FREE SIMULATOR EXAMPLE · ADVANCED

Pico W — WebSocket-controlled LED

This example hand-rolls a WebSocket server on the Pico W: the browser opens an upgrade request, then sends ON/OFF frames that toggle an LED on GP15 in real time. No library — the upgrade handshake, key validation and frame masking are all in the source.

Board	Raspberry Pi Pico W (RP2040 + WiFi)
Difficulty	advanced
Category	communication
Libraries	None — just the Arduino core
Runs in	Sensbench browser simulator — free, no account

The exact circuit you get when you open this example — already wired, ready to run.

The MicroPython script

# main.py
# WebSocket LED Control — MicroPython

import socket, network, time, ubinascii, uhashlib
from machine import Pin

led = Pin(15, Pin.OUT); led.value(0)

SSID = "SENSBENCH-GUEST"
PASSWORD = ""

def ws_accept(key):
    GUID = "258EAFA5-E914-47DA-95CA-C5AB0DC85B11"
    return ubinascii.b2a_base64(uhashlib.sha1((key + GUID).encode()).digest()).strip().decode()

def ws_decode(data):
    if len(data) < 6: return ""
    payload_len = data[1] & 127
    mask = data[2:6]
    payload = data[6:6 + payload_len]
    return bytes(payload[i] ^ mask[i % 4] for i in range(len(payload))).decode()

wlan = network.WLAN(network.STA_IF); wlan.active(True); wlan.connect(SSID, PASSWORD)
while not wlan.isconnected():
    time.sleep(1)
print("Connected!", wlan.ifconfig()[0])

server = socket.socket()
server.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
server.bind(("0.0.0.0", 80)); server.listen(1)

while True:
    conn, addr = server.accept()
    try:
        raw = conn.recv(1024).decode()
        if "Sec-WebSocket-Key" in raw:
            key = raw.split("Sec-WebSocket-Key: ")[1].split("\r\n")[0].strip()
            conn.send(("HTTP/1.1 101 Switching Protocols\r\nUpgrade: websocket\r\n"
                       "Connection: Upgrade\r\nSec-WebSocket-Accept: %s\r\n\r\n" % ws_accept(key)))
            while True:
                data = conn.recv(1024)
                if not data: break
                msg = ws_decode(data)
                if msg == "ON": led.value(1); reply = "LED IS ON"
                elif msg == "OFF": led.value(0); reply = "LED IS OFF"
                else: reply = "OK"
                conn.send(bytearray([0x81, len(reply)]) + reply.encode())
    finally:
        conn.close()

How it works

The script accepts a TCP connection and performs the WebSocket handshake by hand: it validates the browser’s Sec-WebSocket-Key against the magic GUID, replies with the computed Sec-WebSocket-Accept, and from then on parses incoming frames. The browser sends “ON” or “OFF” as text frames, and the LED on GP15 follows.

The frame parsing is the protocol’s core: text frames arrive with a small header plus a payload, and the server unmasked the client’s data (clients must mask, servers must not — a rule this hand-rolled code implements explicitly). It is the same exchange a WebSocket library hides, written out so the upgrade and masking rules are visible.

Before you build it for real

Get the parts

Related examples

FAQ

Why WebSocket instead of plain HTTP?

HTTP is request-response: each toggle needs a full page reload or a background fetch. WebSocket keeps one connection open and pushes frames in both directions instantly — the difference between a light switch that feels wired and one that feels like a web form. This example hand-rolls the upgrade to show the protocol underneath.

Why is the LED on GP15 instead of the onboard one?

The onboard Pico W LED goes through the CYW43 chip, which complicates the wiring story this example wants to keep simple. GP15 is a plain GPIO — one pin, one LED, no WiFi-chip indirection — so the WebSocket protocol is the only thing to learn here.