Lab: ESP32 Wi-Fi sensor with MicroPython

Aim and what you need

The aim is a working Wi-Fi sensor node: an ESP32 microcontroller running MicroPython that reads temperature and humidity and joins your network. Later labs send these readings to an MQTT broker.

• An ESP32 development board with a USB data cable (some cables carry power only).
• A DHT22 sensor module, jumper wires and a breadboard.
• Python 3 on your computer, plus the esptool and mpremote tools.
• The current generic ESP32 MicroPython firmware file from the official MicroPython download page.
• A Wi-Fi network for IoT devices, ideally separate from your main one.

Steps

1. Wire the sensor as in Figure 2 with the board unplugged.
2. Install the tools and find the serial port (/dev/ttyUSB0 or /dev/ttyACM0 on Linux, COM3 or similar on Windows).
3. Erase the flash and write the firmware. Newer esptool releases use hyphenated commands; older ones use esptool.py and underscores.
4. Create secrets.py and main.py as below and copy them to the board.
5. Reset the board and watch the output in the REPL (the interactive prompt).

python3 -m pip install esptool mpremote
esptool --chip esp32 --port /dev/ttyUSB0 erase-flash
esptool --chip esp32 --port /dev/ttyUSB0 --baud 460800 write-flash -z 0x1000 ESP32_GENERIC-<version>.bin

mpremote connect /dev/ttyUSB0 fs cp secrets.py :secrets.py
mpremote connect /dev/ttyUSB0 fs cp main.py :main.py
mpremote connect /dev/ttyUSB0 reset
mpremote connect /dev/ttyUSB0 repl

# secrets.py  (add this file to .gitignore)
WIFI_SSID = "<your-iot-ssid>"
WIFI_PASS = "<your-wifi-password>"

# main.py
import time, network, dht
from machine import Pin
import secrets

wlan = network.WLAN(network.STA_IF)
wlan.active(True)
wlan.connect(secrets.WIFI_SSID, secrets.WIFI_PASS)
for _ in range(20):
    if wlan.isconnected():
        break
    time.sleep(1)
print("Wi-Fi:", wlan.isconnected(), wlan.ifconfig())

sensor = dht.DHT22(Pin(4))
while True:
    try:
        sensor.measure()
        print("temp_c", sensor.temperature(), "humidity", sensor.humidity())
    except OSError as e:
        print("sensor error", e)
    time.sleep(5)

How to verify and troubleshoot

The REPL should print Wi-Fi: True with an IP address from your IoT network, then a temperature and humidity line every five seconds. Breathe on the sensor and the humidity should rise. Your router's client list should also show the ESP32.

• Port not found: try another cable, install the USB-to-serial driver (often CP210x or CH340) and check permissions (on Linux, the dialout group).
• Flashing fails to connect: hold the BOOT button while esptool starts, or lower the baud rate.
• Wi-Fi never connects: the ESP32 supports 2.4 GHz only; check the SSID and that the network is not 5 GHz only or WPA3-only.
• Sensor error or odd values: check the data pin number and the pull-up resistor; DHT22 needs about two seconds between reads.

Clean-up and next steps

Unplug the board before changing wiring. If you give the board away or reuse it, run erase-flash so your Wi-Fi password does not leave with it: the ESP32's flash is not encrypted by default, so anyone with the board can read it. Next, run a broker in the Mosquitto lab and publish your readings using MicroPython's umqtt.simple module, then secure them in the MQTT TLS lab.