Mini Wi-Fi Smart Relay – Single Channel

This project is a compact, Wi-Fi-enabled relay controller designed for smart home automation and conventional wall-switch integration.

With a PCB measuring just 35 × 38 mm, the module combines an AC-to-DC power supply, a single relay output, an ESP32 microcontroller, and two wall-switch sensing inputs.

The design is similar in concept to the Sonoff MINI R4, providing a compact hardware platform for controlling electrical loads through wireless automation while retaining the ability to detect conventional wall-switch inputs.

Key Features

Compact dimensions: 35 × 38 mm

Input voltage: Designed for 85–265V AC, 50/60Hz

Relay outputs: 1 channel

Wall-switch inputs: 2 sensing inputs

Microcontroller: ESP32-based

Connectivity: Wi-Fi

Power supply: Integrated AC-to-DC conversion

PCB design: Optimized for a small footprint

The module is powered from an 85–265V AC supply through the LIN and N terminals. LOUT provides the relay-controlled live output for an external load, while the two N terminals share the neutral connection.

S1 and S2 are mains-referenced wall-switch sensing inputs, allowing the ESP32 to detect external switch states.

The PCB measures 35 × 38 mm and has been physically manufactured and tested.

Design Overview

The board integrates three main functions:

1. Power Supply

An onboard AC-to-DC power supply allows the module to operate directly from mains voltage without requiring an external DC adapter.

2. Relay Control

A single relay provides switching capability for an external electrical load. The relay is controlled through the onboard microcontroller.

3. Wall-Switch Detection

Two sensing inputs allow the microcontroller to monitor external wall switches, providing flexibility for smart-switch applications.

Applications

Smart home automation

Wi-Fi lighting control

Wall-switch integration

IoT relay controllers

Embedded electronics development

Compact automation projects

Hardware Testing

This PCB has been physically manufactured and tested.

The design is based on a working hardware prototype rather than an unverified schematic or simulation-only project.

PCB dimensions: 35 × 38 mm

Pinout

ESP32 Pin Assignments

GPIO3 REL1 Relay control

GPIO0 LED1 Status LED

GPIO9 SW2 User/BOOT pushbutton

GPIO18 S1OUT Wall-switch input 1

GPIO19 S2OUT Wall-switch input 2

EN SW1 Hardware reset button

TXD0 TX UART transmit

RXD0 RX UART receive

Additional Connected Pins

ESP32 GPIO Pin Connection

GPIO2 10 kΩ pull-up to 3.3 V

GPIO8 10 kΩ pull-up to 3.3 V

3V3 3.3 V supply

GND Ground

GPIO2, GPIO8, and GPIO9 are ESP32-C3 strapping pins, so their states during reset matter.

Summary

- Relay: GPIO3

- LED: GPIO0

- Reset: EN

- Button: GPIO9

- Wall-switch detection: GPIO18, GPIO19

- UART: TXD0, RXD0

- Pull-ups: GPIO2, GPIO8

Important Notes

This design operates with potentially hazardous mains voltage and is intended for experienced electronics developers.

The shared PCB design should not be treated as a certified, ready-to-install consumer product. Appropriate electrical protection, insulation, enclosure design, load-rating verification, and regulatory compliance are required for its intended application.

Firmware is not included.