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Wiring Guide

WCMCU-3935 to TTGO Wemos ESP32 OLED

Overview

The AS3935 sensor communicates via I2C and shares the bus with the built-in OLED display. Both devices use GPIO4 (SCL) and GPIO5 (SDA) — note these are reversed from the ESP32 standard pinout.

WCMCU-3935 Pin Description (11 pins)

Pin Connection Description
VCC 3.3V Power — join with SI, A0, A1, EN_V
GND GND Ground — join with CS
SCL GPIO4 I2C clock
MOSI GPIO5 I2C data (SDA in I2C mode)
MISO Not connected SPI only
CS GND Must be pulled LOW
SI (S1) 3.3V Interface select: HIGH = I2C
IRQ GPIO13 Interrupt output
EN_V 3.3V Enable internal voltage regulator
A0 3.3V I2C address bit 0 = 1
A1 3.3V I2C address bit 1 = 1 → address 0x03

Simplified: 5 Wires from ESP32

ESP32 3.3V  ──────────────┬──── VCC
                          ├──── SI   (HIGH = I2C mode)
                          ├──── A0   (address bit 0)
                          ├──── A1   (address bit 1)
                          └──── EN_V (voltage regulator ON)

ESP32 GND   ──────────────┬──── GND
                          └──── CS   (must be LOW)

ESP32 GPIO5 ─────────────────── MOSI (SDA)

ESP32 GPIO4 ─────────────────── SCL

ESP32 GPIO13 ────────────────── IRQ

I2C Bus Configuration

ESP32 GPIO5 (SDA) ──────────────── OLED SDA (0x3C)
                  └──────────────── AS3935 MOSI (0x03)

ESP32 GPIO4 (SCL) ──────────────── OLED SCL (0x3C)
                  └──────────────── AS3935 SCL (0x03)

Both devices share the same I2C bus — no conflict since addresses differ (0x3C vs 0x03).

OLED Built-in Pins

Signal GPIO Note
SDA GPIO5 Non-standard for ESP32
SCL GPIO4 Non-standard for ESP32
RST GPIO16 Required for initialization
Address 0x3C Fixed

I2C Address Selection (AS3935)

A1 A0 Address Status
0 0 0x00 FORBIDDEN by datasheet
0 1 0x01 Valid
1 0 0x02 Valid
1 1 0x03 Used in this project

Power Analysis

Component Current
ESP32 (active WiFi) ~240 mA
OLED SSD1306 ~20 mA
AS3935 (listening) ~0.085 mA
Total ~260 mA

The AS3935 draws ~1000x less current than the ESP32 — negligible load.

EMI Notes

The ESP32 generates electromagnetic interference that the AS3935 may detect as noise or disturbers.

Best practice: Mount the WCMCU-3935 sensor at least 50 cm away from the ESP32 using extension wires. Use twisted pair for I2C lines.

Alternatively, disable WiFi/Bluetooth in firmware:

WiFi.mode(WIFI_OFF);
btStop();