humiditemp

ESP32 + SHT40 temperature/humidity sensor that reports to Home Assistant over MQTT. Entities show up automatically through MQTT discovery.

Wiring

SHT40 breakout	ESP32
VIN / VCC	3V3
GND	GND
SDA	GPIO21
SCL	GPIO22

Keep the sensor a few cm away from the ESP32 (short cable), as the ESP32 warms up when WiFi is on.

Setup

1. Home Assistant needs an MQTT broker (the Mosquitto add-on) and the MQTT integration. Create an HA user for the sensors to log in with.

2. Non-secret settings (SSID, broker IP, MQTT user, pins, intervals) are in config.h. Passwords come from 1Password: the wifi_item / mqtt_item IDs in the justfile point at items whose password field is used. just writes them to secrets.h (gitignored) using the op CLI.

3. Build and flash (just + arduino-cli + op):

   just build                     # fetch secrets and compile
   just flash                     # compile, upload and open the serial monitor
   PORT=/dev/ttyUSB1 just flash   # other serial port
   just clean                     # remove build/ and secrets.h
   

   build/ contains the compiled firmware with the passwords embedded; it is gitignored.

The device appears in HA as "Humiditemp XXXXXX" (last six digits of the MAC address), with Temperature, Humidity and WiFi signal entities. Rename it in HA as you like. Every board gets a unique ID, so the same firmware can be flashed onto all of them.

Deep sleep (battery or mains)

Set DEEP_SLEEP 1 in config.h. This works the same on a power supply, where it simply saves energy and keeps the sensor cooler; leave BATTERY_ADC_PIN at -1 there. Each wake reads the sensor (before WiFi warms the board), connects, publishes one reading and sleeps for the rest of SLEEP_INTERVAL_S. A wake typically takes 0.5–1.5 s. The WiFi channel and access point are kept in RTC memory between wakes to skip scanning.

• HA marks the sensors unavailable if three wakes in a row are missed (expire_after).
• Discovery is re-sent after a cold boot and then once a day, so the sensors come back on their own even if the broker loses its retained messages.
• Battery voltage: wire the battery through a divider (e.g. 2× 100 kΩ) to an ADC1 pin (GPIO32–39) and set BATTERY_ADC_PIN. ADC2 pins don't work with WiFi on.

Hardware matters more than firmware for battery life:

• Typical dev boards (DevKitC etc.) draw 5–15 mA while "asleep" because of the USB-serial chip, the power LED and the regulator. Use a low-power board (e.g. FireBeetle ESP32, LOLIN D32, TinyPICO) or a bare module with a low-quiescent LDO (e.g. MCP1700, HT7333).
• Cut the power-LED jumper on the SHT40 breakout (Adafruit: jumper on the back).
• For debugging, flash with DEEP_SLEEP 0. In sleep mode, serial output only appears during the short wakes.

MQTT topics

• humiditemp/<id>/state: {"temperature":21.53,"humidity":45.20,"rssi":-61,"battery":3.98} (battery only when enabled)
• humiditemp/<id>/status: online / offline (last will, always-on mode only)
• homeassistant/sensor/<id>/<key>/config: discovery messages, retained