LEDs for semaphore signals

Graeme Dunbar · August 22, 2024, 21:55

Something I meant to ask at this evening's meeting but forgot relates to LEDs for semaphore signals.

I was wondering what sort of LED is used in the Dapol semaphore signals? (I regret not buying one to try out recently when I was in Cheltenham and visiting the Cheltenham Model Centre.)

The "obvious" answer is a white LED shining through the coloured lenses, but white LED light is not a very good white. Also the spectacles are red and the (roughly) correct blue rather than green. That is like the prototype, relying on a yellowish light shining through a low saturation (or rather density) blue lens to give blue-green so I was wondering if there was anything unusual about the LEDs used.

It seems there is very little green in most white LEDs light so I was wondering if the blue emitter and yellow phosphor in a white LED shining through blue spectacle gave a good enough representation of signal green?

Does anyone have any experience of shining LED light through coloured gels?

Most of my stock of LEDs are ancient and I'm not sure they would give a fair comparison with what migh be used today.

I found what seems to be a fair representation of the spectrum of white LED light:
https://electronics.stackexchange.com/questions/125500/cool-white-led-bulbs-are-they-full-spectrum

Hugh Norwood · August 23, 2024, 19:52

I'd guess you could try either a yellow LED or a (warm) white LED?

Or use sweetie wrappers as coloured gels

Keith Lawrence · August 23, 2024, 20:55

Is there any reason why you can't/don't want to use red and green leds? I have wired smleds here that are only 1mm or less in size which would surely do the job?

Keith

Graeme Dunbar · August 23, 2024, 21:57

Quote from: Keith Lawrence on August 23, 2024, 20:55Is there any reason why you can't/don't want to use red and green leds? I have wired smleds here that are only 1mm or less in size which would surely do the job?

Keith

I did wonder about that. There are red/green bi-colour LEDs available. Lit together they should give a yellowish light but also the two components should shine through the appropriate coloured lens with the other being largely blocked.

The down side is that bi-colour LEDs rarely give a good combined colour and seen directly appear as two indense dots of the separate colour. Frosting the LED lens helps a bit though.

Graeme Dunbar · August 23, 2024, 22:01

Quote from: hughn on August 23, 2024, 19:52I'd guess you could try either a yellow LED or a (warm) white LED?

Or use sweetie wrappers as coloured gels

Current thoughts are using tinted glue 'n glaze or similar. I've seen the ink from felt tip pens being used as a dye by extracting it using alcohol and letting the alcohol mostly evaporate.

Keith Lawrence · August 24, 2024, 08:50

[/quote]

I did wonder about that. There are red/green bi-colour LEDs available. Lit together they should give a yellowish light but also the two components should shine through the appropriate coloured lens with the other being largely blocked.

The down side is that bi-colour LEDs rarely give a good combined colour and seen directly appear as two indense dots of the separate colour. Frosting the LED lens helps a bit though.

[/quote]

I have both 3mm bi-colour leds and bi-colour smleds here, the 3mm ones may be both too big round (equivilant scale 9 inches across?) & too deep, even bending the anode & cathode immediately behind the body they're 5mm+ deep. Whereas the smleds (pre-wired) are only 1mm across & the same if not less deep, at that size I would doubt there would be a visual differential between red & green.
If you're interested I'll check out exactly where I got them and give the link ?

Keith

Keith Lawrence · August 24, 2024, 09:32

Still can't recall where I got the bicolour smleds (the brain cell's a bit lethargic at the mo' ) but these are worth a look at - https://tinyurl.com/ye2xyhah
Various sizes and colours.

Keith

Graeme Dunbar · August 24, 2024, 10:49

Quote from: Keith Lawrence on August 24, 2024, 08:50I have both 3mm bi-colour leds and bi-colour smleds here, the 3mm ones may be both too big round (equivilant scale 9 inches across?) & too deep, even bending the anode & cathode immediately behind the body they're 5mm+ deep. Whereas the smleds (pre-wired) are only 1mm across & the same if not less deep, at that size I would doubt there would be a visual differential between red & green.
If you're interested I'll check out exactly where I got them and give the link ?
Keith

Thanks Keith. The link would be helpful.

I'd like to give soldering up my own a go too and have found a few options for LEDs down to 0402 size (1 mm x 0.5 mm). The smallest bi-colour LEDs I've found are 0603 size (1.6 mm x 0.8 mm) - and that would be a challenge to solder!

I think the single colour LEDs should be solderable with some practise and a suitable jig but placing and soldering two parallel wires about 0.5 mm apart at each end of a bi-colour LED is beyond me.

Graeme Dunbar · August 24, 2024, 10:57

Quote from: Keith Lawrence on August 24, 2024, 09:32Still can't recall where I got the bicolour smleds (the brain cell's a bit lethargic at the mo' ) but these are worth a look at - https://tinyurl.com/ye2xyhah
Various sizes and colours.

Keith

A slightly different application but I think it was DCC Concepts that sold red/white bi-colour LEDs for (e.g.) tram lights. I don't see them on their web site, but I found this supplier:

https://www.railwayscenics.com/colour-common-anode-white-diffused-tower-reqs-resistor-p-3784.html

It is a pity it is a cool white.

Hugh Norwood · August 24, 2024, 17:02

Layouts4U do prewired red green bicolour nano LEDS