This tutorial shows how to build a web server that serves HTML and CSS files stored on the ESP8266 NodeMCU filesystem (SPIFFS) using Arduino IDE. Instead of having to write the HTML and CSS text into the Arduino sketch, we’ll create separate HTML and CSS files.

The web server we’ll build shows how to control the ESP8266 outputs and how to display sensor readings. As an example, we’ll control an LED and display sensor readings from a BME280 sensor.

You can use the concepts learned in this tutorial to control any output or display sensor readings from other sensors.

Project Overview

Before going straight to the project, it’s important to outline what our web server will do, so that it’s easier to understand.

ESP8266 Web Server using SPIFFS page HTML CSS Demonstration smartphone

The following figure shows a simplified diagram to demonstrate how everything works.

Before proceeding with this project, make sure you check all the following prerequisites.

1. Install ESP8266 Board in Arduino IDE

We’ll program the ESP8266 using Arduino IDE, so you must have the ESP8266 add-on installed. Follow the next tutorial to install it:

2. Filesystem Uploader Plugin

To upload files to the ESP8266 SPI Flash Filesystem (SPIFFS), we’ll use the Filesystem Uploader Plugin. Install the plugin in your Arduino IDE:

3. Installing Libraries

One of the easiest ways to build a web server using files from the filesystem is using the ESPAsyncWebServer library. 

Installing the ESPAsyncWebServer library

This library is not available to download through the Arduino IDE libraries manager. So, you need to follow the next steps to install the library:

Alternatively, you can go to Sketch > Include Library > .zip Library and select the previously downloaded library.

Installing the ESPAsyncTCP

The ESPAsyncWebServer library also needs the ESPAsyncTCP library to operate properly. Follow the next steps to install the ESPAsyncTCP library:

Alternatively, you can go to Sketch > Include Library > .zip Library and select the previously downloaded library.

Installing BME280 libraries

In this tutorial, we’ll display readings from a BME280 sensor (Guide with ESP8266). You need to install the following libraries:

You can install these libraries through the Arduino IDE Libraries Manager. Go to Sketch > Include Libraries > Manage Libraries. Then, search for the libraries’ name to install them.

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Schematic Diagram

Connect all the components by following the next schematic diagram.

BME280 ESP8266
Vin 3.3V

Organizing Your Files

To build the web server you need three different files. The Arduino sketch, the HTML file and the CSS file. The HTML and CSS files should be saved inside a folder called data inside the Arduino sketch folder, as shown below:

Creating the HTML File

Create an index.html file with the following content or download all project files here:

Because we’re using CSS and HTML in different files, we need to reference the CSS file on the HTML text.

The <link> tag tells the HTML file that you’re using an external style sheet to format how the page looks. The rel attribute specifies the nature of the external file, in this case that it is a stylesheet—the CSS file—that will be used to alter the appearance of the page.

The type attribute is set to “text/css” to indicate that you’re using a CSS file for the styles. The href attribute indicates the file location; since both the CSS and HTML files will be in the same folder, you just need to reference the filename: style.css.

In the following line, we write the first heading of our web page. In this case we have “ESP8266 Web Server”. You can change the heading to any text:

Then, add a paragraph with the text “GPIO state: ” followed by the GPIO state. Because the GPIO state changes accordingly to the state of the GPIO, we can add a placeholder that will then be replaced for whatever value we set on the Arduino sketch.

To add placeholder use % signs. To create a placeholder for the state, you can use %STATE%, for example.

You attribute a value to the STATE placeholder in the Arduino sketch.

Then, create an ON and an OFF buttons. When you click the on button, we redirect the web page to to root followed by /on url. When you click the off button you are redirected to the /off url.

Finally, create three paragraphs to display the temperature, humidity and pressure.

We use the %TEMPERATURE%, %HUMIDITY% and %PRESSURE% placeholders. These will then be replaced by the actual temperature readings in the Arduino sketch.

Automatic Updates

We also add a bit of JavaScript in our HTML file that is responsible for updating the temperature readings without the need to refresh the web page.

The following snipet of code is responsible for the temperature.

To update the temperature, we have a setInterval() function that runs every 10 seconds.

Basically, it makes a request in the /temperature URL to get the latest temperature reading.

When it receives that value, it updates the HTML element with temperature id.

In summary, this previous section is responsible for updating the temperature asynchronously. The same process is repeated for the humidity and pressure readings.

Creating the CSS File

Create the style.css file with the following content or download all project files here:

This is just a basic CSS file to set the font size, style and color of the buttons and align the page. We won’t explain how CSS works. A good place to learn about CSS is the W3Schools website.

ESP8266 Asynchronous Web Server Sketch

Copy the following code to the Arduino IDE or download all project files here. Then, you need to type your network credentials (SSID and password) to connect the ESP8266 to your local network.

Continue reading to learn how the code works, or skip to the next section.

First, include the necessary libraries:

You need to type your network credentials in the following variables:

Create an instance that refers to the BME280 sensor called bme:

Next, create a variable that refers to GPIO 2 called ledPin, and a String variable to hold the led state: ledState.

Create an AsynWebServer object called server that is listening on port 80.

Get Sensor Readings

We create three functions to return the sensor readings as strings: the getTemperature(), getHumidity() and getPressure() functions.

Here’s how the getTemperature() function looks like (the other functions are similar).

If you want to display temperature in Fahrenheit degrees, you just need to uncomment the corresponding line in the getTemperature() function:

To learn more about interfacing the BME280 sensor with the ESP8266, you can read the following tutorial:


The processor() function attributes a value to the placeholders we’ve created on the HTML file. It accepts as argument the placeholder and should return a String that will replace the placeholder. The processor() function should have the following structure:

This function first checks if the placeholder is the STATE we’ve created on the HTML file.

If it is, then, accordingly to the LED state, we set the ledState variable to either ON or OFF.

Finally, we return the ledState variable. This replaces the STATE placeholder with the ledState string value.

If it finds the %TEMPERATURE% placeholder, we return the temperature by calling the getTemperature() function created previously.

The same happens for the %HUMIDITY% and %PRESSURE% placeholders by calling the corresponding functions:


In the setup(), start by initializing the Serial Monitor and setting the GPIO as an output.

Initialize the BME280 sensor:

Initialize SPIFFS:

Wi-Fi connection

Connect to Wi-Fi and print the ESP8266 address:

Async Web Server

The ESPAsyncWebServer library allows us to configure the routes where the server will be listening for incoming HTTP requests and execute functions when a request is received on that route. For that, use the on method on the server object as follows:

When the server receives a request on the root “/” URL, it will send the index.htmlfile to the client. The last argument of the send() function is the processor, so that we can replace the placeholder with the value we want – in this case the ledState.

Because we’ve referenced the CSS file on the HTML file, the client will make a request for the CSS file. When that happens, the CSS file is sent to the client:

You also need to define what happens on the /on and /off routes. When a request is made on those routes, the LED is either turned on or off, and the ESP32 serves the HTML file.

In the HTML file, we’ve written a JavaScript code that requests the temperature, humidity and pressure on the /temperature, /humidity, /pressure routes, respectively, every 10 seconds. So, we also need to handle what happens when we receive a request on those routes.

We simply need to send the updated sensor readings. The updated sensor readings are returned by the getTemperature(), getHumidity() and getPressure() functions we’ve created previously.

The readings are plain text, and should be sent as a char, so, we use the c_str() method.

In the end, we use the begin() method on the server object, so that the server starts listening for incoming clients.

Because this is an asynchronous web server, you can define all the requests in the setup(). Then, you can add other code to the loop() while the server is listening for incoming clients.

Uploading Code and Files

Save the Arduino sketch as ESP8266_SPIFFS_Web_Server or download all project files here.

Finally, upload the files to your board. Go to Tools > ESP8266 Data Sketch Upload and wait for the files to be uploaded.

Then, press the Arduino IDE upload button to upload the code to the ESP8266.

When everything is successfully uploaded, open the Serial Monitor at a baud rate of 115200. Press the ESP8266 on-board RST button, and it should print the ESP8266 IP address.

Open a browser and type your ESP8266 IP address. The following web page should load.

Press the ON and OFF buttons to control the ESP8266 on-board LED. You can also visualize the latest sensor readings. The sensor readings are updated automatically without the need to refresh the web page.

Wrapping Up

Using the ESP8266 SPI Flash File System (SPIFFS) is specially useful to store HTML and CSS files to serve to a client – instead of having to write all the code in the Arduino sketch.

This content was originally published here.