LEDs Join Beads and Sequins to Make a Fashion Statement

Clothing and fabric designers are going to town incorporating LEDs into garments and fashion accessories. Although in its infancy, with some designs more in the realm of the hobbyist than Hugo Boss, it’s a growing business, nonetheless. Determining when and if a technology fashion will take hold in the consumer market is unpredictable at best. Yet, embroidering LEDs into fabrics, like sequins or beads, could be a high volume business, despite the fact that LEDs on clothing are not yet on the radar of the market reports for wearable electronic devices.


Figure 1: Left: Sequined gloves with 30 red, green and blue LEDs (3 per finger tip) for dance routines and street performances, runs from two CR2032s per glove. Right: The Visijax electronic cycle jacket with 23 high-intensity LEDs runs from 3 AAA batteries.

Outside of the fashion industry, more practical applications for wearable LEDs include high visibility straps, buckles and patches, which are incorporated into outdoor gear for cyclists, emergency services personnel and other security clothing.

The critical constraints for LEDs in the clothing industry are small size, low cost, robustness, low power and ‘sewability’. Curiously, leaded LEDs are as popular as surface-mount devices, as the leads provide the means of sewing the lights onto a fabric using a conductive thread to provide the electrical interconnects.

This article will look at some of the latest white and colored LEDs that suit clothing applications. Cree’s XLamp^® ML-E devices, for example, meet the need for brightness, while ROHM’s PicoLEDs are among the smallest, cheapest, lowest-power devices available.

Meanwhile, from a project development perspective, Arduino is proving to be a popular tool for developing the interactive and control aspects of strings and arrays of fabric-mounted LEDs. Evaluation and development kits available from Arduino and Olimex, typically based on the ATmega and ATtiny microcontrollers from Atmel, will be highlighted to show how they can be used in this type of application.

Haute couture to high visibility

A web search for wearable LEDs will pull up a huge array of information and images, from the gaudy and glitzy to the fashionable and functional. The crass flashing ties, hair noodles and belt buckles of a few years ago are giving way to more subtle and practical uses of incorporating LEDs into fashion and sports clothing and accessories. There continues, of course, to be increasing demand for outlandish designs for discos, concerts and gala performances. To serve this growing industry, there are a host of wearable technology specialists, combining textiles and technology, as well as hobbyist and gadget suppliers of specially-packaged and board-mounted LEDs and their associated circuitry.

The market is expanding as LEDs, controllers and power sources become smaller, cheaper, lower power and more reliable. Battery power is currently the limiting factor. Some wearable applications can exploit CR2032 or CR2450 coin cell batteries, but designers have to take care when selecting LEDs, as these batteries are current-limited. For higher current designs, more bulky battery options are AA and AAA as well as the CR123 camera battery. There will always be a compromise between number, type and brightness of LEDs, battery size and lifetime.

Typically, white and blue LEDs have higher forward voltages than other colors, at 2.9 to 3.2 V, while red LEDs tend to be the lowest at around 2 V. Running multiple LEDs in parallel can make it more difficult to operate from single coin cells, which is why some applications will use two in series. It should be noted also that for a given temperature, a small change in forward voltage can produce a significant change in forward current.

Forward voltage and forward current requirements can vary dramatically with LED die size, die material, and even die lot variations, as well as temperature. Typically, 3 V coin cell batteries deliver 4 to 9 mA. While finding LEDs rated at 3 V or less is not usually a problem, the majority have forward current figures in the tens or often hundreds of mA. Resistors are the lowest cost and most commonly used current limiting devices in this type of application. However, designers should be aware that resistors generate heat. A further consideration is that too little current and voltage is likely to result in reduced light output from the LED.

Technology and textiles

Take as an example, the light show gloves as illustrated in Figure 1.¹ The design features ten sets of RGB LEDs (three per fingertip), which can be set to flash, slowly change color or burn steadily. Each glove operates on two CR2032 coin cell batteries.

In contrast, the Visijax² cycling jacket incorporates no less than twenty-three high-intensity LEDs. Six white LEDs form two three-diode clusters at the front, five red LEDs on the back and six amber LEDs on each arm, with motion sensors to provide an automatic signaling system. Luminous intensity of the LEDs is 3800 mcd (white), 1200 mcd (red) and 1100 mcd (amber). The power source is three AAA batteries.

Employing a completely different design approach, wearable light company Vega³ has developed the Edge, comprising three LEDs mounted on a small circuit board, packaged in a leather case with reflective strip and strong magnets to hold the unit in place on clothing or on a bag, for example. The device is powered by a single CR2032 battery claimed to last for 20 hours.

Vega Edge has opted to use high brightness, Cree XLamp ML-E LEDs. These devices are specified to produce up to 58 lm at 500 mW. Typical devices include the MLEAWT-A1-CJCA cool white (6200K) and a diffused 5000K version, the MLEAWT-A1-R250. Typical forward voltage for the white lights is 3.2 V at 150 mA (parallel), 9.6 V sy 50 mA (series).

Size, cost and power trade-off

The ML-E family is supplied in a 3.45 x 3.5 mm package, but if size is a critical factor then even smaller packaged devices are available. Cree’s XLamp XQ-E series in white and colored variations measure just 1.6 x 1.6 mm in a 0606 package. The coolest white version, at 5700K, is the XQEAWT-0-BEE2, while a warmer white version, the XQEAWT-0-HCE6 has a CCT of 3500K. Lumen output is 116 lm/W and 102 lm/W, respectively. Typical forward voltage is 2.9 V at 350mA.

If cost and the smallest possible power source are more important than lumen output, as they might be for high volume use as decorative effects on fabrics for example, then LED indicator chips could be the answer. ROHM’s picoLED series, claimed to be the brightest and most efficient LEDs in their class, measure just 1.0 x 0.6 mm and come in a wide range of colors as well as RGB versions. Typical forward voltage is 2.9 V at 5 mA for most colors. For some red and orange devices, typical forward voltage is 1.9 V at just 1 mA. However, millicandela ratings are correspondingly low, from 2.1 to 7.6 mcd.

A fashionable pink model, the SMLP12HBC7W1, has a diffused lens, delivering 22 mcd, with a forward voltage of 2.9 V at 5 mA. Blue, red, green and yellow versions are also available. For its white picoLEDs, ROHM uses InGaN technology. The SMLP13WBC8W1, for example, has a CCT of 7700K, producing 150 mcd; yet forward voltage remains reasonably low at 2.9 V at 5 mA. These ultra-low-profile (just 0.2 mm), ultra-low-power devices are ideal for sewing into clothing.


Figure 2: A graduate project garment inspired by fashion designer Leah Buechly, co-designer of the LilyPad Arduino board, and featured in Craft magazine. This design uses Lumex SML-LX1206SIC-TR high-intensity red surface-mount LEDs rated at 70 mcd, with a forward voltage of 2 V at 20 mA. LEDs are sewn on using conductive thread.

Tinkering kits

One of the most popular development platforms for creating wearable LED designs is the Arduino development environment (IDE) with hardware based on the ATtiny or ATmega328 microcontrollers from Atmel and open source software. Key benefits of the Arduino IDE are low cost, ease of use for the non-specialist, and the fact that it plugs directly into the USB port of a computer. It provides power management and built-in voltage regulation, and can be connected to an external power source up to 12 V, and will regulate it to both 5 V and 3.3 V.

The Arduino Starter Kit is a comprehensive toolbox comprising the latest Arduino Uno board, twenty-nine LEDs in white, red, green, yellow, blue and RGB, a selection of capacitors, resistors and transistors and many other devices. Documentation helps the user explore the Arduino’s potential to control the physical world through sensors and actuators, by stepping through a number of projects.

For developers already familiar with Arduino, an extensive range of development boards and evaluation platforms is available. A wide range of accessories is also available, including LED boards in the TinkerKit series. Wearable device developers have already designed the LilyPad, specifically for textiles and clothing, which can be easily sewn onto fabrics and is even washable. The board uses the Atmel ATmega microcontroller (Figure 2).

The standard Arduino Uno board in SMD format is a good start point for development purposes, as is the smaller Arduino Nano, measuring just 18.5 x 43 mm, which can be powered via the Mini-B USB connection or a 5 V regulated external power supply.

Another option, inspired by Arduino, is the Olimexino-85-Kit and Olimexino-85-ASM from Olimex, based on Atmel’s ATtiny85 microcontroller. The kit is supplied with the microcontroller pre-loaded with the micronucleus tiny85 bootloader, and a selection of components ready to solder, plus a user manual. The ASM version is ready loaded and tested. Smaller, simpler and less powerful than the Arduino Uno, at 32 x 20 mm, the design has been used for wearable LED trinkets.

Low-power micros

The choice of microcontroller can have an impact on power consumption of a wearable LED configuration. Both the ATmega and ATtiny85 microcontrollers are small and extremely power efficient. The ATmega48A, for example, is an 8-bit risc microcontroller with a host of peripherals and six sleep modes. Operating voltage is from 1.8 to 5.5 V and power consumption at 1 MHz, 1.8 V, 25°C is 0.2 mA in active mode and 0.1 µA in power-down mode. The ATtiny85, with fewer peripherals and I/O, has an active mode power consumption of 300 µA at 1 MHz, 1.8 V, and just 0.1 µA at 1.8 V in power-down mode.

Energy harvesting

Ultimately, wearable LED-based devices are likely to be combined with energy-harvesting techniques. Already close to commercialization are fabrics that will be able to generate energy through solar or piezoelectric techniques. Additionally, energy storage is also likely to be incorporated into textiles, with fibers that can be woven to form batteries with supercapacitor-like features.

When these types of developments come to fruition, the power and energy storage challenges of LED-based wearable devices will all but disappear. LEDs could then bring a whole new meaning to flashy suits and color-coordinated accessories.

References

1. Lights & Décor for All Occasions website.
2. Visijax website.
3. Vege Wearable Light website.