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How do you prove an LED is actually lit?
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Brumby:

--- Quote from: james_s on January 27, 2023, 01:30:41 am ---..... unless they mean an indicator on a control panel somewhere.
--- End quote ---
That's what I was thinking.


--- Quote ---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.

--- End quote ---
Fair point.

Perhaps the Op could clarify. (or am I just showing my ignorance?)
T3sl4co1l:
Point of reference, I don't have any photodiodes handy but a T1-3/4 red (clear lens, superbright) measures 1.58V (OC) and 0.26mA (SC) under about 1W of white light (high eff. white LED).  (In comparison, the forward-bias drop is 1.56V (~0.1mA?).  It's photovoltaic-ing pretty hard I would say...)  It won't take much die area to get a proper Si photodiode sinking (or sourcing, as the case may be!) much more current.

I suppose the biggest downside to a self-switched detector is, if multiple diodes in series are needed, they either need dedicated optics, or you lose specificity by trying to focus one LED onto all three.  Upside to dedicated optics is, one lens per diode means one diode per LED in the string, perhaps a redundancy feature.

Speaking of LEDs, upside with them is, a very narrow angle type means it's exactly as sensitive on-axis, and insensitive otherwise.  You still need a fixture/jig/holder to mount a THT LED or photodiode in this way (well, unless you put an SMT PD on flex circuit, which might be nice), but you need one anyway for the lens, so an integrated lens part like this might actually be a good solution.

I'd check the LED's properties in daylight, but, uh, *y'know*.  Also it's been snowy here so I might not get direct sunlight for a while anyway. :P

Tim
jmh:

--- Quote from: Brumby on January 27, 2023, 06:01:29 am ---
--- Quote ---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.

--- End quote ---
Fair point.

Perhaps the Op could clarify. (or am I just showing my ignorance?)

--- End quote ---

Exactly right. Traffic lights (for example) need to spread light horizontally across a junction as well as vertically to cater for cars and trucks etc. Railway signal lamps are very directional, low wattage things.

As to photodiodes this may actually be a way forward. These lamps are designed to prevent phantom lighting by having the insides behind the outer lens completely matt black. There is a hole in the centre and then the coloured discs and behind that some glass and the bulb and reflector. A photodiode placed near the bulb would get only very little external light even if the sun were shining at the lamp.

Brumby:
Today, I learned something - and the logic of it seems so obvious.

Thank you.
james_s:
This has a lot of information on American railroad signals. https://railroadsignals.us/  I assume there must be similar sites out there for signals used in other parts of the world.
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