first upload

examples/Master/Master.ino

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+#include <Wire.h>
+#include <I2CRelay.h>
+#include <I2CMasterUtils.h>
+
+RelayI2CMaster relayMaster(0x08); // Default address, can be changed via setAddress()
+
+void setup() {
+    Serial.begin(115200);
+    delay(1000);
+    relayMaster.begin();
+    Serial.println("Enter:");
+    Serial.println("  s                 - to scan I2C bus");
+    Serial.println("  ch <old> <new>    - to change slave address");
+    Serial.println("  <relay> <on/off>  - to set relay (e.g. 3 on)");
+}
+
+void loop() {
+    if (Serial.available()) {
+        String input = Serial.readStringUntil('\n');
+        input.trim();
+
+        if (input == "s") {
+            I2CMasterUtils::scanI2C();
+        } else if (input.startsWith("ch")) {
+            uint8_t oldAddr, newAddr;
+            if (I2CMasterUtils::parseChangeAddressCommand(input, oldAddr, newAddr)) {
+                I2CMasterUtils::changeSlaveAddress(oldAddr, newAddr);
+                relayMaster.setAddress(newAddr); // Update master's address
+                Serial.print("Changed address to 0x");
+                Serial.println(newAddr, HEX);
+            } else {
+                Serial.println("Invalid command format. Use: ch <old_addr> <new_addr>");
+            }
+        } else {
+            int spaceIndex = input.indexOf(' ');
+            if (spaceIndex > 0) {
+                String relayStr = input.substring(0, spaceIndex);
+                String stateStr = input.substring(spaceIndex + 1);
+
+                int relayNum = relayStr.toInt();
+                bool state = false;
+
+                stateStr.toLowerCase();
+                if (stateStr == "on") state = true;
+                else if (stateStr == "off") state = false;
+                else {
+                    Serial.println("Invalid state! Use 'on' or 'off'.");
+                    return;
+                }
+
+                if (relayNum >= 1 && relayNum <= 8) {
+                    relayMaster.setRelay(relayNum, state);
+                    Serial.print("Relay ");
+                    Serial.print(relayNum);
+                    Serial.print(" turned ");
+                    Serial.println(state ? "ON" : "OFF");
+                } else {
+                    Serial.println("Invalid relay number! Use 1-8.");
+                }
+            } else {
+                Serial.println("Invalid command! Format: <relay_number> <on/off>");
+            }
+        }
+    }
+}

examples/Slave/Slave.ino

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+#include <Wire.h>
+#include <EEPROM.h>
+
+// EEPROM settings
+#define ADDR_EEPROM_LOC 0
+#define EEPROM_SIZE 64
+
+// Relay pins: GPIO0 to GPIO7
+const int relayPins[8] = {0, 1, 2, 3, 4, 5, 6, 7}; // GPIOs 0-7
+
+uint8_t i2cAddress;
+
+// Unified I2C receive event
+void receiveEvent(int howMany) {
+  if (howMany >= 2) {
+    uint8_t first = Wire.read();
+    uint8_t second = Wire.read();
+
+    // Check for address change command
+    if (first == 0xAA) {
+      // Change I2C address and store in EEPROM
+      uint8_t newAddr = second;
+      EEPROM.write(ADDR_EEPROM_LOC, newAddr);
+      EEPROM.commit();
+      Serial.print("I2C address changed to: 0x");
+      Serial.println(newAddr, HEX);
+      delay(100);
+      ESP.restart();
+      return;
+    }
+
+    // Otherwise, treat as relay control
+    int relay = first;   // Relay number: 1-8
+    int state = second;  // State: 1=ON, 0=OFF
+    if (relay >= 1 && relay <= 8) {
+      digitalWrite(relayPins[relay - 1], state ? LOW : HIGH); // Active LOW relays
+    }
+  }
+}
+
+void setup() {
+  Serial.begin(115200);
+  EEPROM.begin(EEPROM_SIZE);
+
+  // Read I2C address from EEPROM or use default
+  i2cAddress = EEPROM.read(ADDR_EEPROM_LOC);
+  if (i2cAddress == 0xFF || i2cAddress == 0) i2cAddress = 0x08; // Default address
+
+  Serial.print("Using I2C address: 0x");
+  Serial.println(i2cAddress, HEX);
+
+  // Initialize relay pins
+  for (int i = 0; i < 8; i++) {
+    pinMode(relayPins[i], OUTPUT);
+    digitalWrite(relayPins[i], HIGH); // Relays OFF initially (active LOW)
+  }
+
+  // Start I2C slave
+  Wire.begin(i2cAddress);
+  Wire.onReceive(receiveEvent);
+}
+
+void loop() {
+  // Nothing needed here for basic operation
+}