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Comment by Namidairo | Hacker News Reader

> The only downside is that external light can actually affect the chip operation (light hitting the die releases electrons at PN junctions creating errors in circuit operation ) .. .however here the Faraday metal shield helps prevent this.

Brings me back to when it was discovered that high intensity flashes could make a Raspberry Pi 2 reboot...[1]

[1] https://www.raspberrypi.com/news/xenon-death-flash-a-free-ph...

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Many PN junction based components such as diodes and transistors are photosensitive; I recall after reading this as a kid on an old electronics magazine, I took a 2N3055, a pretty common power bipolar transistor back then, uncapped it to expose the die then connected it to my analog tester and saw it generating current when exposed to a lamp. The effect is common also in devices one could never suspect to be sensitive to light such as LEDs. They can produce light but also detect it. A few years ago someone published a project online of a MCU based keyboard (actually a button board) made of PWM driven LEDs that can both produce light and during their off cycle detect it, so that one turns them on and off just by touching them with a finger screening them from surrounding light. I attempted to search for that project without luck.

I remember that project too. Might have been Paul Dietz at MERL.If you're curious, look up Paul Dietz "low cost bidirectional sensing using LEDs" and Forrest Mimms "LED Sun photometry".

> Forrest Mimms

There's a name I haven't seen in a long time. Source of endless insight into electronics parts and schematics.

He did a series of columns for Make Magazine. A compilation book titled "Make: Forrest Mims' Science Experiments" was published in 2016.

https://www.makershed.com/products/forrest-mims-science-expe...

A self-directed project I did for a university course was to make a Morse code transmitter and receiver. I was able to use the same LED for both.

By using the same LED as a receiver too?

https://makezine.com/projects/how-to-use-leds-to-detect-ligh...

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