GPIO0 and GPIO15 are used to decide what source to boot the ESP8266 from. In normal operation (boot from flash) GPIO0 is pulled high and GPIO15 is pulled low, each through a resistor.
Even though after boot you can use the two pins, they have to be wired so that the are active low for GPIO0 which is pulled high by the resistor, and active high for GPIO15 which is being pulled low.
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Hello, I made a few edits. Now GPIO15 works for me and the LED lights up.
by Jeff Child
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