Open Source Compact Wi-Fi Temp/Humidity Sensor (ESP32-C3 Pro Mini + HTU21D) with WebSerial Browser Installer and Friction-Fit Case

[u/LeanMCU]

Dear fellow tinkerers,

Just wanted to share a weekend project that, over a number of weeks, turned into a polished little device. I wanted a fleet of cheap, reliable temperature and humidity sensors around the house, so I designed this sensor using an ESP32-C3 Super Mini and an HTU21D module, written using the Arduino ESP32 core.

Quick Specs & Highlights:

• The Code: Built on the Arduino framework for ESP32, leveraging libraries for seamless I2C communication and Wi-Fi stability.
• Hardware: ESP32-C3 Super Mini + HTU21D. Total cost per unit from AliExpress is under $10.
• Assembly: Dead simple. Just 4 wires (VCC, GND, SDA, SCL) directly bridged between the boards.
• Case: Custom 3D-printed friction-fit enclosure (no screws or brass inserts required). I tested it with high speed PLA (Bambu Lab PLA basic) and Sunlu PETG
• Flashing: To save people from installing toolchains, I set up a GitHub web installer. You can flash the precompiled Arduino firmware directly from your browser over WebSerial. It includes a captive portal to configure Wi-Fi, Home Assistant, and ThingSpeak.
• Calibration: It allows you to calibrate temperature and humidity, and also set the level of data smoothing to get nice charts.
• BOM & Hardware Notes
  • Microcontroller: ESP32-C3 Pro Mini (~$4-$5 on AliExpress)
  • Sensor: HTU21D I2C Module (~$3-$4 on AliExpress)
  • Note on the sensor: When ordering from other AliExpress vendors, just double-check user reviews to ensure people are receiving true HTU21D chips rather than mislabeled clones!

The project is entirely open-source. You can find the source code and web flasher on my GitHub account, and the 5-minute full build video on my YouTube channel (links in my bio).

 

Any feedback regarding your experience building it, highly appreciated.

[u/enifuts]

I haven't seen $4 sensors at ikea You may get a tuya for $2 but every one i look at is limit 1 For me it would be great to use rechargeable and some type of sleep state with changeable check times to extend battery life