Project: a Wi-Fi temperature and humidity monitor

What you will build and what you need

You will build a small sensor node for a server cupboard, greenhouse or bedroom. It measures temperature and relative humidity and sends a JSON message to an MQTT broker once a minute. A dashboard or alert rule can then warn you when a comms room gets too hot.

Design

Wire the DHT22 pin 1 to 3.3 V, pin 2 (data) to GPIO 4 with the pull-up to 3.3 V, and pin 4 to ground. Mount the sensor away from the ESP32 itself, because the board's own heat will raise the reading by a degree or more. Readings go to the topic home/<room>/<device-id>/env, which keeps permissions simple: each device can publish only under its own ID.

Build steps

1. Flash MicroPython to the ESP32 with esptool and connect with a tool such as Thonny or mpremote.
2. Install the MQTT client library on the board: mpremote mip install umqtt.simple.
3. Create the broker user for this device and copy the broker's CA certificate (in DER format) to the board as ca.der.
4. Create secrets.py on the board with the Wi-Fi and MQTT credentials. Never commit it to Git.
5. Upload main.py below, reset the board and watch the topic with mosquitto_sub.
6. Check the readings against a trusted thermometer and note any offset.

import time, json, ssl, network, machine, dht
from umqtt.simple import MQTTClient
import secrets   # SSID, WIFI_PASS, MQTT_HOST, MQTT_USER, MQTT_PASS, DEVICE_ID

sensor = dht.DHT22(machine.Pin(4))
TOPIC = "home/lab/" + secrets.DEVICE_ID + "/env"

def wifi_up():
    wlan = network.WLAN(network.STA_IF)
    wlan.active(True)
    if not wlan.isconnected():
        wlan.connect(secrets.SSID, secrets.WIFI_PASS)
        for _ in range(20):
            if wlan.isconnected():
                break
            time.sleep(1)
    return wlan.isconnected()

def mqtt_client():
    ctx = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
    ctx.verify_mode = ssl.CERT_REQUIRED
    ctx.load_verify_locations(cafile="ca.der")
    c = MQTTClient(secrets.DEVICE_ID, secrets.MQTT_HOST, port=8883,
                   user=secrets.MQTT_USER, password=secrets.MQTT_PASS,
                   keepalive=120, ssl=ctx)
    c.connect()
    return c

while True:
    try:
        if wifi_up():
            sensor.measure()
            msg = {"t": sensor.temperature(), "h": sensor.humidity()}
            c = mqtt_client()
            c.publish(TOPIC, json.dumps(msg), qos=1)
            c.disconnect()
    except Exception as e:
        print("error:", e)
    time.sleep(60)

The program reconnects each cycle, which is simple and robust. For battery power, replace time.sleep with machine.deepsleep(60000) so the board sleeps between readings.

Security checklist and next steps

• Unique credentials per device, stored in secrets.py, not in the main code or in Git.
• TLS with certificate verification to the broker; no plain port 1883 outside the lab.
• Broker ACL allowing this device to publish only to its own topic.
• Separate network: put sensors on an IoT VLAN or SSID that cannot reach your laptops.
• Disable unused services: if you enabled WebREPL during development, turn it off.
• Plan updates: keep a note of the firmware version and update MicroPython periodically.

Common mistakes are reading the DHT22 more often than every two seconds, which returns errors, and placing the sensor in direct sunlight. Next, feed the data into the MQTT dashboard project and add an alert when the temperature passes a limit you choose.