..... 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.
QuoteRailway 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?)
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.
Nobody 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.
Indeed 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.
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.
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.
Just in case it's remotely of interest this is one of the lamps taken out of it's housing.
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?
What if the sense circuit fails in such a way as to give a false good reading?
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.
So 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?
On the other hand, given that the indicated price is £1.45 per bulb...