ESP-01S

Overview

The ESP-01S is a bare-bones ESP8266 WiFi module with minimal hardware. It’s one of the smallest and cheapest ESP8266 modules available but requires external components for programming and operation.

Generic ESP-01S

Test Status

Hardware Features

Module: ESP-01S
CPU: ESP8266EX single-core, 80MHz
RAM: 80KB
Flash: 1MB (typically)
- My board: GT25Q80A 8Mbit SPI Nor Flash
- Chip Marking: 580A-UGLI
GPIO Pins: 4 available (2 easily accessible)
WiFi: 802.11 b/g/n
SPI: Hardware SPI
I2C: Software-implemented
I2S: Interfaces with DMA
UART: 1 interface
Operating Voltage: 3.3V

Additional Hardware Features

Built-in LED on GPIO01
No built-in USB-to-UART (requires external programmer with CH340G or similar)
No built-in voltage regulator (requires external 3.3V LDO)
No reset or boot buttons
As bare bones as it gets

Board Pinout

The ESP-01S has 8 pins total:

Power Pins

3V3 - 3.3V input (requires stable, regulated 3.3V)
GND - Ground

GPIO Pins

GPIO0 - General purpose I/O, boot mode selection (pulled HIGH for normal boot, LOW for programming)
GPIO2 - General purpose I/O, boot mode pin (must be HIGH during boot)
TX - GPIO1, UART TX
RX - GPIO3, UART RX
RST - Reset pin (active LOW)
CH_PD - Chip enable (must be HIGH for operation)

Basic Configuration

Basic configuration for ESP-01S with 1MB flash.

esphome:
  name: my-esp01s

esp8266:
  board: esp01_1m

output:
  - platform: gpio
    pin: GPIO01
    id: builtin_led

light:
  - platform: binary
    name: "Built in LED"
    output: builtin_led

Programming Setup

The ESP-01S requires an external USB-to-UART adapter for programming:

Connect USB-to-UART adapter:
- Adapter TX → ESP-01S RX
- Adapter RX → ESP-01S TX
- Adapter 3.3V → ESP-01S 3V3 and CH_PD
- Adapter GND → ESP-01S GND
Enter programming mode:
- Connect GPIO0 to GND
- Power cycle or pull RST to GND briefly
- GPIO0 can be disconnected after entering programming mode
Normal operation:
- GPIO0 should be HIGH (or floating with pull-up)
- CH_PD must be HIGH
- RST should be HIGH (or floating with pull-up)

Important Notes

⚠️ Power Requirements:

Must use a stable 3.3V power supply
Can draw up to 300mA during WiFi transmission
Some USB-to-UART adapters cannot supply enough current - use external LDO for operation

⚠️ Boot Mode Pins:

GPIO0 must be HIGH during normal boot, LOW for programming mode
GPIO2 must be HIGH during boot
Both should have pull-up resistors

⚠️ Limited GPIOs: With only 2 easily accessible GPIOs (GPIO0 and GPIO2), this module is best suited for simple WiFi-to-serial applications.

⚠️ No Built-in Protection: No reverse polarity protection, overvoltage protection, or current limiting. Handle with care.

⚠️ I2C: I2C is implemented in software, so any GPIO can be used, but GPIO5 and GPIO4 are the default pins. They are not available on the board, so if used, SDA and SCL pins need to be specified.

Use Cases

The ESP-01S is ideal for:

WiFi-to-serial bridges
Simple WiFi sensors with minimal GPIO requirements
Retrofit WiFi connectivity to existing devices
Projects where size is critical

Not recommended for:

Projects requiring many GPIO pins
Beginners (consider D1 Mini or ESP32 DevKit instead)
Applications requiring easy reprogramming

Troubleshooting

Cannot enter programming mode:

Ensure GPIO0 is connected to GND before powering on
Check that CH_PD is HIGH
Verify 3.3V power supply is stable

Module doesn’t boot:

Check that GPIO0 and GPIO2 are HIGH (or floating with pull-ups)
Verify CH_PD is connected to 3.3V
Ensure power supply can handle current spikes (300mA+)

Random resets or crashes:

Almost always a power supply issue
Add 100µF capacitor near 3.3V pin
Use proper 3.3V regulator rated for 500mA+

Other Images

Back of the module: ESP-01S Back

With the adapter: ESP-01S with Adapter

Note on this adapter - it requires GPIO0 (IO0) to be set to ground before plugging it in to enter boot mode. I got them with modules, but I have a separate programmer.

My ESPHome Workshop

Connection Types