Wifi

Wi‑Fi

What makes the ESP32 (and ESP8266) such great IoT devices is their ability to connect to the internet over Wi‑Fi. The Arduino platform for the ESP32 already includes a few libraries that make this very easy to use.

The simplest sketch you can use is the following:

#include <WiFi.h>

#define WIFI_SSID      "The name of your Wi-fi network"
#define WIFI_PASSWORD  "the password of your Wi-fi network"

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  // Make sure the built-in LED is off on start-up.
  digitalWrite(LED_BUILTIN, HIGH);

  // Let the ESP32 connects to an access point
  WiFi.mode(WIFI_STA);

  // Your WeMos tries to connect to your Wi-fi network
  WiFi.begin(WIFI_SSID, WIFI_PASSWORD);

  // Keep the while-statement alive as long as we are NOT connected
  // to the Wi-fi network.
  while (WiFi.status() != WL_CONNECTED) {
    delay(1000);
  }

  // If we get out of the while-loop it means that we have a connection!
  // Now move on to the loop-function to blink the LED.
}

void loop() {
  digitalWrite(LED_BUILTIN, LOW); // LED on
  delay(500);
  digitalWrite(LED_BUILTIN, HIGH); // LED off
  delay(500);
}

From top to bottom, here is what happens:

• Include the header file WiFi.h, a standard library for the ESP32 that provides functions to connect to the internet.
• The WiFi mode is set to WIFI_STA, which means the ESP32 acts as a station and will look for an access point.
• The WiFi.begin(..., ...) function connects to the network using the given SSID and password.
• The while loop keeps the program waiting until the Wi‑Fi connection is established.
• And finally in the loop() function:
  • LED on
  • Wait…
  • LED off
  • Wait…

Because the while loop in the setup() function keeps repeating as long as there is no connection, you only see the LED blinking when there actually is a Wi‑Fi connection.

GET request

Once you’re connected to the internet you can, just like a web browser, send requests to retrieve or send data.

The following example prints the received data from the server over the serial interface, similar to printing text in Java/Python/…. While the sketch is running you can open the Serial Monitor to see the text:

Make sure you’ve selected 115200 baud as the speed:

You can now make a GET request with the following sketch:

#include <WiFi.h>
#include <HTTPClient.h>
#include <WiFiClient.h>

#define WIFI_SSID      "The name of your Wi-fi network"
#define WIFI_PASSWORD  "the password of your Wi-fi network"

void setup() {
  // Initialize the Serial connection at a speed of 115200 b/s
  Serial.begin(115200);

  // Let the ESP32 connects to an access point
  WiFi.mode(WIFI_STA);

  // Your ESP32 tries to connect to your Wi-fi network
  WiFi.begin(WIFI_SSID, WIFI_PASSWORD);

  // Keep the while-statement alive as long as we are NOT connected
  // to the Wi-fi network.
  while (WiFi.status() != WL_CONNECTED) {
    delay(1000);
  }
}

void loop() {
  // Initialize a Wi‑Fi client & HTTP client
  WiFiClient client;
  HTTPClient httpClient;

  // Set the URL the request should be made to.
  httpClient.begin(client, "http://koffiepunthva.nl/api");

  // Make the GET request; this returns the HTTP status code.
  int httpCode = httpClient.GET();

  // Check if the response is OK.
  if(httpCode == HTTP_CODE_OK) { // HTTP_CODE_OK == 200
    // Print the body of the GET response.
    String payload = httpClient.getString();
    Serial.println(payload);
  } else {
    Serial.println("Unable to connect :(");
  }

  delay(5000);
}

What you can see in this code:

• The setup() function is almost the same as in the previous example, except that the LED initialization has been removed.
• In the loop() function:
  • WiFiClient and HTTPClient are used to set up the request.
  • httpClient.begin(): prepares the request to http://koffiepunthva.nl/api/
    • Making requests to an HTTPS server is more complex, so this example uses an HTTP server.
  • httpClient.GET(): receives and checks the HTTP status code.
  • httpClient.getString(): retrieves the data (payload) from the response.

JSON parsing

When you work with API endpoints you often deal with structured text; a common format is JSON. An example of JSON:

{
    "coffee": 0,
    "tea": 0,
    "others": 0,
    "chocolate": 0,
    "choco_creme": 0,
    "water": 0
}

To parse this text into something useful you can use a library. You can install it by going to the Arduino IDE menu Sketch > Include library > Manage libraries..., searching for ArduinoJson and installing that library:

Follow these steps to integrate the JSON parser into the GET‑request sketch:

Include the library:

#include <ArduinoJson.h>

Declare a buffer in your program that stores the data; make sure it is large enough for the content:

DynamicJsonDocument jsonBuffer(1024);

Deserialize (parse) the payload of the GET request into jsonBuffer:

deserializeJson(jsonBuffer, payload);

Read a value from the JSON, for example coffee:

int coffeeCupsCount = jsonBuffer["coffee"];
Serial.print("Cups of coffee: ");
Serial.println(coffeeCupsCount);

Or tea:

int teaCupsCount = jsonBuffer["tea"];
Serial.print("Cups of tea: ");
Serial.println(teaCupsCount);