Controlling GPIO through web interface in Raspberry Pi - Step-by-Step Execution

Concept Flow - Controlling GPIO through web interface

User opens web page

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Web server receives request

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Parse user input (e.g., button press)

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Send command to GPIO pin

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GPIO pin state changes (ON/OFF)

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Web server sends updated page/status

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User sees updated GPIO state on web page

User interacts with a web page, which sends commands to the Raspberry Pi's GPIO pins via a web server, changing pin states and updating the page.

Execution Sample

Raspberry Pi

from flask import Flask, render_template, request
import RPi.GPIO as GPIO

app = Flask(__name__)
GPIO.setmode(GPIO.BCM)
GPIO.setup(18, GPIO.OUT)

@app.route('/', methods=['GET', 'POST'])
def index():
    if request.method == 'POST':
        if 'on' in request.form:
            GPIO.output(18, GPIO.HIGH)
        elif 'off' in request.form:
            GPIO.output(18, GPIO.LOW)
    state = GPIO.input(18)
    return render_template('index.html', state=state)

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=5000)

A simple Flask web server that controls GPIO pin 18 based on user button presses on a web page.

Execution Table

Step	Action	Input	GPIO Pin 18 State	Web Response
1	Server starts	None	LOW (default)	Web page loads with LED OFF
2	User presses ON button	POST with 'on'	HIGH	Page reloads showing LED ON
3	User presses OFF button	POST with 'off'	LOW	Page reloads showing LED OFF
4	User refreshes page	GET request	LOW	Page shows LED OFF
5	User presses ON button again	POST with 'on'	HIGH	Page reloads showing LED ON

💡 Server runs continuously; stops on manual shutdown.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5
GPIO Pin 18 State	LOW	HIGH	LOW	LOW	HIGH
Request Method	None	POST	POST	GET	POST
Request Form	None	{'on'}	{'off'}	None	{'on'}

Key Moments - 3 Insights

Why does the GPIO pin state change only after a POST request and not on GET?

Because the code changes GPIO output only when the request method is POST (see execution_table rows 2 and 3). GET requests just read and display the current state without changing it (row 4).

How does the web server know which button was pressed?

The server checks the form data in the POST request for 'on' or 'off' keys (execution_table rows 2 and 3). This tells it whether to set GPIO HIGH or LOW.

What happens if the user refreshes the page after turning the LED on?

A GET request is sent, which does not change GPIO state but shows the current state (HIGH if LED is on). This is shown in execution_table row 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the GPIO Pin 18 state after step 3?

A LOW

B HIGH

C Undefined

D Flashing

💡 Hint

Check the 'GPIO Pin 18 State' column in execution_table row 3.

At which step does the server receive a GET request?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at the 'Request Method' variable in variable_tracker after Step 4.

If the user presses the ON button twice in a row, how would the GPIO state change in the table?

A It stays LOW both times.

B It toggles between HIGH and LOW.

C It stays HIGH after the first press and remains HIGH after the second.

D It causes an error.

💡 Hint

GPIO output is set explicitly on POST; pressing ON twice keeps it HIGH (see execution_table steps 2 and 5).

Concept Snapshot

Controlling GPIO through web interface:
- Use a web server (e.g., Flask) to handle HTTP requests.
- On POST, read form data to decide GPIO pin state.
- Use RPi.GPIO to set pin HIGH or LOW.
- On GET, show current GPIO state.
- Update web page to reflect pin status.

Full Transcript

This example shows how a Raspberry Pi can control a GPIO pin using a web interface. The user opens a web page served by Flask. When the user presses ON or OFF buttons, the browser sends a POST request with form data. The server reads this data and sets GPIO pin 18 HIGH or LOW accordingly using the RPi.GPIO library. The server then reloads the page showing the current LED state. GET requests only display the current state without changing it. Variables like GPIO pin state and request method change step-by-step as the user interacts. This method allows easy remote control of hardware through a simple web page.