The HC-05, a Bluetooth to serial bridge, can be found for around $5 on the internet and therefore may be the cheapest way to add Bluetooth connectivity to your project. However, its default setting… ...

The combination of a transparent OLED display, Arduino Nano Every microcontroller, HC-05 Bluetooth module, and a small LiPo battery results in a functional and versatile piece of wearable technology.

They figured uploading code to the microcontroller over Bluetooth would be a good idea, but their adapter, a cheap HC-06 module, had no way of resetting the microcontroller; it just provided Tx ...