ESP-NOW: two ESP32 boards talking without a router

Two ESP32 boards sending messages to each other directly, without a router and without the Internet: a remote control in the garage, a sensor at the far end of the garden. This is ESP-NOW, a protocol by Espressif. Here you press a button on one board and the LED lights up on the other.

// ESP-NOW, step zero: prints the board's MAC address. You need the one of the receiving board.
#include <WiFi.h>

void setup() {
  Serial.begin(115200);
  WiFi.mode(WIFI_STA);             // the MAC must be read with Wi-Fi in station mode
  delay(500);
  Serial.print("MAC address: ");
  Serial.println(WiFi.macAddress());
}

void loop() {}

// ESP-NOW, the receiving board: turns the LED on or off according to the message.
#include <WiFi.h>
#include <esp_now.h>

const int LED = 2;                 // the LED on the board

typedef struct {                   // identical to the transmitter's
  int counter;
  bool on;
} Message;

#if ESP_ARDUINO_VERSION_MAJOR >= 3   // since version 3.0 the first argument is a structure, not the MAC
void received(const esp_now_recv_info_t* info, const uint8_t* data, int length) {
#else
void received(const uint8_t* mac, const uint8_t* data, int length) {
#endif
  if (length != sizeof(Message)) return;   // not one of our messages
  Message msg;
  memcpy(&msg, data, sizeof(msg));
  digitalWrite(LED, msg.on ? HIGH : LOW);
  Serial.printf("Message no. %d: LED %s\n", msg.counter, msg.on ? "on" : "off");
}

void setup() {
  Serial.begin(115200);
  pinMode(LED, OUTPUT);
  WiFi.mode(WIFI_STA);
  if (esp_now_init() != ESP_OK) {
    Serial.println("ESP-NOW does not start");
    return;
  }
  esp_now_register_recv_cb(received);
}

void loop() {}

// ESP-NOW, the board with the button: each press sends a message to the other board, no router needed.
#include <WiFi.h>
#include <esp_now.h>

uint8_t receiver[] = {0x24, 0x6F, 0x28, 0x00, 0x00, 0x00};   // write the MAC of the receiving board here
const int BUTTON = 13;             // D7: button between this pin and GND

typedef struct {                   // the message: the same on both boards
  int counter;
  bool on;
} Message;
Message msg = {0, false};

#if ESP_ARDUINO_VERSION >= ESP_ARDUINO_VERSION_VAL(3, 3, 0)   // since version 3.3 the function receives other data
void sent(const wifi_tx_info_t* info, esp_now_send_status_t status) {
#else
void sent(const uint8_t* mac, esp_now_send_status_t status) {
#endif
  Serial.println(status == ESP_NOW_SEND_SUCCESS ? "Delivered" : "Not delivered");
}

void setup() {
  Serial.begin(115200);
  pinMode(BUTTON, INPUT_PULLUP);
  WiFi.mode(WIFI_STA);
  if (esp_now_init() != ESP_OK) {
    Serial.println("ESP-NOW does not start");
    return;
  }
  esp_now_register_send_cb(sent);
  esp_now_peer_info_t peer = {};
  memcpy(peer.peer_addr, receiver, 6);
  peer.channel = 0;                // 0 = the channel the board is already on
  peer.encrypt = false;
  esp_now_add_peer(&peer);
}

void loop() {
  if (digitalRead(BUTTON) == LOW) {
    msg.counter++;
    msg.on = !msg.on;              // each press: on, off, on...
    esp_now_send(receiver, (uint8_t*)&msg, sizeof(msg));
    delay(300);                    // wait for the button to stop bouncing
  }
}