Using USB Bluetooth receiver with speaker

Thread Starter

tjohnson

Joined Dec 23, 2014
611
The speaker I'm talking about is the one I posted in the completed projects collection:


I want to add Bluetooth support to it (usually after I think I've completed a project I decide I want to add something :rolleyes:), so I bought this cheap USB Bluetooth receiver. My plan was to remove the plastic casing and the USB plug and add it to my circuit, but I found out that this doesn't work. When the speaker and the Bluetooth receiver are plugged into the same power source, there seems to be a ground loop, because no sound comes out of the speaker.

The instructions on the plastic packaging for the Bluetooth receiver say:
  • For speakers without a USB port, connect the 5V USB plug to a charger, then connect the speaker's AUX port.
  • For speakers with a USB port, connect the device to the speaker's USB port, then connect the speaker's AUX port.
So basically I have two options for how to power this Bluetooth receiver. I can either connect it to a cellphone charger or USB power bank, or I could add a USB port to my speaker which would require a DC-to-DC converter to prevent the ground loop. I think I would also need to use a voltage regulator for this second option because it might be problematic to supply 9V to a 5V USB device, but I'm not sure how to design a circuit to do all this.

The first option is certainly simpler than the second one, but it's not as practical as I want. I don't like having to carry around a charger or power bank to use Bluetooth with my speaker. What do you think is the best way to power both the Bluetooth receiver and the speaker from a single power source? Is there some way I'm not thinking of?
 
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sailorjoe

Joined Jun 4, 2013
363
You are missing a voltage regulator to power the Bluetooth. Place the regulator between the battery and SW1A, pin 3, of your speaker circuit. Then which ever way the switch is set, you'll have 5 Volts at SW1A, pin 2. Wire that pin to the power input of the Bluetooth, and ground to ground. If this is done correctly, there shouldn't be a ground loop. Then wire the audio out from the Bluetooth to the audio in of your speaker.
Here's an easy regulator for you.
https://www.sparkfun.com/datasheets/Components/LM7805.pdf
 

Thread Starter

tjohnson

Joined Dec 23, 2014
611
You are missing a voltage regulator to power the Bluetooth. Place the regulator between the battery and SW1A, pin 3, of your speaker circuit. Then which ever way the switch is set, you'll have 5 Volts at SW1A, pin 2. Wire that pin to the power input of the Bluetooth, and ground to ground. If this is done correctly, there shouldn't be a ground loop. Then wire the audio out from the Bluetooth to the audio in of your speaker.
Here's an easy regulator for you.
https://www.sparkfun.com/datasheets/Components/LM7805.pdf
Thanks for the helpful answer. Would an LM7805 output 5V to the Bluetooth receiver with either a 9V or a 5V input?

If so, I'll want to order one. I was thinking of ordering from AliExpress, since I don't like having to pay shipping for such a cheap item and I don't have much access to electronics locally. Are they reputable enough that you wouldn't discourage buying simple ICs like this from them?
 

sailorjoe

Joined Jun 4, 2013
363
The regulator is only between the battery and the switch. When you switch to 5 V, you don't need the regulator. And the 5 V is powering your speaker and your Bluetooth.
You can order from AliExpress, but it usually takes a month to get the parts, and you will pay shipping, usually. I've used them a few times, and except for delivery time, I've always received what I ordered.
 

Thread Starter

tjohnson

Joined Dec 23, 2014
611
The regulator is only between the battery and the switch. When you switch to 5 V, you don't need the regulator. And the 5 V is powering your speaker and your Bluetooth.
You can order from AliExpress, but it usually takes a month to get the parts, and you will pay shipping, usually. I've used them a few times, and except for delivery time, I've always received what I ordered.
That makes sense. It seems like I'm going to need to add a DC blocking cap as well (I asked this question on Stack Exchange).
 

Thread Starter

tjohnson

Joined Dec 23, 2014
611
It makes sense to me that I should use a voltage regulator, but there's still an additional problem. It definitely seems like a grounding problem, because even when I use a 9V battery to power the speaker, there is still no audio if the USB cable is plugged into the same power source as the Bluetooth receiver.

I was told the following on StackExchange:
A ground loop causes noise or hum, not no audio. Something else may be wrong.
...
I notice there is no DC blocking cap in the input. If the Bluetooth module output have a DC level on it then that would screw up the amp when grounds were connected
and this diagnosis seems accurate to me, but I still have some questions about it. What uF value would you recommend using for this circuit? And would I need to use a resistor along with it like this? (I asked these questions there as well, but they seem to have gotten lost in the comments.)
 
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