ESP-01 ESP8266 Serial WiFi Transceiver Module

Before the NodeMCU and other development-board versions of the ESP8266 existed, the ESP-01 was the chip in its rawest usable form: an 8-pin module exposing UART, GPIO and not much else, designed to be commanded over serial from a separate microcontroller rather than programmed as a standalone board. That makes it the right choice specifically when a project already has an Arduino or other MCU handling logic and just needs to bolt WiFi onto it through a couple of serial wires, rather than replacing the whole design with an ESP32.

1MB of flash and the same ESP8266EX chip that powers larger boards means it runs the familiar AT command set over UART, so an existing Arduino sketch adds WiFi connectivity by sending text commands rather than a full code rewrite. Its compact 8-pin footprint is also why it needs a breadboard adapter to use conveniently - the pin spacing was never meant for a direct breadboard fit.

Common uses are adding WiFi to an existing Arduino-based project without redesigning the whole board, IoT retrofit projects, and any build where a separate microcontroller already handles the application logic.