Jameco Electronics Puzzler Solution: Light to Light

By Forrest M. Mims III

Light-to-light converter's missing component

After several hours of experimenting, Prof. Lichtmeister suddenly thought of an ultra-simple 2-component solution to the problem: He connected a red AlGaAs LED to a second identical LED. When a flashlight was pointed at one LED, the second LED emitted a dim glow. Thus, light from the flashlight was transformed into light from the second LED.

How It Works

Prof. Lichtmeister's parts cabinet had several types of components that, when illuminated by a flashlight, could light an LED. But only one type of component would perform this function on its own and without using two or more components to achieve the forward voltage necessary to drive an LED. For example, a silicon solar cell provides only 0.6 volt. Achieving the voltage required to forward bias a typical LED would require a series string of three silicon solar cells.

Figure 2 shows the two AlGaAs red LEDs connected directly together. The LEDs are connected anode to anode and cathode to cathode. When light from a flashlight with either an incandescent bulb or a white LED is directed at the first LED, the second LED will glow dimly. This arrangement works, because the voltage provided by the illuminated LED matches the necessary forward voltage of the second LED.
Light Mystery Component
Figure 2. The mystery component in Figure 1 is a second LED.


During experiments in college in 1966 I first learned that a pn junction can both emit and detect near-infrared. When LEDs were commercialized, I learned that they, too, can both emit and detect light. I also found that laser diodes can both emit and detect light.

My first application for LEDs as both detectors and emitters was a variety of light wave communication systems. Beginning in 1990, I began seriously using LEDs as detectors in a variety of sun photometers that measure haze and the total column amount of water vapor in the atmosphere. My first such instrument has been used to make measurements at solar noon since 4 February 1990 on days the sun has not been blocked by clouds.

Going Further

You can easily experiment with various kinds of LEDs. For best results, use super bright orange and red LEDs.

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1. Forrest M. Mims III, Light Emitting Diodes, Howard W. Sams & Co., 1973, pp. 118-119.

2. Ibid., Sun Photometer with Light-Emitting Diodes as Spectrally Selective Detectors, Applied Optics , 31, 33, 6965-6967, 1992.

3. Ibid., An inexpensive and stable LED Sun photometer for measuring the water vapor column over South Texas from 1990 to 2001, Geophysical Research Letters 29, 20-1 to 20-4, 2002.

4. Ibid., Five years of photosynthetic radiation measurements using a new kind of LED sensor, Photochemistry and Photobiology 77, 30-33, 2003.

5. Ibid., LED Sun Photometry, Optics & Photonics News 20 , 32-38 (2009).