Freestyle Libre 1
To use your Libre as a CGM that is getting new BG values every 5 minutes without having to scan the sensor, you need to buy an NFC to Bluetooth bridge (commercially available devices, based on the obsolete LimiTTer project).
Libre 2, Libre 1 US and Libre Pro
Verify the bridge and the app you want to use are compatible with your sensor.
Several bridges are available on the market:
MiaoMiao Reader (version 1, 2 or 3) also available on AliExpress.
Blucon Nightrider (mind recent firmware versions might not be compatible with all apps, the vendor app does not broadcast data to AAPS)
Bubble (or Bubble Mini) from European vendors (Bubblan, BubbleShop) or for Russian users here. Also available on AliExpress.
Atom for Russian users.
Historically it is possible to use a specific watch, the Sony Smartwatch 3 (SWR50) which has an NFC chip that can be enabled and used as an NFC collector. However the custom NFC to Bluetooth bridges listed above offer a less complex solution and would be used by the majority of those wanting to use their Libre 1 (non-US) as a CGM.
Sony Smartwatch 3 (SWR50) https://github.com/pimpimmi/LibreAlarm/wiki/
As it currently stands, if using Libre 1 as BG source you cannot activate ‘Enable SMB always’ and ‘Enable SMB after carbs’ within the SMB algorithm. The BG values of Libre 1 are not smooth enough to use it safely. See Smoothing blood glucose data for more details.
1. Using xDrip+
xDrip+ supports Miaomiao, Bubble, Blucon, Atom and LibreAlarm.
You can safely download the latest APK (stable) unless you need recent features, in which case you should use the latest Nightly Snapshot.
Follow setup instructions on xDrip+ settings page.
You also need OOP2 for Libre 1 US (and Libre 2 EU).
Select xDrip+ in ConfigBuilder, BG Source.
2. Using Glimp
Glimp supports Miaomiao, Blucon and Bubble for Libre 1 and Libre 2 EU.
You will need Glimp version 4.15.57 or newer. Older versions are not supported.
Install Glimp.
Select Glimp in in ConfigBuilder, BG Source.
3. Using Tomato
Tomato is the vendor app for Miaomiao.
Install Tomato and follow the vendor instructions.
Select Tomato in ConfigBuilder, BG Source.
4. Using Diabox
Diabox is the vendor app for Bubble.
Install Diabox. In Settings, Integration, enable Share data with other apps.
Select xDrip+ in ConfigBuilder, BG Source.