SENSORS/ACTUATORS

INTERNET OF THING (IoT)

Communication between two ESP32

This tutorial instructs you how to create connection between two ESP32 via TCP/IP over WiFi and exchange data. One ESP32 acts as a TCP client and the other acts as TCP server. We also learn in detail by an example:

ESP32 #1 connects to a button and acts as TCP client.
ESP32 #2 connects to an LED and acts as TCP server.
Two ESP32 connects to each other though the internet .
When the button on ESP32 #1 is pressed, the LED on ESP32 #2 is on.

Hardware Used In This Tutorial

2	×	ESP-WROOM-32 Dev Module
2	×	Micro USB Cable
1	×	(Optional) DC Power Jack
1	×	Breadboard
1	×	Jumper Wires
1	×	(Recommended) ESP32 Screw Terminal Adapter

Or you can buy the following sensor kits:

1	×	DIYables Sensor Kit (30 sensors/displays)
1	×	DIYables Sensor Kit (18 sensors/displays)

Disclosure: some of these links are affiliate links. We may earn a commission on your purchase at no extra cost to you. We appreciate it.

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:

Methods	Distance
SPI	very short
I2C	very short
UART (TTL)	very short
UART (RS-232/485/422)	short
Bluetooth	short
LoRa	long
WiFi	unlimited^(*)

※ NOTE THAT:

(*):

If we connect both ESP32 to the Internet, the communication distance is unlimited
If we does not connect both ESP32 to the Internet, but we connect them to the same LAN network, the communication distance is restricted within the LAN network.

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)

※ NOTE THAT:

UDP protocol is beyond the scope of this tutorial.

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

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.

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

Video Tutorial

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How to connect two ESP32 via Internet

See How to connect two devices via Internet

Learn More

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