Still a very rough draft. When I turn this into a main article,
1. decrement the level for each heading
3. sources & footnotes!
Implementation of LEDs as Photodiode Light Sensors
The properties of light emitting diodes (LEDs) can be exploited to render them useful for light detection as well as emission. This capability has been demonstrated and used in a variety of applications including ambient light detection and bidirectional communications. This implementation of LEDs is important because functionality can be added to designs with only minor modifications, usually at little or no cost.
An LED is simply a diode that has been doped specifically for efficient light emission and has been packaged in a transparent case. Therefore, if inserted into a circuit in the same way as a photodiode, which is essentially the same thing, the LED will perform the same function. Additionally, the LED can be multiplexed in such a circuit, such that it can be used for both light emission and sensing at different times. In Dietz Et. Al., a scheme for implementing this multiplexing is presented:
- An LED is connected to two bidirectional CMOS I/O pins on a microcontroller (or a microprocessor with an i/o bus).
- To emit light, both of the i/o pins are set to output mode, and the LED is driven with current in the forward direction, resulting in current flow through the LED and emission of light.
- To detect ambient light:
- The i/o pins are set to output mode, and the diode is driven in the reverse-bias direction, such that the diode inhibits the flow of current and the LED's inherent capacitor is charged.
- The i/o pins are set to high-impedance CMOS input mode.
- The diode leaks current at a rate proportional to the incident light, as incident photons cause electrons to leap across the band gap.
- The time it takes for this leakage current to discharge the LED's inherent capacitor is measured and is inversely proportional to the incident light.
Several applications for this technology have been suggested and/or implemented, ranging from use as simple ambient light sensors to full bidirectional communications using a single LED. Most of these applications benefit from this technology because of the cost reduction or design simplicity gained by using the same component for multiple functions.
Ambient Light Sensors
LEDs have been used as ambient light sensors. Dietz Et. Al. describes an application in which a remote control keypad backlight would be turned on by capacitive proximity sensors only in the absence of ambient light. The LED used for the backlight was also used as the ambient light sensor. This resulted in increased functionality for no increase in manufacturing costs.
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.
One major limitation of this scheme is the fact that a single LED can only operate as a half-duplex transceiver. A single LED can either transmit or receive information at one time, not both simultaneously. A simple way to put this is that an LED transceiver behaves like a walkie-talkie, in contrast to a telephone. This means that it takes a considerable amount of time for two devices to "talk" to each other.
- Dietz, Paul, William Yerazunis, Darren Leigh. 2003. Very Low-Cost Sensing and Communication Using Bidirectional LEDs. MITSUBISHI ELECTRIC RESEARCH LABORATORIES. http://www.merl.com/papers/docs/TR2003-35.pdf (Accessed October 17, 2008)
- Bent, Sarah, Aoife Moloney and Gerald Farrell. 2006. LEDs as both Optical Sources and Detectors in Bi-directional Plastic Optical Fibre Links. ISSC 2006, Dublin Institute of Technology. http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4123923&isnumber=4123844 (Accessed October 17, 2008)