlighting of the LCD, characters and symbols to a “halo” effect around the keypad buttons that was both aesthetically appealing and functional. (See Fig. 2)

In addition, raised mechanical features had to be designed into the backlight to provide a solution for positioning the LCD as well as a slotted holder for the Zebra strip elastomeric connector between the LCD and the PC board.

Working closely with Carrier’s program engineer, Tim Comerford, GLT employed its patented MicroLens™ light extraction technology and optical modeling and prototyping expertise to provide the solution Carrier was looking for.

In the MicroLens™ process, micro-optical elements are molded directly into the top and bottom of an ultra-thin light guide and the light emission angle is optimized to increase the efficiency of the LED light sources.

“The number of LEDs had to be minimized, which required efficient use of the available LED light,” says Mike Roher. “LED placement was flexible during the concept phase of the design, and GLT was engaged early enough that there was flexibility in placement.”

Optimizing Light Distribution

GLT’s R&D and Optical Engineers were ready to meet this challenge. “The first part of our optimization of Carrier’s design was to choose the right LEDs,” notes Bob Ezell, senior R&D engineer at GLT. “We chose two side-emitting high-brightness LEDs (HBLEDs) that would best optimize the light coupling to the edge-lit light guide, then went about improving the distribution of the light. We took their LCD panel model, ran a baseline to see where the light was going, and created LED models for the GLT optical ray trace software.”

GLT designed a custom geometry into the tooling of the light guide to better control the spread of the light from the LEDs, and added custom optical lens array features in front of the LEDs to spread the light more widely than it was in Carrier’s original design.

“The normal spread of light from LEDs is 90º,” explains Ezell. “The lens arrays we added increased it to a range of 110º - 120º and improved the uniformity.” GLT also optimized the position of the LEDs, which are located in the lower left hand corner of the light guide. “We shifted them to the right a little bit from their original location,” said Ezell.

The entire MicroLens™ light guide is only 1 mm thick, a key to the slim, low profile design of the Edge® Programmable Thermostat. “Our design goal was to minimize thermostat thickness; one of the key drivers in previous designs was the depth required to support backlighting”, notes Mike Roher. “Our design goal was minimum thickness, and the direction to GLT was the same.” The result was a fully integrated “plug-and-play” backlight unit complete with LCD backlighting, multiple discrete button/graphic illumination, mechanical features, exceptional thinness, and maximum utilization of the fewest LEDs possible – all included in one simple assembly.

Design Flexibility

Carrier was able to leverage GLT’s backlighting technology for use in the Edge® Non-Programmable Thermostat, which has a smaller display, less programmable functionality, and is aimed at a different demographic.

The Edge® Programmable Thermostat’s display is also capable of variations in the shading of the backlight, which further enhances its visual appeal. “Leveraging the strengths of LED technology, the GLT product offering, and Carrier's innovation, we were able to include a patented variable backlight adjustment into the Edge product line,” notes Roher. “This allowed us to provide adjustable backlight levels.”

Carrier was also able to leverage GLT’s backlighting technology for use in the Edge® Non-Programmable Thermostat, which has a smaller display, less programmable functionality, and is aimed at a different demographic (people who are home all or most of the day) than the Programmable version (which is more useful for customers like working couples who are out of the house all day).

It has three buttons (Home-Away-Sleep) on the outside and the same three buttons on the top of the inside (Schedule-Fan-Mode) as the Programmable, but without the side buttons and 6 soft-key buttons along the bottom (days, time period, heating set point, cooling set point, etc. – functions that people who are home all or most of the time don’t really need).

“We chose to strategically illuminate the legends above the buttons for the Non-Programmable version to keep design consistency where possible between it and the Programmable thermostats,” explains Roher. “The backlighting technology that GLT provided enabled us to maintain the core functionality and design philosophy between the two while simply scaling the design for the difference in LCD area. We liked that a lot.”