Wemos Attic Environmental Monitor

Firmware for a Wemos D1 mini (ESP8266) with an Adafruit SHT40 and up to three DS18B20 temperature probes. Its MQTT and Home Assistant conventions follow the feather-esp32s3-solar-wx-station project: retained status and device inventory, non-retained JSON state, and retained Home Assistant discovery.

Initial release: v0.1.0

Wiring

Adafruit SHT40 over STEMMA QT / Qwiic (I2C)

SHT40	Wemos D1 mini	ESP8266 GPIO
VIN	3V3	—
GND	G	—
SDA	D2	GPIO4
SCL	D1	GPIO5

STEMMA QT and Qwiic both carry I2C, but verify the cable's wire order if mixing connector brands. Power this sensor from 3.3 V.

Three DS18B20 probes (one shared 1-Wire bus)

DS18B20 wire	Wemos D1 mini
VDD, normally red	3V3
GND, normally black	G
DATA, normally yellow/white	D5 / GPIO14

Connect all three probes in parallel. Add one 4.7 kΩ pull-up resistor between DATA and 3V3 near the Wemos. Use normal three-wire powered mode rather than parasite power. Probe cable colors are not standardized, so confirm them from the probe vendor.

The Wemos D1 mini uses 3.3 V logic. Do not put 5 V on D1, D2, or D5.

Configure and upload

1. Install PlatformIO in VS Code or use PlatformIO Core.
2. Copy include/secrets.example.h to include/secrets.h.
3. Enter the Wi-Fi and MQTT settings in include/secrets.h.
4. Optionally change MQTT_BASE_TOPIC, reporting interval, or probe names in include/config.h.
5. Connect the Wemos by USB and run pio run --target upload.
6. View startup and readings with pio device monitor.

Home Assistant MQTT discovery must be enabled. The device uses these topics:

Purpose	Topic	Retained
State	sensors/attic01/environment/state	No
Availability/LWT	sensors/attic01/environment/status	Yes
Device inventory	sensors/attic01/environment/device_info	Yes
Device inventory status	sensors/attic01/environment/device/availability	Yes
Discovery	homeassistant/sensor/attic01_environment_*/config	Yes

Identifying the DS18B20 probes

On startup, the serial monitor prints each probe's unique 16-character ROM address. The firmware sorts these addresses so Air Handler Probe 1, 2, and 3 remain stable across restarts. Before permanently mounting them:

1. Connect all probes and note the three serial addresses.
2. Warm one probe by hand while watching MQTT or Home Assistant.
3. Label that physical probe with its entity number.
4. Repeat for the other two probes.

If a probe is replaced with a different ROM address, re-identify the physical ordering. To use location-specific names such as “Return Air” or “Supply Air,” edit DS18B20_NAMES in include/config.h before uploading.

MQTT state example

{
  "attic_temperature": 31.42,
  "attic_humidity": 58.17,
  "air_handler_1": 19.31,
  "air_handler_2": 14.88,
  "air_handler_3": 24.56,
  "rssi": -57,
  "uptime_seconds": 3600
}

Temperatures are reported in Celsius; Home Assistant can display them in the unit configured for the system.