Sensing and Communication Using Bi-Directional LEDs
Light Emitting Diodes (LEDs) are inexpensive and widely used as light sources. What is less well known is that LEDs are fundamentally photodiodes and as such are also light detectors. We have developed a novel microprocessor interface circuit that can alternately emit and detect light using only an LED, a resistor and two digital I/O pins.
Background & Objective: This project began as an effort to create a smart backlighting system for television remote controls. A low-power capacitive proximity sensor detects active handling which in turn controls the backlight. To save battery life, the backlight should not be turned on in bright conditions. But adding a separate light sensor would require a new mechanical design for the remote, adding considerable cost. Our solution was to use the backlight LED itself as the light sensor. We developed a simple microprocessor interface technique that uses one additional digital I/O pin, but no other additional components compared to those need to simply light the LED. Since the circuit draws only microwatts of power, it has a minimal impact on battery life.
Technical Discussion: The LED microprocessor interface technique we have developed has far broader implications than simply controlling a backlight. By pointing two LEDs at each other, we can transmit data back and forth. This is trivially easy to do for distances on the order of a centimeter. The result is that almost any LED connected to a microprocessor can be thought of as a potential two-way communications port. We think of this technology as solving "the last centimeter problem" - you have two devices right next to each other, but they have no way to communicate. With LED Comm (LED-based communications), there is a link available, almost for free. Ãƒâ€š Ãƒâ€š We have been looking into a variety of applications for this very fundamental technology. One opportunity is to turn the power light on appliances into a service port - download status and upload new firmware. Cell phones could swap phones numbers via their backlights. LED-based devices could replace RFID tags in many applications with benefits such as true peer-to-peer communications and a much smaller, far less expensive reader.
Dietz, P.H.; Yerazunis, W.S.; Leigh, D.L., “Very Low Cost Sensing and Communication Using Bidirectional LEDS”, International Conference on Ubiquitous Computing (UbiComp), October 2003 (UbiComp 2003, TR2003-035)
Modification Date: July 7, 2008