a series 20ohm resistor,
Approximately 0.7 over the led
Your series resistor is WAY too small. You are drawing 145 mA (2.9 Volts / 20 Ohms) from a pin that has an absolute maximum rating of 40 mA!
I'm not sure why the led is dropping so much, perhaps because the resistor was so low.
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via Egeo 16