Sample development Python scripts for reading data from the Sencor SWS 500 Outdoor Thermo/Humidity Meter Bluetooth LE device.

The goal of this project is to push the weather data extracted from the device to some advanced collector/integrator services (like ThingsBoard or Home Assistant).

There are three sample scripts implementing the BLE communication: one uses the bluepy interface, an other one uses the multi-platform bleak library, and the last one uses pygatt.

The sencor-pygatt.py implementation has built-in MQTT support (with client certificate authentication) for integrating it with Home Assistant easily.

There is also an archived and deprecated APK communicating with the device called Sencor Meter (com.teusoft.sencor_meter). It is available for download at Apkpure. It was useful for reversing the data format, but it was not working on "recent" (LineageOS 15.1 ~ Android 8.1) Android devices.

The sencor-bleak-enum.py script scans for the Sencor SWS 500 device (identifies it by Model Number NGE76), lists the available services and enumerate the service details (characteristics and descriptors).

The characteristic values enumerated from the Device Information service gives some description about the device:

The Temperature Measurement (notify) characteristic under the Health Thermometer service can be used to enumerate the Thermo/Humidity data.

The device stores data from the last 24 hours and the whole archive is served once a client registers for the notify characteristic.

The data format is 5-byte binary object (v[0:4]) for every observation:

The count shows when the data was recorded (count=1 (with type=0) means the last observation). The channel2 is the hardware switch on the device (it was used by the deprecated Android app for distinguish up to 3 different devices).

There is also a Battery Level (read, notify) characteristic under Battery Service to enumerate the current battery state (in a percent scale).

NOTE: The bluetooth interface on the device (due to battery saving) goes to sleep and wakes up only for short durations between the longer sleep periods. This is the reason why the scripts have to retry and sometimes gives error messages (what is normal).

Also, this makes almost impossible to poll the data reliable using a Raspberry device if the WiFi is enabled while the Bluetooth is active because of some interference. If the internal WiFi is enabled, a separate Bluetooth Dongle should be used (or separate WiFi device + internal Bluetooth).

In my setup I use the sencor-pygatt.py script for getting outdoor temperature on an Asus RT-AC68U DD-WRT device with a BLE USB Dongle (ID 0a12:0001), connecting to my Home Assistant instance running on a Raspberry Pi 3. (This multi-device setup is needed for covering Wi-Fi + BLE ranges.)

Customizing the DD-WRT by installing Entware is highly recommended.

Unfortunately, even with Entware, DD-WRT lacks Bluetooth modules and related device drivers. Bluetooth support should be compiled and added manually.

The DD-WRT Development wiki page contains information about compiling extra kernel modules for a running firmware.

Fetch the complete DD-WRT source tree:

svn co svn://svn.dd-wrt.com/DD-WRT

Also get the binary toolchain for cross-compiling.

Add the appropriate toolchain binary folder to the path (the arch is ARM Cortex A9 for the Asus RT-AC68U device, it may be something else for others):

PATH=$PATH:/home/builder/toolchain/toolchain-arm_cortex-a9_gcc-8.2.0_musl_eabi/bin

Dive into the appropriate Linux kernel source directory in the DD-WRT source tree (the kernel version is 4.4.x for the current DD-WRT firmware on the Asus RT-AC68U):

cd DD-WRT/src/linux/universal/linux-4.4

Start from a compatible kernel configuration:

cp .config_northstar_smp .config

And configure the Bluetooth modules for compile (using ARCH=arm env var for make is necessary)

ARCH=arm make menuconfig

If there is some issues about missing or improperly configured modules, feel free to remove it from the Kconfig (e.g. from drivers/net/wireless/Kconfig), only the Bluetooth modules are important now.

After configured, build should be the next step:

ARCH=arm make modules

If build succeeds, copy the bluetooth modules to the DD-WRT firmware (to jffs for persistance):

lib/crc16.ko
net/bluetooth/bluetooth.ko
drivers/base/regmap/regmap-core.ko
driver/bluetooth/btbcm.ko
driver/bluetooth/btintel.ko
driver/bluetooth/btrtl.ko
driver/bluetooth/btusb.ko

Using insmod, the modules can be loaded to the kernel on the DD-WRT device.

For making the module loading persistent across reboots, a startup script file (with executable permissions) could be created in /jffs/etc/config/02-bluetooth.startup:

#!/bin/sh
#

/sbin/insmod /opt/root/bluetooth-kmod/crc16.ko
/sbin/insmod /opt/root/bluetooth-kmod/regmap-core.ko
/sbin/insmod /opt/root/bluetooth-kmod/bluetooth.ko
/sbin/insmod /opt/root/bluetooth-kmod/btbcm.ko
/sbin/insmod /opt/root/bluetooth-kmod/btintel.ko
/sbin/insmod /opt/root/bluetooth-kmod/btrtl.ko
/sbin/insmod /opt/root/bluetooth-kmod/btusb.ko

/opt/bin/hciconfig hci0 up
/opt/bin/btmgmt le on

For /opt folder working (across reboots) I had to bind-mount it during startup (with /jffs/etc/config/01-mount.startup):

#!/bin/sh

/bin/mount -o bind /jffs/opt /opt

For running sencor-pygatt.py, dependencies should be installed:

opkg install bluez-libs
opkg install bluez-utils
opkg install bluez-utils-extra
opkg install python3
opkg install python3-pip
pip install pygatt
pip install pexpect
pip install paho-mqtt

Launching the script from a /jffs/etc/config/*.startup file is preferred. I use it with screen -d -m for making the output and logging available on a screen session.