The secret feature in every LED: a solar cell

Did you know that all LEDs have a secret feature? Not only can they convert electricity into light, they can also do the opposite: turn light into electricity, just like little solar cells!
In this video, Jeff looks at the use of LEDs not only as power-generation devices, but also as photodiodes to detect transmitted light signals. It's a little bit of practical electrical engineering on the lab bench.
Extended discussion of the video topic:
It turns out that the open-circuit voltage you get from an unpowered LED in bright light is usually a couple hundred millivolts lower than the forward voltage when you're driving it. Also, the spectral sensitivity of an LED used in a receive mode is relatively narrow, generally centered at a slightly longer^H^H^H^H^H^H EDIT: shorter wavelength than its peak emission wavelength. If you want to learn more about that effect, I recommend the paper by Filippo et al. (2017) "LEDs: Sources and Intrinsically Bandwidth-limited Detectors"
All of this is well-known, of course, even if LED datasheets pretty much never discuss the characteristics of their components in a receive mode. Semiconductors in general have long been known to be sensitive to light: that's part of the reason for the black epoxy cases surrounding many ICs.
Also, in case you're wondering what the deal is with that red LED on the PCB of the Casio SL-310SV calculator, I believe it's there to regulate the voltage from the normal solar cell in case the calculator is in very bright light (e.g., outside).

[@paulramasco6769]

Forrest Mims demonstrated this in the early 80s, that's where I first found this out. Awesome video.

[@ericwazhung]

Great overview/demos, and cool idea re: Tx/Rx.

[@zoeyzhang9866]

Welcome to UA-cam! Wondering if any custom PCBs may help for the upcoming content? Would love to sponsor if needed. (PCBWay zoey)