Raspberry Pi Pico W MQTT: Publish Sensor Data and Receive Commands

# main.py - Pico W MQTT node: publish temperature, subscribe to LED commands
import network
import time
import ubinascii
import machine
from machine import Pin, ADC
from umqtt.simple import MQTTClient
import secrets

BROKER = 'test.mosquitto.org'
PORT = 1883
CLIENT_ID = b'pico-' + ubinascii.hexlify(machine.unique_id())
BASE = b'plearn/pico1/'                 # change to something unique
T_TEMP = BASE + b'temp'
T_STATUS = BASE + b'status'
T_LED_SET = BASE + b'led/set'
T_LED_STATE = BASE + b'led/state'
PUBLISH_EVERY_MS = 10_000

led = Pin('LED', Pin.OUT)
temp_adc = ADC(4)

def chip_temp():
    total = sum(temp_adc.read_u16() for _ in range(32)) / 32
    v = total * 3.3 / 65535
    return 27 - (v - 0.706) / 0.001721

def wifi_connect():
    wlan = network.WLAN(network.STA_IF)
    wlan.active(True)
    if not wlan.isconnected():
        wlan.connect(secrets.SSID, secrets.PASSWORD)
        for _ in range(40):              # wait up to 20 s
            if wlan.isconnected():
                break
            time.sleep(0.5)
    if not wlan.isconnected():
        raise RuntimeError('Wi-Fi not connected')
    print('IP:', wlan.ifconfig()[0])

def on_message(topic, msg):
    # Called from check_msg() when a subscribed message arrives
    print('RX', topic, msg)
    if topic == T_LED_SET:
        cmd = msg.strip().lower()
        if cmd in (b'on', b'1'):
            led.on()
        elif cmd in (b'off', b'0'):
            led.off()
        elif cmd == b'toggle':
            led.toggle()
        client.publish(T_LED_STATE, b'on' if led.value() else b'off', retain=True)

def mqtt_connect():
    c = MQTTClient(CLIENT_ID, BROKER, port=PORT, keepalive=60)
    c.set_callback(on_message)
    # If the Pico disappears, the broker publishes 'offline' for us
    c.set_last_will(T_STATUS, b'offline', retain=True)
    c.connect()
    c.publish(T_STATUS, b'online', retain=True)
    c.subscribe(T_LED_SET)
    print('MQTT connected as', CLIENT_ID)
    return c

wifi_connect()
client = mqtt_connect()
last_pub = time.ticks_ms() - PUBLISH_EVERY_MS
failures = 0

while True:
    try:
        client.check_msg()               # non-blocking: handle any incoming message
        if time.ticks_diff(time.ticks_ms(), last_pub) >= PUBLISH_EVERY_MS:
            last_pub = time.ticks_ms()
            payload = '{:.1f}'.format(chip_temp())
            client.publish(T_TEMP, payload)
            print('TX', T_TEMP, payload)
        failures = 0
        time.sleep_ms(100)
    except OSError as e:
        # Network or broker problem: wait, then rebuild the connection
        failures += 1
        print('Connection lost:', e, 'attempt', failures)
        if failures > 5:
            machine.reset()              # last resort: clean restart
        time.sleep(5)
        try:
            wifi_connect()
            client = mqtt_connect()
        except OSError as e2:
            print('Reconnect failed:', e2)
        except RuntimeError as e3:
            print(e3)