This is a paid asset, but now you can download Android & Microcontrollers / Bluetooth Free.
Detail this asset from Unity Store: Original Link
Android & Microcontrollers / Bluetooth v3.9 (Latest version)
This plugin allows you to add Bluetooth connection between your Android game and Bluetooth enabled devices. For example, Microcontrollers (e.g. Arduino, PIC and Atmel) connected to serial Bluetooth modules (e.g. HC-05 and HC-06). Moreover, it supports Android to Android connection and multiple connections at the same time.
Features
– One click setup.
– Comes with a tutorial ( build your Bluetooth Joystick ).
– Comes with a Bluetooth terminal.
– Android to Android connection.
– Multiple devices/modules at the same time.
– Works with any other plugin.
– Doesn’t need Unity pro.
– Comes with full source code and documentation.
– Playmaker beta support.
Don’t know what a serial Bluetooth module is : check out this tutorial
– The Asset supports connecting to multiple Android devices or modules at the same time, so you can build a multiplayer game.
– The Asset doesn’t support BLE, but it works with any other BLE plugin in case you want to expand later .