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I have seen a few examples of wiring a red LED directly from the RPI GPIO pins, however, I need to wire a GREEN LED and am having trouble calculating the resistor needed. I believe that a green LED uses 3.3 volts and at 20ma, so if I use the 5V power on pin 1 would I end up with 5V - 3.3V = 1.7.

1.7 V = 20 mA × R

or rephrased:

1.7 V / 20 mA = R

and when we solve that we get:

1.7 V / 20 mA = 1.7 V / 0.020 A = 85Ω ?

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  • There are two common chemistries used when making green LEDs. The traditional gives a warmer green and is more related to the red, yellow and orange and requires roughly 2 volts to operate. The green used in an RGB LED is typically colder in order to improve the gamut, and has a higher voltage, but are also available as discrete LEDs. You should figure out which one you have.
    – pipe
    Commented Dec 9, 2018 at 22:48

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I have powered green, red, yellow, and multi-colored LEDs direct from the (3V3) GPIO. So a green LED forward voltage of 3V3 seems unlikely.

Anyhow putting 20mA through a modern LED would be absurdly bright.

Try something in the range 300-500 ohms (or higher) and use PWM on the GPIO to vary the brightness if needed.

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You are correct and need to consider I = " Forward Current" and V = " Forward Voltage" in the equation.

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