Read values from an Arduino via HTTP

In this tutorial I want to expand the Arduino Web Server tutorial to read the values measured by a sensor, so we can just open a page on our browser and see the data.

For example we’re going to measure the temperature using a DHT11 sensor, and we’re going to measure the distance from an object using a proximity sensor.

We light up the built-in LED on the Arduino by reaching out to the /on URL, and we turn it off by opening the /off URL. Anything else does nothing.

This is the code from the other tutorial:

#include <SPI.h>
#include <WiFiNINA.h>

WiFiServer server(80);

void setup() {
  char ssid[] = SECRET_SSID;
  char pass[] = SECRET_PASS;

  Serial.begin(9600);
  while (!Serial);

  int status = WL_IDLE_STATUS;
  while (status != WL_CONNECTED) {
    Serial.print("Connecting to ");
    Serial.println(ssid);
    status = WiFi.begin(ssid, pass);
    delay(5000);
  }

  Serial.print("IP address: ");
  Serial.println(WiFi.localIP());

  server.begin();
}

void loop() {
  WiFiClient client = server.available();
  if (client) {
    String line = "";
    while (client.connected()) {
      if (client.available()) {
        char c = client.read();
        Serial.write(c);

        if (c != '\n' && c != '\r') {
          line += c;
        }

        if (c == '\n') {
          if (line.length() == 0) {
            client.println("HTTP/1.1 200 OK");
            client.println("Content-Type: text/html");
            client.println("Connection: close");  // the connection will be closed after completion of the response
            client.println();
            client.println("<!DOCTYPE HTML>");
            client.println("<html>");
            client.println("test");
            client.println("</html>");
            break;
          } else {
            line = "";
          }
        }
      }
    }

    client.stop();
  }
}

In the last else you see, we have a full line, so we can check its content before clearing it. In this case we can check for GET /on and GET /off :

if (line.startsWith("GET /on ")){
  digitalWrite(LED_BUILTIN, HIGH);
}
if (line.startsWith("GET /off ")) {
  digitalWrite(LED_BUILTIN, LOW);
}

That’s it! Now load the code on the Arduino and call the /on URL, or the /off URL.

I reserved a static IP to the Arduino using my local network router, and I named it arduino.local in my /etc/hosts file, so reaching out to http://arduino.local/on turns the LED on, and to http://arduino.local/off turns the LED off.

Here’s the complete program:

#include <SPI.h>
#include <WiFiNINA.h>

WiFiServer server(80);

void setup() {
  char ssid[] = SECRET_SSID;
  char pass[] = SECRET_PASS;

  Serial.begin(9600);
  while (!Serial);

  int status = WL_IDLE_STATUS;
  while (status != WL_CONNECTED) {
    Serial.print("Connecting to ");
    Serial.println(ssid);
    status = WiFi.begin(ssid, pass);
    delay(5000);
  }

  Serial.print("IP address: ");
  Serial.println(WiFi.localIP());

  server.begin();
}

void loop() {
  WiFiClient client = server.available();
  if (client) {
    String line = "";
    while (client.connected()) {
      if (client.available()) {
        char c = client.read();
        Serial.write(c);

        if (c != '\n' && c != '\r') {
          line += c;
        }

        if (c == '\n') {
          if (line.length() == 0) {
            client.println("HTTP/1.1 200 OK");
            client.println("Content-Type: text/html");
            client.println("Connection: close");  // the connection will be closed after completion of the response
            client.println();
            client.println("<!DOCTYPE HTML>");
            client.println("<html>");
            client.println("test");
            client.println("</html>");
            break;
          } else {
            if (line.startsWith("GET /on ")){
              digitalWrite(LED_BUILTIN, HIGH);
            }
            if (line.startsWith("GET /off ")) {
              digitalWrite(LED_BUILTIN, LOW);
            }

            line = "";
          }
        }
      }
    }

    client.stop();
  }
}

Written on Aug 25, 2021

I wrote 17 books to help you become a better developer, download them all at $0 cost by joining my newsletter

C Handbook
Command Line Handbook
CSS Handbook
Express Handbook
Git Cheat Sheet
Go Handbook
HTML Handbook
JS Handbook
Laravel Handbook
Next.js Handbook
Node.js Handbook
PHP Handbook
Python Handbook
React Handbook
SQL Handbook
Svelte Handbook
Swift Handbook

JOIN MY CODING BOOTCAMP, an amazing cohort course that will be a huge step up in your coding career - covering React, Next.js - next edition February 2025