nRF52840 SHT41 BTHome Beacon

An ultra-low-power Bluetooth LE temperature and humidity beacon for the Pro Micro nRF52840 (nice!nano v2 compatible) with a Sensirion SHT41 sensor, built on Zephyr / the nRF Connect SDK.

The device broadcasts BTHome v2 advertisements, so it is discovered automatically by Home Assistant's BTHome integration (and any other BTHome-compatible receiver). It never accepts connections: it only advertises.

How it works

Every 60 s the firmware:

Switches on the SHT41's supply from a GPIO (P0.17) and waits 3 ms for it to start.
Takes one high-repeatability temperature / humidity measurement over I2C.
Puts the I2C pins into a disconnected "sleep" state and cuts sensor power.
Measures the supply voltage (VDDH) with the SAADC, right after the sensor load.
Updates the advertising payload.

Between measurements the CPU sits in System ON idle while the radio sends a non-connectable advertisement every 10 s (the payload therefore carries the same values for six consecutive adverts). If a sensor read fails, the last good values keep being advertised.

Power-saving measures, mostly configured in app.overlay and prj.conf:

Sensor supply gated through a power-domain-gpio node, driven by Zephyr's runtime device power management (pm_device_runtime_get/put).
I2C bus enabled only during transfers; SDA/SCL disconnected while idle so the breakout's pull-ups can't back-feed the unpowered sensor.
DC/DC converters enabled on both REG0 (VDDH → VDD) and REG1 (VDD → core).
0 dBm TX power, 10 s advertising interval.
UART, USB, console, logging and unused peripherals (I2C1, SPI2, PWM0) disabled.

Hardware

Signal	nRF52840 pin	SHT41 breakout
Sensor power	P0.17	VIN / VCC
I2C SDA	P0.20	SDA
I2C SCL	P0.22	SCL
Ground	GND	GND

The SHT41 is at I2C address 0x44. Battery voltage is read internally from VDDH (VDDH/5 input), so no external divider is needed — power the board from a cell connected to the battery / VDDH input.

The sensor supply pin uses high drive (NRF_GPIO_DRIVE_H0H1). Keep the breakout's total load small; if your board has large bulk capacitance or a power LED, remove it or increase startup-delay-us in app.overlay.

Project layout

.
├── CMakeLists.txt
├── prj.conf        # Kconfig: BLE broadcaster, drivers, PM, console off
├── app.overlay     # Devicetree: sensor power domain, I2C, ADC, regulators
├── README.md
└── src/
    └── main.c      # Measurement loop and BTHome payload

Building

Requires the nRF Connect SDK (or upstream Zephyr) with west.

west build -b nice_nano/nrf52840 -p

On older SDKs that use the pre-hardware-model-v2 board names, use -b nice_nano_v2 instead.

Flashing

The nice!nano ships with a UF2 bootloader:

Double-tap the reset button (or briefly short RST to GND twice). A USB drive named NICENANO appears.
Copy build/zephyr/zephyr.uf2 onto the drive. The board reboots into the new firmware.

Because USB and serial are disabled in the application, the board will not enumerate over USB while running. Use the double-tap reset each time you want to reflash. You can also flash over SWD with a J-Link or similar probe using west flash.

Advertising payload

Advertised name: SHT41 (set by CONFIG_BT_DEVICE_NAME). The device uses its identity address, so it appears with a stable MAC.

Service data (UUID 0xFCD2):

Bytes	Meaning
`D2 FC`	BTHome UUID (little-endian)
`40`	BTHome v2, unencrypted, regular interval
`0C` + 2	Voltage, uint16 LE, 1 mV
`2E` + 1	Humidity, uint8, 1 %
`45` + 2	Temperature, sint16 LE, 0.1 °C

Example: D2 FC 40 0C 1B 0C 2E 2D 45 E7 00 decodes as 3.099 V, 45 % RH, 23.1 °C. The full advertisement (flags + service data + name) is 23 bytes, inside the 31-byte legacy limit.

Configuration

What	Where
Measurement interval	`MEAS_INTERVAL` in `src/main.c`
Advertising interval	`ADV_INTERVAL` in `src/main.c` (0.625 ms units, legacy max 10.24 s)
Device name	`CONFIG_BT_DEVICE_NAME` in `prj.conf`
TX power	`CONFIG_BT_CTLR_TX_PWR_*` in `prj.conf`
Pins	`sens_pwr` and `pinctrl` nodes in `app.overlay`
Sensor power-up delay	`startup-delay-us` in `app.overlay`
ADC resolution / averaging	`&adc` channel in `app.overlay`

Home Assistant

With a Bluetooth adapter or ESPHome Bluetooth proxy in range, the BTHome integration discovers the device automatically and creates temperature, humidity and voltage sensors. Expect the first values within about 10 s of power-up, and new readings every 60 s.

Troubleshooting

Nothing shows up: check that the board is powered from the battery input and wait at least one advertising interval (10 s). The first advertisement is sent even if the initial sensor read fails, with zeros for temperature and humidity.
Temperature/humidity stuck at 0: the sensor isn't responding. Check the wiring, the I2C address and that P0.17 actually powers the breakout.
Voltage reads 0: the ADC read failed; check the &adc node and that CONFIG_ADC=y.
Can't flash: the application disables USB, so you must enter the bootloader with a double reset first.

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