Electronics Puzzler: Light to Light Converter

By Forrest M. Mims III

The world's simplest electronic light-to-light converter

Physics professor Marcus Lichtmeister planned to illustrate a lecture with demonstrations of how light can be intercepted by certain phosphors or various optoelectronic sensors and transformed into entirely new light. His demonstrations included a light-sensitive phosphor card, CCD and CMOS video cameras and ultraviolet and near-infrared image converters. The optoelectronic devices were much more complex than the phosphor card, and Prof. Lichtmeister wanted an ultra-simple demonstration of how an LED would glow when biased by a forward current provided by a suitable sensor.

He rummaged through the optoelectronic drawer in the parts cabinet on his workbench and found two silicon solar cells, several cadmium sulfide photo resistors, a couple of AlGaAs red LEDs, some silicon phototransistors and half a dozen silicon photodiodes. In his parts cabinet he found some transistors, miniature chokes and assorted resistors and capacitors. How did he combine the smallest number of components to do what the phosphor card did?

Puzzle Hints
Prof. Lichtmeister stayed up late designing several circuits that converted incoming light into enough current to drive an LED. But these circuits were much more complicated than a phosphor card. At midnight the proverbial LED flashed in his mind, and he quickly built an all electronic light-to-light converter by using only an LED and one other component from his parts cabinet. The resulting circuit is shown in the image above.

What is the component represented by the puzzle piece in image above?

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