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From: Rich <rich@example.invalid>
Newsgroups: comp.os.linux.misc
Subject: Re: Anybody Seen a Simple LED "Fail-Over" Circuit ?
Date: Tue, 26 Nov 2024 13:34:36 -0000 (UTC)
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186282@ud0s4.net <186283@ud0s4.net> wrote:
> Critical Redundancy - One LED fails, another takes over ?
> 
> Consider traffic lights, warning lights, similar.
> 
> It's not as simple as dividing the drive current in half because LED 
> brightness is not strictly linear to the current.

LED's are, at a low level, 'current' responsive lights.  Driving them 
with a current source is the best way to drive them.

> Searches really don't bring up much here.
> 
> Yea, there are more complex solutions ...  but what can be done with 
> the fewest, simplest, most robust parts ?

fewest, simplest, most robust -- you get to pick two....

The simplest (if you can assume the upstream power supply will be 
functional [1]) is to drive each in parallel with their own current source 
(fixed current driver). I.e.:

          PSU
           |
   +-------+-------+
   |               |
 driver          driver
   |               |
  LED             LED
   |               |
   +-------+-------+
           |
          Gnd


Then if one led (or its driver) fails, the other continues to operate, 
because it does not depend upon the first one.

But this is far from 'fewest' parts, as you need one driver per led.  
While some driver chips can be had for pennies each in 1K quantities, 
that still adds to the BOM cost in the end.

> Not strictly Linux, but we DO sometimes wanna drive external 
> displays.  Usenet electronics groups ...  dismal at this point.
> 
> LEDs are great, but never "forever".  They DO fail - but for some 
> safety apps you can't just HAVE things go black.

Most LED's that fail do so because they are being driven hard [2] 
(right at the limits that they are rated for, if not well beyond 
sometimes).  If you derate your drive by a fair amount you'll find they 
do, in fact, appear to last nearly forever.  But then you will need 
more LED's for an equivalent amount of lumens of light output.


[1] redundant PSU's are a different matter

[2] And they are being driven hard because the Shenzen engineer 
optimized for lowest BOM cost posible without regard to lifespan of the 
device.