[support-gang] XO brightness key operation

Hal Murray hmurray at megapathdsl.net
Tue Apr 10 02:26:06 EDT 2012

> As I have a tiny understanding of LEDs, I recall something about being able
> to detect light with them.

If you shine light on a reverse biased diode, it will leak in proportion to 
the intensity of the light.  The mechanism is a photon occasionally kicks 
loose an electron and the reverse-bias field pushes the electron toward the 
positive terminal.  Enough electrons like that and you have a small current 

Normally, that's not a problem since diodes and transistors are packaged in 
opaque plastic.  LEDs are packaged in clear plastic to let the light out.  
But that means you can also also get light in.


There is an old story about one of the early SUN boxes.  It had just booted 
the OS (or something great like that) when somebody took a picture of it.  It 
crashed.  The camera used a flash.  The UV ROMs with the software didn't have 
any stickers over the transparent lids, or maybe there was no lid over the 
CPU chip.  The light from the flash was enough to screw things up.

(If anybody has a good URL for that story, please let me know.  I didn't find 
it with a bit of searching.)

These are my opinions, not necessarily my employer's.  I hate spam.

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