Author Topic: How do you prove an LED is actually lit?  (Read 1347 times)

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Online Brumby

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Re: How do you prove an LED is actually lit?
« Reply #25 on: Today at 12:20:14 am »
..... take over an amp at 12V .....

These would be outdoors, then?

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.
 

Offline james_s

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Re: How do you prove an LED is actually lit?
« Reply #26 on: Today at 01:30:41 am »
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.
 

Online eti

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Re: How do you prove an LED is actually lit?
« Reply #27 on: Today at 02:04:18 am »
They tend to emit light - the clue is in their very name. 😁
 

Offline Infraviolet

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Re: How do you prove an LED is actually lit?
« Reply #28 on: Today at 03:14:57 am »
Just a thought, regarding the original post's LDR idea, there is a way this could be done.

If the LED were made to flash at a very high (human imperceptible) frequency then a phototransistor (LDR wouldn't be fast enough) or photodiode circuit could be set up facing the bulb with bandpass filtering so it would only amplify signals at the expected frequency of the LED's flicker, this would be immune to other light sources so long as they didn't also match that flicker rate or an integer multiple of it. That said, with the LED "bulb" in question apparently running off AC or DC it likely contains internal circurity to smooth things out, the only way you'd be able to get a very fast flicker would be to redesign those internals, unless the bulb already has a naturally existing one which you might detect, and if it does have a naturally existing one then depending how "smart" the design is this frequency might adjust depending on things like how hot the bulb gets as it is left running.

Thermal detection on heatsinks probably isn't good enough, there is substantial time lag between bulb startup and sink heating, and between bulb failure and sink cooling.

In the end, I'd look to design for yourself as much of the unit as you can, so you can have things like fast flashing done within it controlled by your own electronics, rather than having the whole 12V unit being a commericla item with inbuild controls. With low enough duty cycles you could even do occasional ultra-fast flashes when the bulb is supposed to be off, and still detect if these fail. That is to say you could detect a failed LED when the LED is desired to be off, not only be able to detect a failure when it should be on but isn't. Knowing it has failed while it is off and knowing that before you need it to be on soudns useful to me.
« Last Edit: Today at 03:22:07 am by Infraviolet »
 

Offline james_s

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Re: How do you prove an LED is actually lit?
« Reply #29 on: Today at 04:00:40 am »
That's not a bad idea, LEDs can be pulsed at hundreds of kHz, even MHz, and you could use a high duty cycle to minimize any side effects. Phototransistor or photodiode detector, it's getting hard to even find LDRs anyway. ROHS banned them because of the cadmium IIRC.
 

Offline Marco

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Re: How do you prove an LED is actually lit?
« Reply #30 on: Today at 04:05:45 am »
If the photodiode is right next to the LED it is pretty bright relatively speaking. Don't want to look into a lighting LED from a couple mm.
 

Offline Infraviolet

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Re: How do you prove an LED is actually lit?
« Reply #31 on: Today at 05:14:30 am »
One can always put some darkened filtering material between the bulb and photodiode/transistor to stop the sensor going in to saturation. Particularly important would be ensuring it isn't exposed to enough light to saturate it from non-LED sources (daylight and such), as if this saturates it then it won't be able to detect a signal from the LED that can then be found by bandpass filtering.
 

Online T3sl4co1l

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Re: How do you prove an LED is actually lit?
« Reply #32 on: Today at 05:53:32 am »
It should be straightforward enough to set the sensitivity and orientation so that -- either the LED is lit, or it's inundated by such intensity that you won't be able to read it by eye anyway!

Like I said, LED chips are about the luminosity of the sun; it takes a monumental source, or more concentration than could ever be accidental (from specular reflections, refraction of glass/water/etc.), to fool that.

Tim
« Last Edit: Today at 05:55:24 am by T3sl4co1l »
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Online Brumby

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Re: How do you prove an LED is actually lit?
« Reply #33 on: Today at 06:01:29 am »
..... unless they mean an indicator on a control panel somewhere.
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.
Fair point.

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

Online T3sl4co1l

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Re: How do you prove an LED is actually lit?
« Reply #34 on: Today at 06:11:46 am »
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
« Last Edit: Today at 06:14:25 am by T3sl4co1l »
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Offline karpouzi9

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Re: How do you prove an LED is actually lit?
« Reply #35 on: Today at 06:48:53 am »
You've come to a forum full of electrical engineers to ask them an X-Y question, where

X = how do we save money/time on our railway indicator light solution, which comprises expensive, handmade electromechanical contraptions of metal and glass and basic glue logic and interlocks

Y = how do we make an LED solution for our railway indicator light given that we also don't want to replace the electromechanical glue logic implemented for the glass bulbs

They're going to tell you these things:

- Pay an electrical engineer to invent an LED bulb which, through much effort, replicates exactly the electrical lifecycle diagram of a glass bulb
- Pay an electrical engineer and a team of other engineers and designers to revamp your entire safety system
- Forget about lights, just get with the times and replace every crossing with a mesh radio or LTE system
- There's no need for any of this worrying, just connect a MOSFET's input to a transimpedance amplifier connected to the anode or cathode of the LED, hook that MOSFET and a series resistor across the power supply, the LED won't draw current if it's dead and the MOSFET will shut off and fire up the other LED.

A forum full of electrical engineers will probably never suggest these options:
- Quit your job at the rail company, raise some capital to buy the light bulb company, charge the railway company even more per bulb, and make the bulbs either less reliable so they have to buy more or a little more reliable so the rail company is willing to stick with it.
- Convince a company in China to make the bulbs for you for less.
« Last Edit: Today at 06:51:23 am by karpouzi9 »
 

Online jmh

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Re: How do you prove an LED is actually lit?
« Reply #36 on: Today at 10:45:04 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.
Fair point.

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

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.

 


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