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NNTP-Posting-Date: Thu, 28 Nov 2024 08:11:03 +0000
Subject: Re: Anybody Seen a Simple LED "Fail-Over" Circuit ?
Newsgroups: comp.os.linux.misc
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On 11/27/24 12:12 AM, Computer Nerd Kev wrote:
> Rich <rich@example.invalid> wrote:
>> Computer Nerd Kev <not@telling.you.invalid> wrote:
>>> Rich <rich@example.invalid> wrote:
>>>>
>>>>           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.
>>>
>>> Unless the driver chip fails short-circuit, causing the PSU to shut
>>> down power to both drivers.
>>
>> fewest, simplest, most robust -- you get to pick two.....
>>
>>>> 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.
>>>
>>> If the PSU has regulated voltage output, or LED brightness can vary
>>> with the supply voltage (such as from a battery), then a resistor
>>> would do instead of the LED drivers.
>>
>> Yes, and you still have the same potential for a possible "fail short"
>> with a current limiting resistor, which would then drive that led with
>> too much current.  And if it happens to fail short when overdriven too
>> much, you are back to your 'fail short' for the "drivers".
> 
> You can get "fusible resistors" that are supposed to fail
> open-circuit if overloaded.
> 
>>> I'd expect the drive circuitry and wiring to be as common a point of
>>> failure as the LEDs themselves.  To detect
>>> open-circuit/short-circuit, you could pass a small current through
>>> them and use that to tell whether the LED is OK (current is correct
>>> for the LED's forward voltage drop specification), triggering a
>>> single bulb-failure warning if it's not (possibly simpler in practice
>>> than duplicating every LED on a display panel, even if the total
>>> number of components is similar).
>>
>> Yes, you could design a "detector" that could detect open/short for the
>> LED and/or its driver.  But then that means you've excluded "fewest
>> parts" (at least) from the design selection criteria. And, depending
>> upon how 'robust' you really need to be, you'd need to detect failures
>> of the detection circuitry itself as well.
> 
> 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". If those lights are outdoors
   then half-bright may look like "nothing".

   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.