Wroked by myself...

This section covers the "invisible" side of the project....

While the Web Server (AP) is great for controlling the box with a phone, I wanted to use this specific assignment to experiment with something more robust for the future: device-to-device communication.

I didn't just pick this assignment at random. I have a bigger vision for this project. Right now, I control the box with my phone, but in a dusty CNC workshop, I don't always want to be touching my screen. My goal is to build a physical "Master Remote" that sits right on my CNC machine. I want to be able to press a physical button on that remote and control a network of Smart Boxes which might be on the other side of the LAB and open the correct drawer instantly. To do that, I needed a way for two ESP32 boards to talk to each other directly, instantly, and without relying on a potentially flaky shop Wi-Fi router.

To make this happen, I used ESP-NOW. If you haven't used it before, ESP-NOW is a connectionless communication protocol developed by Espressif. Unlike standard Wi-Fi, where a device has to find a router, negotiate a connection, get an IP address, and then send data (which takes seconds), ESP-NOW is like using a walkie-talkie. Implementing ESP-NOW is a two-step process because it relies on the physical hardware ID (MAC Address) of the chips, not IP addresses. First, I had to find out "who" my boards were. Every ESP32 has a unique MAC address burned into it. I wrote a simple script with the help of GEMINI to force the Wi-Fi hardware on and print this address to the Serial Monitor for the XIAO ESP32S3 Series.

Arduino Code (XIAO_ESP-32S3_MAC_Address)

  
//Walter Lenigan - Fab Academy 2025

#include "WiFi.h"

void setup() {
  Serial.begin(115200);
  
  // Add a small delay to give the board time to initialize.
  delay(500); 

  Serial.println("Attempting to initialize WiFi...");
  
  // Explicitly set the mode to Wi-Fi Station mode.
  // This forces the radio hardware to turn on, which is often necessary
  // to read the MAC address.
  WiFi.mode(WIFI_STA);
  
  // Disconnect any previous connection.
  WiFi.disconnect();

  // A small delay after mode set
  delay(100);

  Serial.print("ESP32 Board MAC Address:  ");
  Serial.println(WiFi.macAddress());
}

void loop() {
  // Nothing to do here
}
  

1. MAC_B1: {0x24, 0x58, 0x7C, 0xE4, 0x0F, 0x30}

2. MAC_B2: {0x24, 0x58, 0x7C, 0xE4, 0x0E, 0x04}

3. MAC_B3: {0x64, 0xE8, 0x33, 0x50, 0x52, 0xDC}

Once I had the MAC addresses of my boards, I wrote the communication firmware. This code allows multiple boards (Board 1, 2, and 3) to chat with each other. The structure struct_message that contains a senderID and the text message. Change #define MY_BOARD_ID to 1, 2, or 3 for each board. To send a message you must firth type the board number (1, 2 or 3) to whom you want to communicate and then “:” (1: LENIGAN).

Arduino Code (ESP-NOW Network Code (CHAT))

  
//Walter Lenigan - Fab Academy 2025

#include 
#include 
#include "esp_wifi.h"

// ======================= SETTINGS =======================

// !!! CHANGE THIS TO 1, 2, OR 3 FOR EACH BOARD !!!
#define MY_BOARD_ID 1  

// Your 3 Boards:
uint8_t mac_table[3][6] = {
  {0x24, 0x58, 0x7C, 0xE4, 0x0F, 0x30}, // Board 1 (ID 1)
  {0x24, 0x58, 0x7C, 0xE4, 0x0E, 0x04}, // Board 2 (ID 2)
  {0x64, 0xE8, 0x33, 0x50, 0x52, 0xDC}  // Board 3 (ID 3)
};

#define WIFI_CHANNEL 1

// ========================================================

typedef struct struct_message {
  int senderID;
  char message[200];
} struct_message;

struct_message dataToSend;
struct_message receivedData;

// ========= THE FIX IS HERE =========
// Your library version (3.3.3) requires 'const wifi_tx_info_t *' 
// instead of the older 'const uint8_t *'.
void OnDataSent(const wifi_tx_info_t *info, esp_now_send_status_t status) {
  // We don't need to use the 'info' variable for this simple chat,
  // but the function signature MUST match what the library expects.
}

// Callback when data is received
// (This one also uses the newer 'esp_now_recv_info' signature)
void OnDataRecv(const esp_now_recv_info * info, const uint8_t *incomingData, int len) {
  memcpy(&receivedData, incomingData, sizeof(receivedData));
  
  Serial.println(); 
  Serial.print("--- Message from Board ");
  Serial.print(receivedData.senderID);
  Serial.println(" ---");
  Serial.println(receivedData.message);
  Serial.println("------------------------");
  Serial.print("Enter 'TargetID:Message': ");
}

void setup() {
  Serial.begin(115200);
  
  // Wait a moment for Serial to start
  delay(1000);
  
  Serial.println();
  Serial.print("Initializing Board ");
  Serial.println(MY_BOARD_ID);

  WiFi.mode(WIFI_STA);
  WiFi.disconnect();

  esp_wifi_set_promiscuous(true);
  esp_wifi_set_channel(WIFI_CHANNEL, WIFI_SECOND_CHAN_NONE);
  esp_wifi_set_promiscuous(false);

  if (esp_now_init() != ESP_OK) {
    Serial.println("Error initializing ESP-NOW");
    return;
  }

  // Register Peers
  esp_now_peer_info_t peerInfo = {};
  peerInfo.channel = WIFI_CHANNEL;
  peerInfo.encrypt = false;

  for (int i = 0; i < 3; i++) {
    // We register everyone except ourselves
    // (Array index 'i' corresponds to Board ID 'i+1')
    if ( (i + 1) != MY_BOARD_ID ) {
      memcpy(peerInfo.peer_addr, mac_table[i], 6);
      if (esp_now_add_peer(&peerInfo) != ESP_OK){
        Serial.printf("Failed to add Board %d as peer\n", i+1);
      } else {
        Serial.printf("Board %d registered as peer.\n", i+1);
      }
    }
  }

  esp_now_register_send_cb(OnDataSent);
  esp_now_register_recv_cb(OnDataRecv);

  Serial.println("Ready!");
  Serial.println("Syntax: Type 'TargetID:Message'");
  Serial.println("Example: '2:Hello' sends to Board 2");
  Serial.print("Enter 'TargetID:Message': ");
}

void loop() {
  if (Serial.available() > 0) {
    String input = Serial.readStringUntil('\n');
    input.trim(); 

    int separatorIndex = input.indexOf(':');

    if (separatorIndex == -1) {
      Serial.println("\nError: Use format 'ID:Message' (e.g. 2:Hello)");
      Serial.print("Enter 'TargetID:Message': ");
      return;
    }

    String targetStr = input.substring(0, separatorIndex);
    String msgStr = input.substring(separatorIndex + 1);
    int targetID = targetStr.toInt();

    // Basic Validation
    if (targetID < 1 || targetID > 3 || targetID == MY_BOARD_ID) {
      Serial.printf("\nError: Invalid Target ID %d.\n", targetID);
      Serial.print("Enter 'TargetID:Message': ");
      return;
    }

    dataToSend.senderID = MY_BOARD_ID;
    msgStr.toCharArray(dataToSend.message, sizeof(dataToSend.message));

    uint8_t *targetMAC = mac_table[targetID - 1];
    
    esp_err_t result = esp_now_send(targetMAC, (uint8_t *) &dataToSend, sizeof(dataToSend));

    if (result == ESP_OK) {
      Serial.printf("\nSent to Board %d: %s\n", targetID, dataToSend.message);
    } else {
      Serial.println("\nError sending data.");
    }
    
    Serial.print("Enter 'TargetID:Message': ");
  }
}