How do you prove an LED is actually lit? - Page 1
Forum section: electronics / general

author: jmh
author_rank: Contributor
timestamp: January 25, 2023, 02:09:19 pm
content: I suspect the answer to this to have something looking at it but I need to find a reliable way to find out if a LED is actually lit.The (12V) LED bulb in question is replacing a filament bulb (I say 'a', there are actually 9 or so in various places). Finding out if a filament bulb is lit is more a matter of seeing if it is taking current and having a fuse able to blow if the bulb fails short. I had hoped we could 'see' what current the LED was drawing but the bulbs we have can take ac or dc and either polarity of the latter, so there is circuitry inside and this may well still draw current even if the LED itself has gone dark (assumed, not experimented). So sensing current flow may not be a reliable indicator.Having an LDR and circuitry (or similar thing to sense light) fitted in such a way that external light cannot influence is only way I can think of to do this unless I am missing something obvious / being daft / or there is a more elegant solution? (going to look at each one is not an elegant solution, nor is the series of cleverly placed mirrors I wondered about!)Thanks

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 25, 2023, 02:25:39 pm
content: Use multiple LEDs with their own driving circuits, that way you have redundancy.

author: themadhippy
author_rank: Super Contributor
timestamp: January 25, 2023, 02:29:02 pm
content: Quoterun a bit of fiber optic cable from each led to one point were you can if all the leds are lit

author: SeanB
author_rank: Super Contributor
timestamp: January 25, 2023, 02:29:49 pm
content: Normally the LED retrofits have a bridge rectifier, a resistor and a led in the package, or simpler ones use 5 chip LED units and a resistor, 4 led in a bridge and the fifth between positive and negative of the bridge, so 3 at a time light. LED's are normally robust, so they rarely go short, but do go open, so simply detecting current flow works, easy for DC by using a sense resistor, that develops around 0.8V at rated current, and a simple resistor to the base of a NPN transistor, so you have an open collector output that pulls low with enough current flow, or use a mux to feed these small voltages into a MCU analogue input, and read the current in.

author: tom66
author_rank: Super Contributor
timestamp: January 25, 2023, 06:41:24 pm
content: You can't do it with purely electronic means, you need to meter the light output. I've seen LEDs fail "dim" but still have about 2.5V across them in the forward condition, usually this is due to the device reaching end of life but can be due to high temperature, etc. For packaged bulbs, I've seen multiple ones fail in a condition where they are partially / dimly lit (especially common with the GU10/MR50 'halogen replacement' ones). Would one bad LED be enough to consider the device failed - probably not - but current would still read out of range. So I think optical testing may be the only way.

author: Fraser
author_rank: Super Contributor
timestamp: January 25, 2023, 06:42:35 pm
content: Sometimes reverting to old fashioned technology remains the best and most appropriate solution….. go back to filament indicator lamps that may easily be monitored for function. The filament lamp has been used as an indicator for many years and should not be shunned when an application benefits from its use. LED’s are excellent technology but they are not a perfect substitute for the humble filament lamp in all applications.Fraser

author: Bud
author_rank: Super Contributor
timestamp: January 25, 2023, 06:44:16 pm
content: It should draw the rated current I'd say. Anything obviously below or above the rated current would indicate a fault.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 25, 2023, 07:00:31 pm
content: Quote from: Bud on January 25, 2023, 06:44:16 pmMonitoring the current should do the trick in most cases and would be the easiest to implement.The direct, but hardish way would be to use a photodiode and some kind of optical splitter to get light from the LED without getting light from the outside. Probably overkill.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 25, 2023, 07:02:47 pm
content: Sounds like a weird requirement. Do you think it's a common failure mode to stop producing light while continuing to pull similar amount of current? I'd say it's far-fetched.But if you have to absolutely prove that, for some weird legal reasons for example, then you of course have to sense the light, using a photo sensor of some kind. But that's the same forprovingan incandescent bulb. And then you need to prove that your light sensor cannot fail, and you need to prove the amount of ambient light coupling to it, ever, in any natural (maybe even supernatural?) conditions, does not exceed your detection threshold.TLDR; what are you actually doing?

author: mariush
author_rank: Super Contributor
timestamp: January 25, 2023, 07:41:00 pm
content: Quote from: jmh on January 25, 2023, 02:09:19 pmSuch 12v LED bulbs typically have one bridge rectifier to make them compatible with 12v AC input, and then you either have enough leds in series and a resistor to limit current, or there's a led driver.Usually if one led brakes down (often the bonding wires get loose or burn out inside the package) then the whole series of leds is broken so you don't get light anymore.Sometimes, you may get two independent "chains" of leds, and in that case if one led fails open breaking the chain (therefore no longer producing light), the other "chain" would still work.Rarely you have multiple leds or filaments in parallelIf there's a led driver and the circuit fails, at most you're gonna have pulses of current draw, but average the current consumption would be very low, I'd say maybe a couple mA at most... much less compared to at least 20-50mA or whatever such bulbs would use.

author: Fraser
author_rank: Super Contributor
timestamp: January 25, 2023, 08:20:49 pm
content: A Google search for “LED failure detector” revealed that On-Semi produce a component specifically designed to detect LED failures and it can even activate a back-up LED. Likely not what the OP wants due to his use of a 12V LED indicator package but it demonstrates that such detection devices exist as a COTS component.https://www.onsemi.com/pub/Collateral/DN05066-D.PDFhttps://www.onsemi.com/pdf/datasheet/hbl5006-d.pdfFraser

author: jmh
author_rank: Contributor
timestamp: January 25, 2023, 11:21:52 pm
content: Quote from: Siwastaja on January 25, 2023, 07:02:47 pmMy issue is I have no idea how these things may fail. A single LED yes, but all the gubbins in these bulbs, no. I would not expect it to pull the same current of course but it may pull some and I don't know the difference between good and bad. I may be worrying too much! This is why I asked rather the my usual cobble bits together and experiment.Quote from: mariush on January 25, 2023, 07:41:00 pmThanks, in which case then current sensing seems to best way to go.Quote from: Siwastaja on January 25, 2023, 07:02:47 pmIt's to modify railway signal lamps to use LEDs instead of filament bulbs. The bulbs, each has 2 filaments / 3 contacts, take over an amp at 12V and are very expensive things and don't last that long. The trouble here is, with a filament bulb we simply use a relay to determine if it is dead, switch to the second filament and raise an alarm. We need to replicate this for the LED - not to switch to a backup LED but to drop a relay. What is supposed to happen then is all the further signals go to danger, and that bit is easy. These signal lamps are somewhat ancient and will not take the 'real railway' drop-in replacement LED units and anyway we could never afford them.(I am of course asking in other relevant places but for me I want to understand how it works / make it work myself rather than just being given a widget)

author: IanB
author_rank: Super Contributor
timestamp: January 25, 2023, 11:33:23 pm
content: I have some LED bulbs that take anything from 12 V to 24 V, AC or DC.When I tested one of them, I found that it had an internal regulator behaving something like an SMPS, so that it would draw constant power at any supply voltage and maintain more or less constant light output. So if I doubled the voltage from 12 V to 24 V, the current drawn was reduced by half.I would guess, if the bulb failed, that the current would change from the nominal value. Most likely it would decrease. If it increased then some internal component would probably overheat and fail, and then the current would decrease after that. Or the electronics might fail short circuit and blow a supply fuse. There are all sorts of possibilities here.The reliable solution I suppose, in a safety critical application, is to continue using the traditional incandescent bulb, or to completely replace the signal lamp fixture with a modern replacement designed to meet regulatory standards for performance and reliability?I believe there is some similar issue with traffic lights on the highway. You might research how they handle the same question?

author: Tomorokoshi
author_rank: Super Contributor
timestamp: January 26, 2023, 02:44:10 am
content: I'm lit, thereforeiamps?Sorry!

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 26, 2023, 02:57:43 am
content: Note that the current sensing method may not work if the failure mode of the LED is a short-circuit (never seen it myself, but I guess that could happpen) and the LED is driven with a constant current source. Obvious, but hey, what do ya know.If it's just powered with a voltage source and a limiting resistor, it should work fine though.

author: Marco
author_rank: Super Contributor
timestamp: January 26, 2023, 03:42:45 am
content: Quote from: jmh on January 25, 2023, 02:09:19 pmExternal light will always influence it, you just need to make sure that the threshold is high enough that only only the lamp can actuate the circuit. So either due to proximity or a lens the lamp needs to put significantly more light on the sensor than stray sunlight.

author: Nusa
author_rank: Super Contributor
timestamp: January 26, 2023, 03:58:46 am
content: For your application you're going to be using higher power LEDs mounted on a heatsink. So a thermal sensor on that heatsink would be an alternate way to sense if a light has failed. No thermal change when a LED is turned on = failed LED.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 26, 2023, 05:18:43 am
content: Quote from: jmh on January 25, 2023, 11:21:52 pmFor that specific application, it might end up easier to design a custom board instead of trying to make it work with off the shelf modules. Maybe PWM the LEDs at a high frequency (several kHz) and detect that the LEDs are in fact pulsing at that rate?Another solution could be to convert the lights to use cheap 12V automotive bulbs.

author: thm_w
author_rank: Super Contributor
timestamp: January 26, 2023, 10:59:37 pm
content: Quote from: Nusa on January 26, 2023, 03:58:46 amThe idea is good but at that point the logic is too complex that you'd want a micro: waiting for heatup, taking into account ambient temperature, etc.I think the current consumption is fine, especially if its a string of LEDs or something with a smart driver. OR if you want to get crazier, monitor current draw and voltage across the LED, and ensure they are both within appropriate windows. Could be done with a comparator or a micro.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 26, 2023, 11:17:14 pm
content: Quote from: jmh on January 25, 2023, 11:21:52 pmAh, now this is something to work with.These would be outdoors, then?It should be possible to arrange a photodiode (no one uses LDRs anymore, they're old, bad, and non-RoHS) to sense some of the optical output, close enough that the intensity of the LED chip(s) overwhelms almost any possible interference. Interference would have to be fairly extreme I think; a white LED these days is typically about as luminous as the sun(!), so any interference would have to be much more intense: a very close welding arc perhaps; lightning strike; or highly focused light (think: Archimedes death ray).As long as the diode senses light of sufficient intensity, turn on a MOSFET to switch a shunt resistor onto the circuit. This replicates the incandescent's current draw.At such intensities, the PD probably looks more like a small resistance than a diode. Or use it photovoltaically, i.e. apply a load resistor and sense the voltage. Maybe use two PDs in series so it can bias a BJT on (instead of using a MOSFET). (Some Schmitt trigger action may still be desirable for clean switching, or the switching transistor can be heatsinked to handle the dissipation during indeterminate levels.)Note that this solution necessarily negates the LED's efficiency; only reliability is affected. (And maybe not all that much, if the resistor can't be placed somewhere away from the LEDs.)This also assumes the LEDs aren't dimmed by variable AC or DC. (PWM is OK.)If changing the sense relays is an option, then I would simply suggest using the LEDs as-is.Also mind that transients can fry LEDs easily; a TVS diode may be handy as well. Just a little SMAJxx something should be fine. How important this is, depends on where we're talking. If this is like trackside infrastructure, I would anticipate lightning induced transients, and maybe use a bigger TVS. If this is on rolling stock, ESD or smaller surges are a more likely threat, which may not need much if any additional hardware; but beware if alternator load dump is required by the customer (in which case a voltage limiter circuit is probably adequate).Tim

author: james_s
author_rank: Super Contributor
timestamp: January 27, 2023, 12:06:47 am
content: 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.

author: TimFox
author_rank: Super Contributor
timestamp: January 27, 2023, 12:19:09 am
content: 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.

author: Brumby
author_rank: Unknown
timestamp: January 27, 2023, 12:20:14 am
content: Quote from: T3sl4co1l on January 26, 2023, 11:17:14 pmIf 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.

author: james_s
author_rank: Super Contributor
timestamp: January 27, 2023, 01:30:41 am
content: 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.

author: eti
author_rank: Super Contributor
timestamp: January 27, 2023, 02:04:18 am
content: They tend to emit light - the clue is in their very name. 😁

author: Infraviolet
author_rank: Super Contributor
timestamp: January 27, 2023, 03:14:57 am
content: 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.

author: james_s
author_rank: Super Contributor
timestamp: January 27, 2023, 04:00:40 am
content: 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.

author: Marco
author_rank: Super Contributor
timestamp: January 27, 2023, 04:05:45 am
content: 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.

author: Infraviolet
author_rank: Super Contributor
timestamp: January 27, 2023, 05:14:30 am
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 27, 2023, 05:53:32 am
content: 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

author: Brumby
author_rank: Unknown
timestamp: January 27, 2023, 06:01:29 am
content: Quote from: james_s on January 27, 2023, 01:30:41 amThat's what I was thinking.QuoteFair point.Perhaps the Op could clarify. (or am I just showing my ignorance?)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 27, 2023, 06:11:46 am
content: 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.Tim

author: jmh
author_rank: Contributor
timestamp: January 27, 2023, 10:45:04 am
content: Quote from: Brumby on January 27, 2023, 06:01:29 amExactly 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.

author: Brumby
author_rank: Unknown
timestamp: January 27, 2023, 11:05:39 am
content: Today, I learned something - and the logic of it seems so obvious.Thank you.

author: james_s
author_rank: Super Contributor
timestamp: January 27, 2023, 07:20:58 pm
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: January 27, 2023, 08:33:52 pm
content: Sort of a similar application is flame detection/combustion monitoring in large burners. A UV photodiode or more typically aHamamatsudischarge tube is used with a viewing port on the flame.To meet functional safety requirements, there is a self-check whereby you turn a UV LED on and off, while watching the sensor's response. This confirm things are not blocked by soot, dirt etc. or that there is a hardware failure. Some implementations use a shutter you can pop up to cover the sensor.OP would have troubles with the extra hardware and conductors in the cables to the gate arms. I can't see monitoring the lamps as being necessary because there are redundant sets at a rail crossing. Monitoring the LED current & voltage would give a lot of coverage, I've only once seen a red LED fail that almost faked the correct VF.Sunlight would be a problem with phototransistor-based monitoring. I'm told most crossing still use primitive relay logic for the lamps/gate arms.OT: TheUVTRONwould make an awesome kitchen fire alarm, it can detect a candle flame 20m away, across a house, sunlight and smoke is ignored.

author: james_s
author_rank: Super Contributor
timestamp: January 27, 2023, 08:49:17 pm
content: Quote from: floobydust on January 27, 2023, 08:33:52 pmNobody said anything about gate arms. From what the OP has said this is for railroad signals, ie traffic signals for the trains, not crossing signals for motorists. Railroad signals are quite specialized, traditionally they were kerosene lamps, then gradually they were electrified, initially using batteries as they were often installed in remote locations.

author: IanB
author_rank: Super Contributor
timestamp: January 27, 2023, 09:21:13 pm
content: Quote from: james_s on January 27, 2023, 08:49:17 pmIndeed this, and from the context I would assume it is for a heritage railway, where funds are likely to be less abundant than at a mainline railway.

author: jmh
author_rank: Contributor
timestamp: January 27, 2023, 09:34:40 pm
content: Quote from: IanB on January 27, 2023, 09:21:13 pmYes it is a heritage line - mostly our own money or what we can scrounge.Quote from: james_s on January 27, 2023, 08:49:17 pmAnd we still use kerosine lamps sometimes where there is no power, although a rechargeable 12V battery and a single LED lasts for several weeks. But those are in some of the mechanical signals.

author: jmh
author_rank: Contributor
timestamp: January 27, 2023, 09:37:15 pm
content: Just in case it's remotely of interest this is one of the lamps taken out of it's housing.

author: floobydust
author_rank: Super Contributor
timestamp: January 27, 2023, 09:50:31 pm
content: 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.

author: Nusa
author_rank: Super Contributor
timestamp: January 27, 2023, 10:30:51 pm
content: Quote from: james_s on January 27, 2023, 12:06:47 amA 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.

author: IanB
author_rank: Super Contributor
timestamp: January 27, 2023, 11:44:46 pm
content: Quote from: jmh on January 27, 2023, 09:37:15 pmIt 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=GBPGiven 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?

author: tom66
author_rank: Super Contributor
timestamp: January 28, 2023, 12:08:52 am
content: 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?

author: AndyBeez
author_rank: Frequent Contributor
timestamp: January 28, 2023, 10:45:17 am
content: 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.

author: AVGresponding
author_rank: Super Contributor
timestamp: January 28, 2023, 10:54:39 am
content: The problem with any current sensing method is that you just can't rule out a failure mode where there's an approximately normal current flowing. The only way I can see meeting the criteria is to directly sense the light output, and even that has its problems. What if the sense circuit fails in such a way as to give a false good reading?

author: jmh
author_rank: Contributor
timestamp: January 28, 2023, 11:35:04 am
content: Quote from: AVGresponding on January 28, 2023, 10:54:39 amFrom what everyone has been saying yes I think sensing the light output is the best way here. At the end of the day its about risk management and mitigation and what The Management agree to. All we can do is give options. Now don't shoot me for not building my own but I found a little relay module on a certain auction site which is trigged from a photosensor which will do exactly what we need. At least it's something I can experiment with and only a few quid. And not my money this time.When I am next there we do have a LED replacement unit from a 'real railway' signal and I'll have a closer look to see how that works. Those are very different signal lamps to our old thing and replace bulbs which are a lot more watts.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: January 28, 2023, 12:29:41 pm
content: Quote from: AVGresponding on January 28, 2023, 10:54:39 amThen simply replace all of this clever technology with the Mark One eyeball.Quote from: jmh on January 28, 2023, 11:35:04 amSo how will you notify the signalman that any light source is out of operation? How do you force a failsafe or stop condition on the block?

author: jmh
author_rank: Contributor
timestamp: January 28, 2023, 01:03:39 pm
content: Quote from: AndyBeez on January 28, 2023, 12:29:41 pmThat's the easy part. LED fails, relay drops, drops the control relays in all following signals which then go to red (their default). Driver stops the train and calls to see what is wrong. Showing anything other than red is always a positive action i.e. a relay is energised to do so.

author: PA0PBZ
author_rank: Super Contributor
timestamp: January 28, 2023, 01:21:27 pm
content: Quote from: IanB on January 27, 2023, 11:44:46 pmI didn't trust the site: quantity 10, price each £ 14.50. And indeed, clicking the 'add to cart' button presents you with the total of £ 145.00

author: MarkS
author_rank: Unknown
timestamp: January 28, 2023, 02:55:09 pm
content: 12v vehicle LEDs are known to get very hot.A simple thermistor might suffice. If it drops below normal operating temps while on, something is wrong. Helps to read the post before posing...

author: Infraviolet
author_rank: Super Contributor
timestamp: January 28, 2023, 09:48:46 pm
content: The OP seems to be in the UK, unless he's in northern Scotland snow is pretty rare here. And I guess the heritage railway would be shut on days if the weather is really bad anyway.The picture of what he is replacing is pretty bulky, so he looks to have plenty of physical volume to work within. In his position I'd look for high powered LEDs/arrays which do NOT have internal driver circuitry, then PWM them very fast and monitor a light sensor for this varying signal. The trickiest part will be finding an LED type which is bright, doesn't have internal driver circuitry (which would prevent PWMing), and can be easily driven from a simple fixed DC voltage level without him needing to build a complex external driver circuit (the way one can need to for some fancy LEDs, especially lasers but it can be the case for simpler high power LEDs too). In the volume of that box he'll also need to work out a way to provide a regulated power source for the logic and op amp chips involved in monitoring the light sensor, he doesn't seem to have said exactly what power source is currently available (only that the LEDs he has tried can take 12V DC or AC).

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: January 29, 2023, 01:54:27 am
content: The LED light assemblies we use here are not all the same they have double sided and different size ones.Some gates are longer with more lights, all lights are DOT spec'd.For the flasher units + (power supply) we had to pass railroad testing including artic tests.Jeff

author: IDEngineer
author_rank: Super Contributor
timestamp: January 30, 2023, 07:22:42 am
content: I'm thinking of two LED arrays, toggled in operation, where during one phase the first array is the emitter and the second array is the detector. During the other phase their roles reverse. Both arrays point in the same direction and detection relies on some internal reflection within the fixture. If you optically associated LED's from opposing phases you could know which devices had reduced output. The advantage here is efficiency... all devices contribute to both emission and detection.

author: peter-h
author_rank: Super Contributor
timestamp: January 30, 2023, 05:34:37 pm
content: An LED, when not lit, is also a photodiode.In some designs, this is used to measure the ambient light and adjust the ON current accordingly.So you could have a piece of photoluminescent material next to the LED which returns the LED light back to it, with the material selected to have a short TC, say 1 second, and you periodically turn off the LED and measure how much light is coming in.One problem would be how to exclude the effect of ambient light, but that should be possible because you will know the rough TC of the luminescent material.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: January 30, 2023, 05:54:46 pm
content: I have seen matrix roadsigns with a photosensor pointing at every LED.A phototransistor will probably do if you can get decent optioncal coupling, so no amplifier needed.If you don't need continuous monitoring, you could connect multiple sensors in parallel and periodically light each LED in turn & look for chnages.

author: BrokenYugo
author_rank: Super Contributor
timestamp: February 01, 2023, 09:30:30 pm
content: The suggestion to modulate the LED and detect that seems best to me, the matter that we're discussing DIY modification of safety equipment aside.That said, you mention there are optics in these lamps. It's been my experience, but by no means a hard and fast rule, that you get bad results retrofitting any lamp that's more complex than a simple reflector. I assume because they're designed to focus on a white hot filament in a fairly precise location, not an array of SMD LEDs. For example there's no real upgrade to halogen car headlights (aside from cleaning fogged housings and minimizing voltage drop in the wiring), whatever else you stick in there inevitably screws up the cutoff, giving the impression of of more light, while blinding oncoming traffic.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 02, 2023, 12:49:17 pm
content: Quote from: IanB on January 27, 2023, 11:44:46 pmIt is not just the costs for the new lamps (which seem to be higher by a factor of 10 - though likely with a discount for larger quanteties). Often much of the costs are for sending out a maintainance team to sometimes relatively remore locations. At least less ladders needed for the rail signals.When introducing the LED trafic lights the longer service intervals were a big plus, not so much the energy savings.Depending on the environment one may still need regular cleaning (e.g. check for spider webs).Modern railways have an electronic signal going to the train as a backup. The optical signal is limited reliability becaus of the human factor anyway, no matter how good the lamps.Normally I would consider lamps with 3 redundant strings and than a somewhat more sensitive system to detect a drop in the current good enough. 2/3 the intensity should be good enough until fixed.A failure with reduced optical power when getting the right current is rare and mainly an issue when running too hot or a series failure with white LED loosing the phosphor. Even if 1 LED in the string would fail, this would be something like 1 in 3 or 4.Modulation of the lamps would be a way to make it easier for an electgronic system to detect them too - but that would be a larger change to the system.Chances are this would need changes in the regulations anyway, no matter how small the change.

author: TomS_
author_rank: Frequent Contributor
timestamp: February 04, 2023, 10:23:00 am
content: Maybe it would be going further than is technically possible or feasible (or even wanted), but if you were inclined to produce your own replacement "bulbs", there are LED drivers available that can detect a variety of failure modes (like opens and shorts), which could then be read back using a small microcontroller and perhaps operate a set of relay contacts that can indiciate a fault to the rest of the signalling system.For example,https://www.ti.com/lit/ds/symlink/tlc5916.pdfI used some of these in a project to drive 16 segment LED displays, although I didnt use the error detection functionality because my shift register doesnt have an ability to read back, but its there.Maybe there are smaller chips available, or maybe these could be built onto remote boards that then drive the LEDs so they dont have to try to be wedged into a tiny space.Just a thought.