Transmitter & Receiver: Instant Signal Control

Why You Need This Project: The Simple Scenario

WHAT WILL YOU LEARN

CIRCUIT CONNECTIONS

This map shows you the physical map for building the project. It shows exactly where to plug in every wire for the buttons, LEDs, screen or any components for every exercises so that the software instructions can correctly control all the hardware pieces.

1. Transmitter (Sender) Setup

Component	ESP32 Dev Module	Component function
LED Control Button (any button)	GPIO4	Press to tell the Receiver to turn its LED ON.
Buzzer Control Button (any button)	GPIO5	Press to tell the Receiver to turn its Buzzer ON. Export to Sheets

2. Receiver (Listener) Setup

Component	ESP32 Dev Module	Component function
Controlled LED	GPIO18	Turns ON when the LED button is pressed on the Transmitter.
Controlled Buzzer	GPIO19	Turns ON (makes noise) when the Buzzer button is pressed on the Transmitter. Export to Sheets

CODE LAB

This section holds the instructions for the ESP32. It’s the “brain” that tells the board exactly what to do, like when to flash a green light, what sound to play, and how to choose a random card. You copy and upload this code so your project can actually work.

Step 1: Install Library

Step 2: Full Code

#include <WiFi.h>

void setup() {
  Serial.begin(115200);

  // Set the device to Wi-Fi Station mode
  // This is required to initialize the Wi-Fi stack and get the MAC address
  WiFi.mode(WIFI_STA);

  // Get and print the MAC address
  String macAddress = WiFi.macAddress();
  
  Serial.println("");
  Serial.println("---------------------------------");
  Serial.println("ESP32 MAC Address Finder");
  Serial.println("---------------------------------");
  Serial.print("MAC Address (colon format): ");
  Serial.println(macAddress);

  // Print the MAC address in the format needed for the ESP-NOW code
  Serial.print("MAC Address (ESP-NOW Code Format): {0x");
  
  for (int i = 0; i < macAddress.length(); i++) {
    // Only print the hex characters, ignore the colons
    if (macAddress.charAt(i) != ':') {
      Serial.print(macAddress.charAt(i));
    }
    
    // Add the 0x and comma separators for the code format
    if (i % 3 == 1 && i < macAddress.length() - 1) {
      Serial.print(", 0x");
    }
  }
  Serial.println("};");
  Serial.println("---------------------------------");
}

void loop() {
  // The MAC address is only retrieved in setup(), so the loop can be empty.
  delay(5000); 
}

You will paste the MAC Address in the Transmitter(Sender) ESP32 code.

Why do you need to get the MAC Address first?

2. ESP32 Transmitter (Sender) Code:

#include <esp_now.h>
#include <WiFi.h>

// 1. **REPLACE THIS with your Receiver's MAC Address**
uint8_t receiverAddress[] = {0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0xFF}; // Your MAC here!

// GPIO Pin definitions
const int LED_BUTTON_PIN = 4;
const int BUZZER_BUTTON_PIN = 5;

// Data structure (MUST be the same on TX and RX)
typedef struct struct_message {
  bool ledState;
  bool buzzerState;
} struct_message;

struct_message myData;
struct_message prevData = {false, false}; // To track previous state for change detection

// Callback function executed when data is sent
void OnDataSent(const uint8_t *mac_addr, esp_now_send_status_t status) {
  Serial.print("\r\nLast Packet Send Status:\t");
  Serial.println(status == ESP_NOW_SEND_SUCCESS ? "Delivery Success" : "Delivery Fail");
}

void setup() {
  Serial.begin(115200);

  // Set up input pins
  pinMode(LED_BUTTON_PIN, INPUT);
  pinMode(BUZZER_BUTTON_PIN, INPUT);

  // Set device as a Wi-Fi Station
  WiFi.mode(WIFI_STA);

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

  // Register the send callback function
  esp_now_register_send_cb(OnDataSent);

  // Register peer (the receiver)
  esp_now_peer_info_t peerInfo;
  memcpy(peerInfo.peer_addr, receiverAddress, 6);
  peerInfo.channel = 0;
  peerInfo.encrypt = false;

  // Add peer
  if (esp_now_add_peer(&peerInfo) != ESP_OK) {
    Serial.println("Failed to add peer");
    return;
  }
}

void loop() {
  // Read current button states
  bool currentLedState = digitalRead(LED_BUTTON_PIN);
  bool currentBuzzerState = digitalRead(BUZZER_BUTTON_PIN);

  // Check if a state has changed
  if (currentLedState != prevData.ledState || currentBuzzerState != prevData.buzzerState) {

    // Update the data structure
    myData.ledState = currentLedState;
    myData.buzzerState = currentBuzzerState;

    // Send message via ESP-NOW
    esp_err_t result = esp_now_send(receiverAddress, (uint8_t *)&myData, sizeof(myData));

    if (result == ESP_OK) {
      Serial.println("Sent with success");
    } else {
      Serial.println("Error sending the data");
    }

    // Update the previous state
    prevData.ledState = currentLedState;
    prevData.buzzerState = currentBuzzerState;
  }

  delay(50); // Small delay to debounce and avoid excessive sending
}

#include <esp_now.h>
#include <WiFi.h>

// GPIO Pin definitions
const int LED_PIN = 18;
const int BUZZER_PIN = 19;

// Data structure (MUST be the same on TX and RX)
typedef struct struct_message {
  bool ledState;
  bool buzzerState;
} struct_message;

struct_message incomingData;

// Callback function executed when data is received
void OnDataRecv(const uint8_t *mac, const uint8_t *incomingDataPtr, int len) {
  memcpy(&incomingData, incomingDataPtr, sizeof(incomingData));
  Serial.print("Bytes received: ");
  Serial.println(len);

  // Control the LED and Buzzer based on the received data
  digitalWrite(LED_PIN, incomingData.ledState);
  digitalWrite(BUZZER_PIN, incomingData.buzzerState);

  Serial.print("LED State: ");
  Serial.println(incomingData.ledState ? "ON" : "OFF");
  Serial.print("Buzzer State: ");
  Serial.println(incomingData.buzzerState ? "ON" : "OFF");
}

void setup() {
  Serial.begin(115200);

  // Set up output pins
  pinMode(LED_PIN, OUTPUT);
  pinMode(BUZZER_PIN, OUTPUT);
  
  // Ensure outputs are off initially
  digitalWrite(LED_PIN, LOW);
  digitalWrite(BUZZER_PIN, LOW);

  // Set device as a Wi-Fi Station
  WiFi.mode(WIFI_STA);

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

  // Register the receive callback function
  esp_now_register_recv_cb(OnDataRecv);
}

void loop() {
  // Everything is handled in the OnDataRecv callback function,
  // so the loop can be empty or used for other tasks.
  delay(10);
}

Step 3: Upload and Test

1. Upload Receiver Code: Upload the code to your Receiver board and keep it powered on.
2. Upload Transmitter Code: Upload the code to your Transmitter board (with the correct MAC).
3. Test: Press the buttons on the Transmitter. The LED and Buzzer on the Receiver should respond instantly!

Troubleshooting Guide

This guide is your fast-fix manual for when errors occur. It helps you quickly diagnose why something isn’t working, like a dark screen or a silent buzzer—and provides simple steps to correct the problem.

Problem	What is Happening?	Simple Solution to Try
TX shows “Failed to add peer”	The MAC address in the TX code is wrong.	1. MAC Address: Double-check the 6-byte address for typos. 2. Format: Ensure it is in the 0xXX, 0xXX, ... format.
TX shows “Delivery Success,” but the RX doesn’t activate outputs.	The data is sent successfully, but the RX wiring is wrong.	1. RX Wiring: Check the LED/Buzzer wiring on GPIO18 and GPIO19. 2. Output States: Ensure your outputs are wired correctly (LED with resistor, buzzer).
The output is always ON, even when the button is released.	The button wiring is backward.	TX Button Wiring: The button must connect the pin (4 or 5)
RX shows data received, but the pin states don’t change.	The received data isn’t being applied to the pins.	Struct Match: Ensure the struct_message is IDENTICAL in both TX and RX files (variable names and types must match exactly).

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