The ESP-01S

An ESP8266 with its flash chip and an antenna on a module 24.8 by 14.3 mm, and eight pins.

What it is

The same chip as the NodeMCU and the D1 mini, and the same programs. What is different is how little of it reaches the outside: two rows of four pins on a 0.1 inch grid.

It does not go in a breadboard

The two rows are only 0.1 inch apart, so pushed into a breadboard both would land in the same five-hole strips and short each other. On the bench it is wired with jumpers or through an adapter, and it is the same here: wire it from its pins.

Pins

Pin	What it is
GND	ground
GPIO2	GPIO 2, and the blue LED
GPIO0	GPIO 0
RX	GPIO 3, UART0's receive
TX	GPIO 1, UART0's transmit
EN	chip enable; low holds the chip off
RST	reset; low holds the chip off
3V3	the 3.3 V supply

Four of the eight are GPIO, and two of those four are the serial port, so a sketch that prints has GPIO0 and GPIO2 left for everything else. There is no A0: the ESP8266's analog input is not brought out on this module.

The blue LED is the S version's, on GPIO 2 and wired from 3.3 V, so it lights when the pin is LOW. EN and RST are both pulled up on the S version, so the module runs with them left alone. Pull either one to ground and it stops.

The programs

Like every ESP8266 board here, the ESP-01S compiles your sketch in the tab: edit it and press Run. The NodeMCU's Blink lights the blue LED.

WiFi

It joins Mokxi's simulated network. The network name and password are properties of the module. There is no radio, no 802.11 and no real internet: Simulated WiFi has the rest.

What is not modeled

• The original blue ESP-01, whose LED is on TX and flickers with the serial port. This is the ESP-01S.
• The boot modes. On a real module GPIO0 low at power-up starts the ROM's flash loader; here a program is loaded by pressing Run, and GPIO0 is an ordinary pin.
• The module's current draw, which is why a real ESP-01 browns out a weak 3.3 V supply. 3V3 here is an ideal rail.

Starter project

ESP-01 blink is the module beside a breadboard, wired with jumpers, running the NodeMCU's Blink. An LED goes from 3V3 through a resistor to GPIO2, the way the module's own blue LED is wired, so the two light together whenever the pin goes LOW.