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

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On 11/28/24 4:06 PM, Computer Nerd Kev wrote:
> 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.

   Well, the circuit I described doesn't use TOO many parts
   and they're all cheap as dirt. a couple FETs and some
   resistors by and large. You WILL get full brightness
   from LED-2 and will NOT be burning LED-2 unless needed.

   Anyway, visualizing The Problem as voltage-range-error
   detection seems the simplest approach. Doesn't need
   microcontrollers or current supplies or anything very
   complex.

   BlackOuts ... yea, they CAN happen (been through some
   lasting nearly three weeks). After a little while almost
   NO tech works - it's back to 1880. However decent design
   at least DOES ease everybody into that old paradigm.

   DO own a few kerosene/oil lamps and had to use them on
   two occasions over the past 20 years. Old tech DOES
   work ... just make sure they can't crash to the floor
   and start a big fire. Oddly, can't get the Net with
   them - what WERE they thinking ???

   Traffic lights - esp since the LED conversions - more
   and more DO have battery-backup that'll last for an
   hour or two. Maybe not cost-effective everywhere, but
   at major intersections ......

> 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.

   As I suggested elsewhere, 'redundancy' can only go SO far.
   A tsunami/earthquake/melt-down ... kinda WAY off the spectrum.

   Oh, car batts are NOT so bad - cheap and work. Fair cap.
   Only maybe 2-3 year lifetime though.

   Ah, Fukushima ... reported last week ... they finally got
   a little robot in there - with a long 'fishing pole' probe
   on it - and recovered a few tiny bits of core material.
   Japan congratulated itself on that. Alas the radiation is
   still SO high that it nukes all modern tech, hence the
   'fishing pole'. Maybe they should look to vacuum tubes
   or something, crude RC ......

   There ARE 'cold-emitter array' valves these days which
   are pretty small - kinda leveraging microchip tech to
   make a mass of tiny 'points' that emit electrons. Then
   you mechanically overlay a grid and anode. Can't do
   TOO much logic/etc with those, but they would probably
   hold up in a hyper-rad environment.

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Thread

Anybody Seen a Simple LED "Fail-Over" Circuit ? "186282@ud0s4.net" <186283@ud0s4.net> - 2024-11-26 02:24 -0500
  Re: Anybody Seen a Simple LED "Fail-Over" Circuit ? rbowman <bowman@montana.com> - 2024-11-26 08:40 +0000
    Re: Anybody Seen a Simple LED "Fail-Over" Circuit ? "186282@ud0s4.net" <186283@ud0s4.net> - 2024-11-27 00:05 -0500
  Re: Anybody Seen a Simple LED "Fail-Over" Circuit ? Bernd Froehlich <befr@eaglesoft.de> - 2024-11-26 09:00 +0000
    Re: Anybody Seen a Simple LED "Fail-Over" Circuit ? "186282@ud0s4.net" <186283@ud0s4.net> - 2024-11-27 00:20 -0500
  Re: Anybody Seen a Simple LED "Fail-Over" Circuit ? Rich <rich@example.invalid> - 2024-11-26 13:34 +0000
    Re: Anybody Seen a Simple LED "Fail-Over" Circuit ? not@telling.you.invalid (Computer Nerd Kev) - 2024-11-27 08:21 +1000
      Re: Anybody Seen a Simple LED "Fail-Over" Circuit ? Rich <rich@example.invalid> - 2024-11-27 01:26 +0000
        Re: Anybody Seen a Simple LED "Fail-Over" Circuit ? Computer Nerd Kev <not@telling.you.invalid> - 2024-11-27 15:12 +1000
          Re: Anybody Seen a Simple LED "Fail-Over" Circuit ? "186282@ud0s4.net" <186283@ud0s4.net> - 2024-11-28 03:11 -0500
            Re: Anybody Seen a Simple LED "Fail-Over" Circuit ? not@telling.you.invalid (Computer Nerd Kev) - 2024-11-29 07:06 +1000
              Re: Anybody Seen a Simple LED "Fail-Over" Circuit ? "186282@ud0s4.net" <186283@ud0s4.net> - 2024-11-29 05:53 -0500
    Re: Anybody Seen a Simple LED "Fail-Over" Circuit ? "186282@ud0s4.net" <186283@ud0s4.net> - 2024-11-27 00:36 -0500
      Re: Anybody Seen a Simple LED "Fail-Over" Circuit ? Rich <rich@example.invalid> - 2024-11-27 06:47 +0000
        Re: Anybody Seen a Simple LED "Fail-Over" Circuit ? "186282@ud0s4.net" <186283@ud0s4.net> - 2024-11-27 23:47 -0500

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