I’m in the process of making low profile yard signals and have completed several single light red/green signals using 3mm red/green bi-colour, reverse polarity LED’s. So what I need to source is a 3mm yellow/green bi-colour, reverse polarity LED.
I’ve gone through several catalogues and haven’t seen any. So, is there such an animal?
Fergie
Seems to me that I read somewhere, and I haven’t tried it, that if you put an AC current through a bi-polar LED, that you will get yellow. As I said, I haven’t tried this, so don’t blame me if the LED goes up in smoke!
Fergie, I think there is a dedicated green / yellow LED on the market. Check Digi-Key. The AC method does work on the red / greens, but the resulting yellow can be “funky” depending on the frequency.
What you are doing when you put AC on a R/G LED is flickering the colors together so rapidly, the eye is tricked into seeing a yellow orange color.
Actually, check the prototype, wouldn’t you want a red / yellow for a yard signal. I’ve never seen green in a yard, except as a switch indicator.
That’s interesting. I thought diodes only conducted in one direction…
I know the Tomar searchlight signals can display red/amber/green. The normal red and green and then if both are wired together you get a nice amber color. It looks great and super realistic if your prototype used single head signals.
So for yellow/green all you would have to do is wire the green and then wire the red and green together to get the amber color. Don’t wire the red by itself if you do not want red.
And here is a big warning for Tomar signals using LEDs. They need very little power so you’ll likely have to use resistors. My transformer puts out 18volts on the output I’m using for all my signals. So resistors were a necessity…otherwise I’d have some smoking signals.
Bi-color LEDs with two leads are actually two LEDs in one package, wired in reverse to each other. DC current of one polarity causes the red to illuminate, and reversing the polarity illuminates the green. As mentioned above, using an AC voltage causes both colors to illuminate, producing an orang-ish yellow color.
I’ve searched all of the specialty LED sources I normally order from, and cannot find a green-yellow LED, only red-green.
Bi-color leds will produce yellow on AC current. Using 60 cycle current turns each led on 50% of the time and the yellow is not the best. Using a electronic oscillator to vary the on off time will improve the yellow. I beleive some commerical signals use this method.
Ok here’s a site that lists bicoloured LEDS http://www.purdyelectronics.com/pdf/LED-SFC.pdf . Hope It helps
They do only conduct in one direction, however one side looks for positive voltage, and the other looks for negative voltage.
Think of a sine wave going between plus and minus whatever voltage. As the current alternates, only one color is on at a time, the other is blocked. At 60 cycles per second of RGRGRGRGRGRG…, your eye sees yellow. If you want it to blink red then green, you need to slow the alternating down to maybe 3 cycles or less.
Very good, they have one listed. AND208YGA
Hello Fergmiester, here is a link on eBay to a store that usually has such items.
http://stores.ebay.com/Hobby-Electronics-and-Accessories_W0QQsspagenameZFAVQ3aFQ3aSLLRQQtZkm
Yellow/green, right angle, in plastic case, $1.27 each:
http://www.digikey.com/scripts/DkSearch/dksus.dll?Detail?Ref=410828&Row=115599&Site=US