nRF52840 SHT41 BTHome Beacon

An ultra-low-power Bluetooth LE temperature and humidity beacon for the Pro Micro nRF52840 (Zephyr board promicro_nrf52840, UF2 bootloader variant) with a Sensirion SHT41 sensor, built on the nRF Connect SDK v3.4.1.

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:

1. Switches on the SHT41's supply from a GPIO (P0.17) and waits 3 ms for it to start.
2. Takes one high-repeatability temperature / humidity measurement over I2C.
3. Puts the I2C pins into a disconnected "sleep" state and cuts sensor power.
4. If the sensor read succeeded, measures the supply voltage (VDDH) with the SAADC, right after the sensor load.
5. 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 battery is not measured and the advertisement is not updated, so the last good values (including voltage) 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

Installing the SDK

The project is built with the nRF Connect SDK (NCS) v3.4.1, installed via Nordic's nrfutil command-line tool and its sdk-manager plugin.

1. Get nrfutil

Download the nrfutil executable for your OS from Nordic's nRF Util page: https://www.nordicsemi.com/Products/Development-tools/nRF-Util

On Linux / macOS, make it executable and put it on your PATH, e.g.:

chmod +x nrfutil
mv nrfutil ~/.local/bin/        # or any directory on your PATH
nrfutil --version

On Windows, place nrfutil.exe in a folder that is on your PATH.

2. Install the sdk-manager command

nrfutil install sdk-manager

3. Install NCS v3.4.1 (SDK + matching toolchain)

nrfutil sdk-manager install v3.4.1

By default this places the SDK in ~/ncs/v3.4.1 and the toolchain alongside it. You can list what is available or installed with nrfutil sdk-manager search and nrfutil sdk-manager list.

Building

Open a shell with the NCS toolchain, load the Zephyr environment, and build from the project directory:

nrfutil sdk-manager toolchain launch --ncs-version v3.4.1 --shell
source ~/ncs/v3.4.1/zephyr/zephyr-env.sh
west build -b promicro_nrf52840/nrf52840/uf2

The /uf2 board variant links the application above the pre-installed UF2 bootloader and produces a zephyr.uf2 image. Add -p to force a pristine (clean) build, e.g. after changing app.overlay or prj.conf.

Flashing

The Pro Micro nRF52840 ships with a UF2 bootloader:

1. Double-tap the reset button (or briefly short RST to GND twice). A USB mass-storage drive appears (often named NICENANO; the name depends on the bootloader your board came with).
2. Copy the generated zephyr.uf2 onto the drive. With NCS's default sysbuild it is at build/<project-folder>/zephyr/zephyr.uf2 (or build/zephyr/zephyr.uf2 if you build with --no-sysbuild). 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.

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, humidity and voltage.
• All values stuck at 0: the sensor has never responded (the battery is only measured after a successful sensor read). Check the wiring, the I2C address and that P0.17 actually powers the breakout.
• Values frozen at an old reading: sensor reads are failing after an earlier success; same checks as above.
• Voltage reads 0 but temperature/humidity update: 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.