Read values from an Arduino via HTTP

In this tutorial, we will expand on the Arduino Web Server tutorial to include reading the values measured by a sensor. This will allow us to view the data by simply opening a page in our browser.

For this example, we will measure the temperature using a DHT11 sensor and measure the distance from an object using a proximity sensor.

To control the built-in LED on the Arduino, we can send requests to the /on URL to turn it on and the /off URL to turn it off. Any other URL will not trigger any action.

Here is the code from the previous 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");
                        client.println();
                        client.println("<!DOCTYPE HTML>");
                        client.println("<html>");
                        client.println("test");
                        client.println("</html>");
                        break;
                    } else {
                        line = "";
                    }
                }
            }
        }

        client.stop();
    }
}

In the last else block, we have a complete line, which we can check for 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 you can load the code onto the Arduino and call the /on URL or the /off URL to control the LED.

I have assigned a static IP address to the Arduino using my local network router, and I have named it arduino.local in my /etc/hosts file. So, by accessing http://arduino.local/on, the LED turns on, and by accessing http://arduino.local/off, the LED turns 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");
                        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();
    }
}

Tags: Arduino, HTTP, sensors, DHT11, proximity sensor