Thanx Bruce; your explanation was a LOT better than my lame attempt to figure it out using Google. Now I have to use the term in a sentence somewhere so I'll remember it . . . .
As the expedition pushed deeper into the gloomy recesses of the store, we began to see shelves draped with festoons of tiny, bright, varicoloured lights. Carruthers nudged me and pointed these out silently with his rifle. I nodded, resolutely despite the chill which had gripped me. We both knew what that meant. Time to guard your wallet, maybe count your ammunition too. The horror. The horror. In this place it was still Christmas.
For that kind of tri-color indicator, they have two choices: First is a double-junction red/green LED - Feed it positive polarity DC, you get red. Negative DC gets you green, and AC gets you yellow.
They do make three-lead LEDs with R G and common, but you would have mentioned three leads...
For indicators, a dropping resistor to get roughly 10ma to 20ma is all you need - just stay well below the maximum. For high-powered illumination you do need to have a current regulator in the circuit, or you can easily cook the LED by driving it too hard.
But they have current regulators out there on a single small chip that can be right next to the LED, they are getting used a lot as the electric sign industry changes over to strips of LED's instead of Neon for lit letters - no glass blowing experience required, no 20KV power supplies, no arcing over, no maintenance for 20 years...
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