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issue: May 2008 APPLIANCE Magazine

Electronics Report
Bright Idea

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Appliance manufacturers can use light to communicate with end users thanks to a new capacitive switch technology.

The switches, created by UK-based companies Design LED Products and IGT Industries, integrate thin, printed lightguide technology with light emitting diodes (LEDs). According to Iain Kyle, business development manager at Design LED Products, the resulting technology gives designers a possible user feedback mechanism that defines the status of the device or switch.

“For example, the Apple iPod uses a ‘click’ sound to tell the user they successfully changed the volume,” Kyle says. “Using light as the feedback mechanism gives the designer more options; [for example, the] level of volume could be indicated by an illuminated scale and/or different colors of light. In the case of a music player, using light would make more sense, as you might not hear the ‘click’ sound when the volume is high.”

Kyle says that while the switches use off-the-shelf LED products and established manufacturing practices, they are unique in terms of design and the “construction recipe” of the lightguide structure. “Traditionally, the problem of evenly illuminating the surface immediately above the capacitive sense pad meant that the light source had to be located on the underside of the sense pad and a hole cut through it to allow the light to escape,” he explains. “As the area of the electrical field detected is very small and the presence of a hole disturbs this field, this made it much harder for the engineer to tune the circuit and make the switch work reliably.”

Design LED’s approach, however, allows the LED and lightguide layer to be placed “above” the capacitive sense pad, which means no holes are required. “We incorporate side-emitting LEDs in a thin, sandwich-type assembly with a flat, printed lightguide structure,” Kyle explains. “By printing scattering patterns at locations within the lightguide, where the light needs to exit, the light scatters in the vertical plane. The design and location of the scattering pattern control the location and uniformity of the escaping light, which can be at some distance from the LED. This sandwich structure can be linked to the drive circuitry using flexible ribbon cable, giving the designer greater flexibility, as they no longer need to place the LEDs on or next to the PCB.”

QTouch sensor chips from UK-based Quantum Research Corp. are connected to the capacitive sense pad. “These can detect the small changes in the electrical field caused by a finger moving over or close by the pad,” Kyle explains.

Design benefits of the switches include a thin form factor (less than 1 mm, including LED) and flexibility (curved shaped lightguides are possible). Kyle says the technology can also be integrated with capacitive and membrane switches and allows designers to have one or multiple lightguides in the same 1-mm-thick layer.

Another key feature, Kyle notes, is that the location and uniformity of the escaping light can be controlled. “The latter is important, as uniform lighting is pleasing to the human eye and brain and perceived as higher quality than light from a nonuniform source,” he says.

A wide range of colors is available, which Kyle says opens up many design opportunities for appliance makers. “Color is used as one of several techniques to better communicate the operational status of the device to the user, in particular to define the mode of operation in multifunction devices or as a status indicator (red means danger, green means okay),” he says. “The highly saturated colors are very noticeable to the eye, so this gives LED-based status panels a ‘presence’ or a level of visibility that exceeds other light sources.”




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