Turn an Espressif ESP32-C3 Dev Board Into a Slick Environmental Monitor — Flashed From Your Browser

Pseudonymous maker "LeanMCU" has released open source firmware and a friction-fit enclosure designed to convert low-cost Espressif ESP32-C3 development boards and TE Connectivity HTU21D environmental sensor breakouts into compact, connected temperature and humidity monitors for Home Assistant.

"I wanted a fleet of cheap, reliable temperature and humidity sensors around the house, so I designed this sensor using an [Espressif] ESP32-C3[-based] Super Mini [development board] and a [TE Connectivity] HTU21D module, written using the Arduino ESP32 core. Total cost per unit from AliExpress is under $10. 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 hardware for the project is entirely off-the-shelf, comprising a cheap development board built around Espressif's low-cost ESP32-C3 wireless system-on-chip — offering one 32-bit core running at up to 160MHz, single-band Wi-Fi, and Bluetooth 5 Low Energy (BLE) connectivity. This is linked to a TE HTU21D, which provides temperature and humidity sensing fed into a custom firmware that can smooth the readings before transmitting them to Home Assistant for recording and display. Everything is housed in a friction-fit 3D-printed case, to keep things neat.

"To save people from installing toolchains," LeanMCU explains of what makes the project stand out from the rest, "I set up a GitHub web installer. You can flash the pre-compiled Arduino firmware directly from your browser over Web Serial. It includes a captive portal to configure Wi-Fi, Home Assistant, and ThingSpeak."

The source code and enclosure 3D-print files are available on GitHub under the reciprocal GNU General Public License 2; additional information is available in LeanMCU's Reddit post.