LEDs as input devices
LEDs are fairly straightforward to use — just put them in series with a current-limiting resistor and apply forward voltage to produce light of whatever color the LED is designed for.
What isn’t as well-known, though, is that LEDs can also be used as photodetectors, sensitive to the color of light that they emit. This is a somewhat nonstandard use of an LED, but requires no more components than a standard blinking-LED project — an LED, a current-limiting resistor, and a microcontroller.
The trick is to run the LED backwards, reverse-biasing it for a short time (a microsecond is more than enough) — then disconnect the input (tristating the microcontroller I/O pin), and time how long it takes for the voltage to be reduced to below the TTL low threshold. Some leakage current will flow across the LED even in darkness, lowering the voltage towards zero with a time constant on the order of perhaps a few hundred microseconds. When brightly illuminated with light of the correct wavelength, however, a much greater photocurrent will flow, lowering the voltage on the pin in a few tens of microseconds.
Originally posted by Druscilla
reply to post by C0bzz
LED Computer Monitors are fairly popular now, replacing those old LCD models with sharper picture clarity.
June 6, 2001
The flicker of fluorescent lights, long a symbol of institutional drear, may give new freedom to the handicapped, thanks to a high-tech startup that sees the bulbs as the perfect transmitters.
Talking Lights, a Cambridge-based MIT spinoff, is developing a local area network that uses fluctuations in fluorescent lights to transmit data. Inventor, company founder and MIT professor Steven Leeb predicts the technology will be a boon for the disabled.
For example, he says, airport lights could direct a blind person carrying a special receiver-worn as a badge or held like a PDA-to the correct gate. Auditorium lights could broadcast enhanced audio to the hearing disabled, or transcriptions to the deaf. And research published this month suggests that the technology could greatly improve the rehabilitation of persons with traumatic brain injury.
In his MIT laboratory, Leeb recently demonstrated his invention. First, he turned on a circular fluorescent light. "See?" he asked. "A normal lamp. You probably have one in your bathroom."
Next, he picked up his receiver-a black box attached to two small speakers. From a few feet away, he pointed the receiver at the lamp. Music blared from the speakers. Tinny, but clear, came the familiar chorus from Handel's Messiah.
At the heart of the device is a new kind of ballast, the component of fluorescent lights that regulates the amount of electricity flowing into the lamp. Magnetic ballasts dim the lamp about every 1/120th of a second-the normal oscillation of alternating current-causing an imperceptible flicker. Newer electronic ballasts speed up the flicker rate to milliseconds, eliminating eyestrain and hum, two complaints long associated with fluorescents.
Since electronic ballasts flicker independently of the current's oscillation, Leeb realized that they could, with some modification, encode data. He designed ballasts that transmit both digital data-by turning the light on and off in short bursts-and analog data, by modulating the light's brightness by degrees.
A basic Leeb-designed ballast can encode a simple repeating signal, such as the location of an emergency exit. A more advanced version includes a modem to read data transmitted over the power line.
Burke and Leeb designed a system to remind brain-injured patients at Spaulding about their schedule. Each participating patient carried a "Personal Locator and Minder," a modified PDA programmed with the patient's schedule. Lights in the patients' rooms, hallways and certain other areas were set up to broadcast location information to the Personal Locator.
When the time of a scheduled event, such as therapy or medication, drew near, the Personal Locator would remind the patient. Using location information from the lights, the Personal Locator would judge whether the patient was proceeding toward the event and give more detailed directions if needed.
Leeb envisions far more applications for his invention. Malls could direct the blind and befuddled alike. Airlines could turn their plane's cabin lights into a data network, without adding to the miles of wire.
"Fluorescent lights are everywhere," he says. "The infrastructure is already in place."
August 24, 2011
Think about it: around the world, there are millions of street lamps, in every city and town on every continent. One visionary has a proposal to put each and every one of these lamps to work for a new purpose beyond illuminating the street below. They could serve as wireless Internet access points, communicating to devices, as well as vehicles.
Harald Haas, a professor of engineering at Edinburgh University, even has a name for this new networking technology: “Li-Fi,” for light-fidelity.
At a recent TED conference, Haas pitched his proposal for Li-Fi data transmission, suggesting that the applications and capacity for data would be limitless — from using car headlights to transmit data, or employing line of sight light sources as data transmitters.
Haas says data can be transmitted via LED bulbs that glow and darken faster than the human eye can see.
The system, which he’s calling D-Light, uses a mathematical trick called OFDM (orthogonal frequency division multiplexing), which allows it to vary the intensity of the LED’s output at a very fast rate, invisible to the human eye. For the eye, the bulb would simply be on and providing light. The signal can be picked up by simple receivers. As of now, Haas is reporting data rates of up to 10 MBit/s per second (faster than a typical broadband connection), and 100 MBit/s by the end of this year and possibly up to 1 GB in the future.
There’s plenty of capacity, he says: “We have 10,000 times more spectrum, 10,000 times more LEDs installed already in the infrastructure. You would agree with me, hopefully, there’s no issue of capacity anymore.” The added bonus, he adds, is that the infrastructure is free, and even would promote more rapid adoption of more energy-efficient LED bulbs. “It should be so cheap that it’s everywhere,” Haas says. “Using the visible light spectrum, which comes for free, you can piggy-back existing wireless services on the back of lighting equipment.”
Plus, there would be wireless access points anywhere there is a light source. Even smartphones, with their LED displays, could serve as data sources...
Wonder how possible it would be to spy on people on a large scale using this technique?
Originally posted by ManFromEurope
But what about your LCD-TV? Much bigger. Many more pixels. Better turn it to the wall if unused, right?
Sorry, just messing with you. The data-link is missing or would clog your bandwidth.