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How do you prove an LED is actually lit?

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james_s:
I'm usually all in favor of LEDs, but in this case it seems like continuing to use a filament lamp is probably the most ideal solution. If you do decide to go with LED then add a photodiode to monitor the actual output. Since railway signals (that I'm familiar with) lack a reflector in order to prevent reflected light from making it appear lit, you should be able to mount a photodiode off to the side where it can see the LED but will not be exposed to direct light from elsewhere. I don't think anything will reach the reliability of failure detection you can get with incandescent, but the LED should be much less likely to fail.

TimFox:
Before retirement, I used commercial x-ray generators that had a large red 24VDC indicator light to indicate "X Rays ON".
The control system was interlocked to prevent x-ray power unless that bulb drew current.

Brumby:

--- Quote from: T3sl4co1l on January 26, 2023, 11:17:14 pm ---
--- Quote from: jmh on January 25, 2023, 11:21:52 pm ---..... take over an amp at 12V .....

--- End quote ---

These would be outdoors, then?

--- End quote ---

If we are looking at an incandescent running in the vicinity of 12W or so, I doubt they would be for outside use.

Seems more like a mission critical safety indication situation.  As such, any solution would require rigorous testing and validation with failsafes.

james_s:
The OP said they're for a railway signal, so I'm taking that as meaning outdoors, unless they mean an indicator on a control panel somewhere. Railway signals are traditionally relatively low wattage incandescent lamps with very directional optics. The train is on a track so you know right where it's going to be and can aim the signal accordingly.

eti:
They tend to emit light - the clue is in their very name. 😁

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