Re: Anybody Seen a Simple LED "Fail-Over" Circuit ?

186282@ud0s4.net <186283@ud0s4.net> wrote:

On 11/27/24 12:12 AM, Computer Nerd Kev wrote:

Depends on the details. Say you have flashing warning lights
driven by a microcontroller which also has spare remapable ADC
inputs: You could add a capacitor in parallel with the LED+resistor
and switch the micro's pin from HIGH digital output into ADC input
mode to turn the LED off. While the light fades from on to off,
measure the discharge of the capacitor - too fast means a short,
too slow means open-circuit. Yet there's only one more component
per LED if you already have a suitably capable microcontroller
there.
 
For traffic lights to look normal, you could flash so quickly that
it's not noticable to the eye (if you've got surplus brightness).
 
Now the problem is that capacitors tend to fail short-circuit more
often than most other common components including LEDs. So you can
detect the failure, but the failure is now more likely.
 

Another commenter's statement of inverting the indicator, where "on"
means "situation normal" and "off" means "abnormal" is probably the
absolute simplest way to go.  But then the "LED indicator" fights human
psychology that senses a new stimuli appearing in the environment (lamp
turning on) far more readily and quickly than noticing that a continual
low level stimuli has disappeared (light has gone out).

 
Flashing to indicate a warning instead of turning permanently off
would help there. Need to retrain everyone to use traffic lights
which always have two lights on if applied to that example
application though, so probably not a solution there.

 
 
  There are 'critical' lights - traffic, railway, nuke-reactor -
  that simply can't show "nothing".

I haven't been let into a reactor control room, but I'm pretty sure
I've caught traffic lights showing nothing during a blackout
before (and ample other times for that matter). The individual
traffic light units usually have redundancy by there being more
than one of them, in Australia at least. You just need a method
for detecting the failure and reporting it so it's fixed as soon
as possible.

I'm not sure what railway crossing lights do, as off is their
normal state. I assume they die too during a blackout and I
therefore slow sufficiently to be able to stop if I see a train
coming as I check both ways before crossing. Where there's little
space to see before reaching the line, sufficient slowing down can
annoy drivers behind. But tough luck to them, I won't trust my
life to a light, especially as I heard relatively recently that a
nearly railway line was closed after multiple crossing lights
failed to activate.

  For the original question, I think using two FETs, an N
  and P, linked to the + on LED-1 can form a "voltage-range
  error" circuit without too many parts. LED-2 is the
  indicator lamp. So, whether LED-1 fails open or closed
  LED-2 still lights full. Attach an extra tiny red led
  or piezo buzzer or whatever to it to indicate fail mode
  if you can't just tell by looking.

Well since you're defining "too many parts", one can't argue
with that. This topic is really too general for a universal
minimum-parts solution. The over-priced indicator lights on a
nuclear reactor control panel probably cost much more individually
than an excessive bunch of redundant components to drive them all.

Though at Fukushima they were apparantly powering instruments with
car batteries when the building lost power, so redundancy only went
so far there. I thinkI remember the movie The China Syndrome showed
a fictional nuclear incident happening because a panel meter needle
was stuck.

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