Forrest Mims' Science Experiments

Chapter 22 How to Use LEDs to Detect Light

Since an electromagnetic telephone receiver can double as a microphone, can a semiconductor light detector double as a light emitter?

That question was on my mind when I was a high school senior in 1962. Back then I didn’t know that quantum effects in a semiconductor are unrelated to the electromagnetic operation of a telephone receiver. If I’d known that, I never would have connected a spark coil across the leads of a cadmium sulfide photoresistor to see if it would emit light. It did—a soft green glow punctuated with bright flashes of green.

During college I found that a silicon solar cell connected to a transistor pulse generator emitted flashes of invisible infrared that could be detected by ...