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If I just want to connect LEDs to the +3.3v pin and ground pin, what limits the number that I can connect? For example using green LEDs with a voltage drop of 2.2 V, I can only connect one with a resistor in series, because 2.2 + 2.2 V > 3.3 V.

But, in parallel, what stops me from connecting many LEDs? The voltage across them will always be 2.2 V, which is less than 3.3 V?

  • The GPIO pins can only source a limited amount of current. If you like to add more leds, you could just add a transistor to allow more current. – Gerben Jul 20 '13 at 19:48
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When you connect things in parallel, the total amount of current would be the same, limited by the current limiting resistor, therefore one LED will shine quite bright, 5 LEDs would be not so bright and 10+ LEDs will be hard to notice if these are ON or OFF.

If you plan to connect more than 2 LEDs, use a transistor, as Gerben noticed in his comment before.

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