I am using a device with an ESP 8266 microcontroller. I have discovered that setting LED_BUILTIN to HIGH turns the built-in LED off and setting LED_BUILTIN to LOW turns the built-in LED on. This seems to be because the microcontroller can sink more current than it can source.

Before I start using this microcontroller, I would like to know which other pins have this reverse behaviour. I've looked up datasheets for the ESP 8266 but I don't see how to discern this information. I would simply write a sketch to test, but I'm not sure what I might damage, and I'm not sure how to determine when an output pin is in the high-impedance state anyway.

On the ESP 8266, how to know which pins are HIGH -> 0v and which pins are HIGH -> 5v? Are the HIGH -> 0v pins necessarily LOW -> 5v? Where is this documented?

  • the pin is not reversed. the attached circuit with the LED reverses the signal.
    – Juraj
    Apr 22, 2020 at 19:09
  • @Juraj: Thank you. The pin is not reversed, but the behaviour (on this board and others) is. Are you saying that the issue is board-specific, so won't be relevant to all ESP 8266 boards? This is the board that I'm using.
    – dotancohen
    Apr 22, 2020 at 19:12
  • it is a Wemos D1 R1
    – Juraj
    Apr 22, 2020 at 19:14
  • the behaviour is not reversed ... a HIGH does not necessarily mean LED on ... a HIGH only means high output
    – jsotola
    Apr 22, 2020 at 19:15
  • Yes, or at lease a clone of one. I see mention on forums of people having this issue with other ESP 8266 boards as well.
    – dotancohen
    Apr 22, 2020 at 19:15

1 Answer 1


The esp8266 must have a pull-up on io 2 for boot configuration. The Wemos D1 boards put the LED in series with the pull-up resistor on io 2. This creates the 'reversed' LED behavior.

enter image description here

The large D1s are retired and it is hard to find a good and readable schematics. The cut out is from D1 mini schematics.

  • I see now, thank you! I take it that this is common, but not necessary, on ESP 8266 boards.
    – dotancohen
    Apr 22, 2020 at 19:51
  • I've found the explanation of the boot configuration and it is lucky that I did, as it is non-trivial and very important. Thank you Juraj!
    – dotancohen
    Apr 22, 2020 at 19:59

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