LED Circuit - LED As Light Sensor - Bidirectional Communications

Bidirectional Communications

LEDs can be used as both emitters and detectors of light, which means that a device having only a single LED can be used to achieve bidirectional communications with another device meeting these requirements. Using this technology, any of the ubiquitous LEDs connected to household appliances, computers and other electronic devices can be used as a bidirectional communications port.

One application for bidirectional communication with a single LED is fiber optic communications. In typical plastic optical fiber communications, a single optical fiber is used only for communication in one direction. This is because a single LED transmitter is placed at one end of the fiber, and a photodiode receiver is placed at the other end. Thus, two fibers are needed for bidirectional communication. However, if a single LED is placed at each end of a fiber, then the optical fiber can carry information in both directions using half the number of components as a typical system. This reduces system weight, cost and complexity.

Another application of this use of LEDs is a proposed alternative to RFID tags called the iDropper, developed by Mitsubishi Electric Research Laboratories in 2003. The iDropper is a small device that consists of a microcontroller, a battery, an LED, and a single push-button. The device records or transmits a small amount of data upon command from the user. Compared to RFID tags, the iDropper is more secure because the user must press a button to reveal personal information, and is similar in cost.

A single LED can only operate as a half-duplex transceiver; it can either transmit or receive information at one time, not both simultaneously. Simultaneous two-way communication requires both a forward and reverse channel and a second LED.