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Bluetooth Hygrometer

About LYWSDCGQ

  • Xiaomi MiJia 'Bluetooth Hygrometer' LYWSDCGQ are hygrometers
  • Has sensors to relay temperature and humidity
  • Uses Bluetooth Low Energy (BLE) and has a limited range
  • An AAA battery is used as power source

Features

  • Read real-time sensor values
  • Read historical sensor values
  • Temperature and humidity sensors

Protocol

The device uses BLE GATT for communication.
Sensor values are immediately available for reading.

BLE & GATT

The basic technologies behind the sensors communication are Bluetooth Low Energy (BLE) and GATT. They allow the devices and the app to share data in a defined manner and define the way you can discover the devices and their services. In general you have to know about services and characteristics to talk to a BLE device.

Endianness

Data structure

Bluetooth payload data typically uses little-endian byte order.
This means that the data is represented with the least significant byte first.

To understand multi-byte integer representation, you can read the endianness Wikipedia page.

Services, characteristics and handles

The name advertised by the devices is MJ_HT_V1.

TODO

Historical data

TODO

Advertisement data

There seems to be two kind of advertisement data broadcasted.
LYWSDCGQ broadcast service data with 16 bits service UUID 0xFE95 and 0xFFFF.

UUID 0xFE95 16-20 bytes messages (with hygrometer data)

Check out the MiBeacon protocol page to get more information on advertisement data for this device.

UUID 0xFFFF 6 bytes messages
Position 00 01 02 03 04 05
Value e7 b9 f8 86 54 48
Bytes Type Value Description
00-05 bytes ?

Reference

[1] sputnikdev/eclipse-smarthome-bluetooth-binding#18

License

MIT