practical electronics
DC Isolated Bluetooth Audio Adapter
A car audio upgrade that solved an audible ground-loop problem with an isolated relay and battery system.
Much-needed Bluetooth upgrades
This project came about when I decided to install a Bluetooth module into my car. I liked my existing head unit and only wanted wireless audio, so replacing the whole unit felt unnecessary.
After installing the Bluetooth adapter and powering it from the car's 12V supply, there was an audible hum. A voltmeter showed stray voltage between audio ground and the 12V supply ground: a classic ground loop. Audio isolators reduced the issue but noticeably muted the sound.
Unsatisfied with both approaches, I built a relay system that disconnected the Bluetooth adapter from the car's 12V power when the car turned on and switched it over to its own battery supply. That isolation let the audio signal pass without the ground loop or low-quality isolators.
When the keys were off, the relay switched back to the car's 12V power so a charge controller could recharge the lithium battery pack. It also gave the module a clean on/off behavior tied to the car.
Results
The upgrade gave me years of reliable wireless audio. If I rebuilt it now, I would likely design a dedicated circuit board and use an isolated DC supply instead of a dedicated battery system.