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How do you prove an LED is actually lit?

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floobydust:
OK we got some details. I get they are railway signal lamps, not crossing lamps, and no redundancy as far as a second lamp.
I thought they would be higher power, the bulbs are apparently 200lm 2,000hrs 10V 18W 18S11/1S but not sure what the UK is using. pic related USA has lots of room in the fixtures due to the optics, and maybe a rheostat?

So far monitoring (LED) current is going to tell you a lot, I like that approach. The LED lamp failure mode depends on the # strings and # LED's in a string, as far as what happens when an LED goes open/short-circuit. It is likely a single high power LED to keep the focal point with the old fixtures, like many modern traffic lights they are no longer using arrays of LED's but a single one.

Nusa:

--- Quote from: james_s on January 27, 2023, 12:06:47 am ---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.

--- End quote ---

A valid analysis. If it's the expensive bulbs that are the real concern here, perhaps explore adapting the fixture to automotive bulbs. There are many dual-filament bulbs designed for road conditions that should be cheap enough if you stock up this decade. I suspect they might last longer than rated in a stable fixture.

IanB:

--- Quote from: jmh on January 27, 2023, 09:37:15 pm ---Just in case it's remotely of interest this is one of the lamps taken out of it's housing.
--- End quote ---

It is of course of enormous interest, since you have started a thread asking about this problem, but you have been strangely starving us of information, making everyone guess and speculate, as you can see above.

Is it perhaps the case the bulbs you are trying to replace are similar to this one?

https://www.proflamps.com/us/en/Products/Applications/Airfield-%26-Railway/p/13800306842296/?currency=GBP

Given that the life expectancy is only 600 hours, I can see why replacing them could become annoying. On the other hand, given that the indicated price is £1.45 per bulb, and each bulb will have to be replaced every 25 days, you will need something very inexpensive as a replacement to provide a reasonable payback on the investment, I would think?

tom66:
I do wonder how they solve this problem in railway engineering;  LED signals are used, as these have superior visibility to halogen signals, and in theory longer lifetimes.  But it is impossible to verify with a simple voltage/current measurement that they are functioning correctly. So are they just multiply-redundant (3/4 parallel strings of LEDs independently driven) or are they actually monitoring light output?

AndyBeez:
I suppose a Hall effect current sensor placed around the LED's power supply line can do the sensing? I assume these are watt rated not milliwatt LEDs? Perhaps an ACS712 current to voltage or an INA219 current to I2C bus, with a jellybean microcontroller doing the pony work. A LoRA module would provide status updates remotely. As for the lifespan of the LED, place the MTBF outside of the service and replace period. btw, stage 'PAR lights' provide a lot of light in a small space.

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