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for 3 LEDs, red blue and green, connected all 3 in parallel at a 9v battery, what resistor should I use? I tried 1K, but no led lights, and then 460 ohms, here just the red and the green function. At the moment I don' t have another resistor, that's why I am asking. Maybe 300, 200 ohms? What should I buy?

Thanks in advance.


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Since the three LED have different forward voltage only one of then will light up if they are connected in parallel. You should either connect them in series or use an individual resistor each of them. And, most LED's would light at least a little when connected to a 9 volt battery through a 1Kohm resistor, not much but sure visible. Sounds like you have another problem too.

Use this LED resistor calculator. It's my fave. You need the Vf (forward voltage) and mA (typical current, not the maximum) rating of your LEDs for this to work. Well, you'll need the Vf rating at least. Most LEDs work fine with 20mA so that's a safe guess if you can't locate that information.