Pinout

ESP8266 NodeMCU pinout

Pins

Module	ESP-12E/ESP-12F (ESP8266EX)
CPU	Tensilica L106, 80 MHz (160 MHz option)
Flash memory	4 MB
Supply	5 V via USB or VIN, onboard 3.3 V regulator
Logic level	3.3 V
GPIO current	12 mA max per pin
Digital I/O	D0–D8 plus RX/TX (D0 has no PWM or interrupt)
Analog input	1 × 10-bit (A0, 0–3.3 V via onboard divider)
Wi-Fi	802.11 b/g/n, 2.4 GHz
USB	Micro-USB, CP2102 (v1.0) or CH340G (v3)

GPIO0 (D3): LOW at reset drops the chip into UART flashing mode; HIGH — the default, usually via an onboard pull-up — boots the sketch normally.
GPIO2 (D4, also the onboard blue LED on most boards): must be HIGH at reset. Conveniently, that is also the LED’s off state, so wiring the LED there doesn’t fight the strap.
GPIO15 (D8): must be LOW at reset for either boot path to succeed — this board usually ships with a pull-down already in place. A pull-up wired here, or an SPI device that idles its chip-select pin HIGH, can be enough to stop the board booting.
Most NodeMCU boards wire their USB-serial chip’s DTR/RTS lines to toggle GPIO0 and RST automatically during upload, so flashing normally needs no button press — on a board without that circuit, hold FLASH (GPIO0 low) while tapping RST, then release FLASH once the upload starts.

A0 has an onboard voltage divider so it reads 0–3.3V — the bare ESP8266 chip’s own ADC input only accepts 0–1.0V, so a design written against the raw chip’s spec does not carry over to this board unmodified.
D0 (GPIO16) is the odd one out: no PWM, no interrupt support, and it is the pin wired to RST to wake the chip from deep sleep — that wire is what makes deep sleep work at all on this board.
The silkscreened "D" numbers (D0–D8) are this board’s own numbering layer, not the underlying GPIO numbers: D0 is GPIO16, D1 is GPIO5, D3 is GPIO0, and so on — a library or tutorial written against "GPIO0" means D3 here, not D0.
ADC readings can get noisy during an active Wi-Fi transmission burst — the single ADC shares board space with the radio. A small filter capacitor on A0, or averaging a few readings, is the usual fix; unlike the ESP32, there is no second ADC to switch to.
Each GPIO is rated for a maximum of 12mA — noticeably less headroom than an AVR board’s 40mA, so driving even a single LED without its own series resistor is closer to the pin’s actual limit here. The same Wi-Fi-transmit-burst-vs-onboard-regulator caution as the ESP32 DevKit above applies: a marginal USB supply plus a transmit peak is the classic cause of a random reset that looks like a code bug.