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By faramon
#64837 Hi,

when connecting NodeMcu to usb power, BLUE LED flashes.
I put code to D1 and D4 pins to switch simple LED with resistor.
Why and when BLUE (I suppose this is for wifi or power LED) LED flashes? I have MQTT and when put gpio.HIGH for D4 pin, my RED LED start to flash but BLUE LED on NodeMcu stops. When gpio.LOW then BLUE starts. When gpio.HIGH for D1, BLUE light is present and also RED lLED from D1. I dont get it, when this BLUE LED starts and why it stops only if gpio.HIGH is present on D4 not D1?
Maybe question is pretty stupid but it is irritating to dont know how this BLUE LED works.
User avatar
By MHeys
#65079 Hi,

The Blue LED on the ESP-12 module (module used in NodeMCU) is connected to the Serial TX of the ESP8266 in reverse bias, so if Serial TX goes Low then the Blue LED is on. It blinks when you power on the board because the ESP8266 spits out it's boot loading message at 74880 baud on startup causing highs and lows.

The Serial TX just so happens to be the same pin as D4, so if you are bringing that Low then it will turn on.

See the Pinout diagram for the NodeMcu to see which peripherals are attached to which pins. ... 266-wisoc/