ESP-01S (ESP8266): specs and pinout
An Ai-Thinker ESP-01S Wi-Fi module on a 2 x 4 pin header, with two usable I/O pins and a UART.
Specs
| Spec | Value | Source |
|---|---|---|
| Supply voltage | 3.0 V to 3.6 V (3.3 V typical) | source Table 1 Main Parameters (p.7); Electrical Characteristics (p.8) |
| Supply current requirement | more than 500 mA available | source Table 1 Main Parameters (p.7); Design Guide Power Supply (p.15) |
| Operating temperature | -20 C to 70 C | source Table 1 Main Parameters (p.7) |
| SPI flash | 8 Mbit default (1 MB) | source Table 1 Main Parameters (p.7) |
| Interfaces | UART / GPIO / PWM; 2 available I/Os | source Table 1 Main Parameters (p.7) |
| UART baud rate | 110 to 4,608,000 bps, default 115200 bps | source Table 1 Main Parameters (p.7) |
| I/O levels | VIL -0.3 V to 0.25 VIO; VIH 0.75 VIO to 3.6 V; IMAX 12 mA | source Electrical Characteristics (p.8) |
| Wi-Fi | 802.11 b/g/n, 2400 to 2483.5 MHz; STA / AP / STA+AP | source Product Overview and Features (p.5-6); Table 1 (p.7) |
| Dimensions | 24.6 x 14.4 x 11.2 (+/-0.2) mm; 11.2 mm includes the pin header height | source Table 1 Main Parameters (p.7) |
Power
| Value | Source | |
|---|---|---|
| Supply | 3.0 V to 3.6 V; supply able to give more than 500 mA | source Power Consumption table (p.9) |
| Current | TX 802.11b 170 mA, 11g 140 mA, 11n 120 mA; RX 50 to 56 mA; modem-sleep 20 mA; light-sleep 2 mA; deep-sleep 20 uA | source Power Consumption table (p.9) |
Pinout
Pin table is from the Ai-Thinker ESP-01S Specification V1.1. The 1M Flash in the listing matches the sheet default of 8 Mbit. The ESP8266 ADC pin is not brought out on this module.
| Pin | Function | Voltage | Notes | Source |
|---|---|---|---|---|
| 1 | GND | Ground | source 4 Pin Definition, Pin Function Definition Table (p.12) | |
| 2 | IO2 | GPIO2/UART1_TXD | source 4 Pin Definition, Pin Function Definition Table (p.12) | |
| 3 | IO0 | GPIO0, download mode: externally pulled low, run mode: floating or externally pulled high | source 4 Pin Definition, Pin Function Definition Table (p.12) | |
| 4 | RXD | UART0_RXD/GPIO3 | source 4 Pin Definition, Pin Function Definition Table (p.12) | |
| 5 | TXD | UART0_TXD/GPIO1 | source 4 Pin Definition, Pin Function Definition Table (p.12) | |
| 6 | EN | Chip enable, active high | source 4 Pin Definition, Pin Function Definition Table (p.12) | |
| 7 | RST | Reset | source 4 Pin Definition, Pin Function Definition Table (p.12) | |
| 8 | VCC | 3.3 V | 3.3V power supply (VDD); external power supply output current is recommended to be above 500 mA | source 4 Pin Definition, Pin Function Definition Table (p.12) |
Build with it
Project ideas for the ESP-01S (ESP8266). Each link opens Soldr with the prompt filled in; Soldr assigns the pins, checks power and shows how to wire every part.
- Wi-Fi temperature and humidity logger: DHT22 data on GPIO2, which must be high at boot; the downloader flashes the module.
- Wi-Fi low-voltage switch: Learning project: low-voltage lamp only, check the relay input works from a 3.3 V pin, keep GPIO0 high at boot. Mains wiring needs a qualified person.
- Serial Wi-Fi modem for a 3.3 V board: Both sides are 3.3 V logic; cross TX and RX and supply the ESP-01S with at least 500 mA.
Common mistakes
- Supply is 3.0 to 3.6 V and I/O is 3.3 V; use a level shifter to a 5 V controller (source: link, p.8 and Design Guide GPIO item 3, p.16).
- A weak supply browns out the module on transmit; the maker asks for more than 500 mA at 3.3 V (source: link, pin 8 description p.12, Power Supply p.15).
- GPIO0 low enters download mode, high or floating runs the program; GPIO2 must be high at boot (source: link, Module Boot Mode Description Table, p.13).
Not covered by the official source
- Table 1 of the sheet lists the module as 24.6 x 14.4 x 11.2 mm; older revisions give different dimensions.
Sources
- link (ESP-01S Specification V1.1, Ai-Thinker, 2025-08-02)