Everything for Electronics
Posted in: Developing Perspectives (December 2011)


By Bryan Bergeron

Still, there’s a lot of energy being wasted by LEDbased lighting — especially if you think of the light they generate as unused bandwidth. Making use of this bandwidth is the basis of LI-FI (Light Fidelity) which has many parallels with WI-FI.

Proponents of LI-FI contend that room light, automobile lights, interior aircraft lights, and street lights all represent under-utilized bandwidth that can carry everything from voice communications and HDTV to intelligent navigation signaling between cars. If you want a great vision of what’s possible with the technology, check out the free TEDtalks podcast, “Wireless Data From Every Lightbulb,” available on iTunes.

Communications with light is nothing new. Roman soldiers used polished shields to reflect sunlight when signaling during battle. Commercial, secure, line-of-sight laser communications systems have been around for decades. I’ve used a DIY He-Ne gas laser and telescope system to communicate over five miles, but at modest bandwidth. And, of course, much of the phone system is based on optical transmission and switching.

IR light signaling has been around for some time. You probably own at least one IR remote. However, as with UV light, you wouldn’t want to bathe a room with IR light because it can damage your vision. The real R&D activity is around visible light. There’s plenty of bandwidth available, and it’s not likely that you’ll inadvertently stare into a bright light and burn your retinas.

As with RF communications, the transmission process is straightforward and doesn’t take much in terms of hardware. Reception is the sticking point. There are issues of inadequate signal strength, how to handle multipath signals formed by light reflecting off of different objects and arriving at the receiver out of time, and interference from other light sources, just to name a few.

So, assuming I’ve piqued your interest, where do you begin? Start simple. Get an ordinary red LED and phototransistor — each coupled to your favorite microcontroller — to communicate with each other. Once you’ve managed a simple simplex serial connection, go for full duplex (two-way) communications.

To give you an example of what’s involved, I’ve been experimenting with parallel data streams using red, green, and blue LEDs. Driving low power red, green, and blue LEDs with an Arduino is trivial. I use external switching transistors with each LED so that I can handle high power LEDs.

On the receiver end, I’m working with two different approaches. The first uses separate phototransistors for red, green, and blue light. I use standard photographic acetate as a filter for each phototransistor. Red, green, and blue transparent film from acetate report covers works just about as well.

I’m also working with color-sensitive light sensors, including the TCS3200-DB color sensor from Parallax. At almost $60, it’s expensive, but it has a built-in array of photodetectors with red, green, and blue filters. It’s worth looking at the spec sheet (downloadable from Parallax.com) to get an idea of how they’re using the chip. Parallax also sells the TSL230R light-to-frequency converter. As with the TCS3200-DB, the $6 chip isn’t designed for color light communications, but it has potential that’s worth exploring.

SparkFun electronics sells an inexpensive TEMT600 ($1.50) light sensor that can be put behind a color acetate filter. As with standard phototransistors, these devices are more sensitive to certain portions of the visible spectrum than others. SparkFun also sells the Avago ADJD-S311- CR999 that has built-in red-green-blue filters ($5). Unfortunately, you’ll have to be skilled at SMT mounting to get at the input and output leads.

Of course, the real work for harnessing broadband light communications is in the software. Checksums and other error-detection mechanisms provision for interference from other visible light transmitters. There’s the approach used by most IR remote controllers — that of modulating the bean at about 38 kHz. Take a look at the spec sheet on the IR receiver TSOP85 — available from SparkFun — to get an idea of what’s typically involved in light receivers.

Of course, if you’re on an unlimited budget, then you could start considering diffraction gratings, prisms, and other commercial-grade tools developed for the fiber optics community. Edmund Scientifics is a good place to look for information and products.

There are obvious technical and behavioral issues that must be addressed before LI-FI becomes ubiquitous. For example, there will be no more tucking your phone in your pocket or purse. You’ll have to expose at least part of the phone to light — perhaps as Bluetooth chest-pin communicators akin to those on Star Trek. Then, there are times when lights are normally out — when you’re sleeping at home or in a plane. There are places and times — the beach or on your bike, for example — where there isn’t normally artificial light.

Clearly Li-Fi isn’t ready to displace the current WI-FI infrastructure. Perhaps you’ll be the experimenter-entrepreneur that works out the kinks and brings LI-FI to market. NV