Communication between two ESP32

Hardware Used In This Tutorial

Or you can buy the following sensor kits:

Communication between two ESP32 - Overview

There are many ways to make ESP32 #1 communicate with ESP32 #2. Depending on the distance, we can choose one of the below ways:

This tutorial is going to use WiFi for communication between two ESP32.

Communication between two ESP32 via WiFi

Two ESP32 can communicate with each other via WiFi if:

• Two ESP32 in the same LAN network. They does not need to connect to Internet
• Two ESP32 in the different LAN networks. They need to connect to Internet

There are two ways that two ESP32 can communicate to each other.

• Two ESP32 communicate with each other directly: one ESP32 is a TCP client, the other is a TCP server.
• Two ESP32 communicate with each other indirectly via a centralized server (e.g. MQTT server): both ESP32 are TCP clients. Refer to ESP32-to-ESP32 Communication via MQTT tutorial.

It needs an application protocol that two ESP32 uses to communicate to each other. For example:

• Self-defined protocol over raw TCP (directly)
• Modbus TCP (directly)
• HTTP (directly)
• Telnet (directly)
• SSH (directly)
• MQTT (indirectly via a server)

Example Application

Let's realize the following application: A button/switch connected to ESP32 #1 controls an LED connected to ESP32 #2 via WiFi.

To make it simple, This tutorial will use a self-defined protocol (Defined by us, no standard)

Self-defined Protocol

We define a simple protocol as follows:

• Create a TCP connection between ESP32 #1 and ESP32 #2
• ESP32 #1:
• TCP client: actively makes TCP connection request to ESP32 #2
• If the switch's state is changed to ON, ESP32 #1 sends a byte (command) with value 1 to ESP32 #2.
• If the switch's state is changed to OFF, ESP32 #1 sends a byte (command) with value 0 to ESP32 #2.
• ESP32 #2:
• TCP server: listens to a TCP connection request from ESP32 #1. If there is a TCP request, it accepts and the connection is created.
• If It receives a byte:
• If the value is 1, Turn LED on
• If the value is 0, Turn LED off

Wiring Diagram

• Wiring Diagram for ESP32 #1: TCP CLIENT + A BUTTON/SWITCH


This image is created using Fritzing. Click to enlarge image

• Wiring Diagram for ESP32 #2: TCP SERVER + AN LED


This image is created using Fritzing. Click to enlarge image

※ NOTE THAT:

It needs touse a resistor for LED if it does not have a built-in resistor. We have dedicated tutorials for button and LED. You can learn more about them in:

• ESP32 - LED tutorial
• ESP32 - Button tutorial

ESP32 Code for ESP32 #1


/* * This ESP32 code is created by esp32io.com * * This ESP32 code is released in the public domain * * For more detail (instruction and wiring diagram), visit https://esp32io.com/tutorials/communication-between-two-esp32 */ // ESP32: TCP CLIENT + A BUTTON/SWITCH #include <ezButton.h> #include <WiFi.h> #define BUTTON_PIN 21 // ESP32 pin GPIO21 connected to button const char* ssid = "YOUR_WIFI_SSID"; // CHANGE TO YOUR WIFI SSID const char* password = "YOUR_WIFI_PASSWORD"; // CHANGE TO YOUR WIFI PASSWORD const char* serverAddress = "192.168.0.180"; // CHANGE TO ESP32#2'S IP ADDRESS const int serverPort = 4080; ezButton button(BUTTON_PIN); // create ezButton WiFiClient TCPclient; void setup() { Serial.begin(9600); button.setDebounceTime(50); // set debounce time to 50 milliseconds Serial.println("ESP32: TCP CLIENT + A BUTTON/SWITCH"); // Connect to Wi-Fi WiFi.begin(ssid, password); while (WiFi.status() != WL_CONNECTED) { delay(1000); Serial.println("Connecting to WiFi..."); } Serial.println("Connected to WiFi"); // connect to TCP server (Arduino #2) if (TCPclient.connect(serverAddress, serverPort)) { Serial.println("Connected to TCP server"); } else { Serial.println("Failed to connect to TCP server"); } } void loop() { button.loop(); // MUST call the loop() function first if (!TCPclient.connected()) { Serial.println("Connection is disconnected"); TCPclient.stop(); // reconnect to TCP server (Arduino #2) if (TCPclient.connect(serverAddress, serverPort)) { Serial.println("Reconnected to TCP server"); } else { Serial.println("Failed to reconnect to TCP server"); } } if (button.isPressed()) { TCPclient.write('1'); TCPclient.flush(); Serial.println("- The button is pressed, sent command: 1"); } if (button.isReleased()) { TCPclient.write('0'); TCPclient.flush(); Serial.println("- The button is released, sent command: 0"); } }

ESP32 Code for ESP32 #2


/* * This ESP32 code is created by esp32io.com * * This ESP32 code is released in the public domain * * For more detail (instruction and wiring diagram), visit https://esp32io.com/tutorials/communication-between-two-esp32 */ // ESP32 #2: TCP SERVER + AN LED #include <WiFi.h> #define LED_PIN 18 // ESP32 pin GPIO18 connected to LED #define SERVER_PORT 4080 const char* ssid = "YOUR_WIFI_SSID"; // CHANGE TO YOUR WIFI SSID const char* password = "YOUR_WIFI_PASSWORD"; // CHANGE TO YOUR WIFI PASSWORD WiFiServer TCPserver(SERVER_PORT); void setup() { Serial.begin(9600); pinMode(LED_PIN, OUTPUT); Serial.println("ESP32 #2: TCP SERVER + AN LED"); // Connect to Wi-Fi WiFi.begin(ssid, password); while (WiFi.status() != WL_CONNECTED) { delay(1000); Serial.println("Connecting to WiFi..."); } Serial.println("Connected to WiFi"); // Print your local IP address: Serial.print("ESP32 #2: TCP Server IP address: "); Serial.println(WiFi.localIP()); Serial.println("ESP32 #2: -> Please update the serverAddress in ESP32 #1 code"); // Start listening for a TCP client (from ESP32 #1) TCPserver.begin(); } void loop() { // Wait for a TCP client from ESP32 #1: WiFiClient client = TCPserver.available(); if (client) { // Read the command from the TCP client: char command = client.read(); Serial.print("ESP32 #2: - Received command: "); Serial.println(command); if (command == '1') digitalWrite(LED_PIN, HIGH); // Turn LED on else if (command == '0') digitalWrite(LED_PIN, LOW); // Turn LED off client.stop(); } }

Quick Instructions

• If this is the first time you use ESP32, see how to setup environment for ESP32 on Arduino IDE.
• Wire a button/switch to ESP32 #1
• Wire an LED to ESP32 #2
• Open Arduino IDE (called Arduino IDE #1)
• Install ezButton library on Arduino IDE
• Connect ESP32 #1 to PC via USB cable and select COM port of ESP32 #1 on Arduino IDE #1
• Open another Arduino IDE window (called Arduino IDE #2) by clicking on Arduino IDE icon on your PC (important!⁽⁾)
• Connect ESP32 #2 to PC via USB cable and select COM port of ESP32 #2 on Arduino IDE #2
• Copy ESP32 #1 code, paste to Arduino IDE #1 and save it (named ESP321)
• Copy ESP32 #2 code, paste to Arduino IDE #2 and save it (named ESP322)
• Upload ESP32 #2 code to ESP32 #2 first
• Open Serial Monitor on Arduino IDE #2, get TCP Server IP address
COM6
Send
ESP32 #2: TCP SERVER + AN LED TCP Server IP address: 192.168.0.2 -> Please update the serverAddress in ESP32 #1 code
Autoscroll Show timestamp
Clear output
9600 baud  
Newline  
• Update TCP Server IP address in ESP32 #1 code
• Upload ESP32 #1 code to ESP32 #1
• Open Serial Monitor on Arduino IDE #1
• Press and hold the button on ESP32 #1 → see LED's state on ESP32 #2 (ON)
• Release button on ESP32 #1 → see LED's state on ESP32 #2 (OFF)
• Do the above process several times.
• See output on both Serial Monitors
• Serial Monitor of ESP32 #1
COM6
Send
ESP32 #1: TCP CLIENT + A BUTTON/SWITCH ESP32 #1: Connected to TCP server ESP32 #1: - The button is pressed, sent command: 1 ESP32 #1: - The button is released, sent command: 0 ESP32 #1: - The button is pressed, sent command: 1 ESP32 #1: - The button is released, sent command: 0 ESP32 #1: - The button is pressed, sent command: 1 ESP32 #1: - The button is released, sent command: 0
Autoscroll Show timestamp
Clear output
9600 baud  
Newline  
• Serial Monitor of ESP32 #2
COM6
Send
ESP32 #2: TCP SERVER + AN LED ESP32 #2: TCP Server IP address: 192.168.0.2 ESP32 #2: -> Please update the serverAddress in ESP32 #1 code ESP32 #2: - Received command: 1 ESP32 #2: - Received command: 0 ESP32 #2: - Received command: 1 ESP32 #2: - Received command: 0 ESP32 #2: - Received command: 1 ESP32 #2: - Received command: 0
Autoscroll Show timestamp
Clear output
9600 baud  
Newline  

※ NOTE THAT:

• (): If you open Arduino IDE #2 window via "File" → "New" or "Open" from Arduino IDE #2 window, you will NOT be able to open two Serial Monitors for two ESP32 in the same PC at the same time.
• There is an alternative to this self-defined protocol. It is the Modbus TCP. The Modbus protocol is standardized, it has many advantages over the self-defined protocol. See more in ESP32 - Modbus tutorial

If you're unfamiliar with how to supply power to the ESP32 and other components, you can find guidance in the following tutorial: How to Power ESP32.