Sorry, this one isn't strictly Arduino, but I wasn't part of any other forums that seem like a good place for basic circuit design advice. I apologize if this seems like a obvious question, but I am used to fixing circuit boards, not designing them.
I ordered a cheap $5, tiny(size not brand) bluetooth 4.1 audio transmitter that is powered off of 3.3-5v. The board also offers usb audio out. The chip is called "KCX_BT_EMITTER V1.1" I also have a old GBA SP adapter that has a pass through for the link cable EXT.1 , and the EXT.2 charging/sound port, as well as a 3.5mm audio out. The adapter is apparently a transmitter that was supposed to have a pair of RF earphones to go with it, but I never had these. The circuit looks quite busy, and perhaps does something more than just being an extension cord on the power side.
Instead of modifying the GBA SP itself, I want to install the bluetooth transmitter in the adapter without removing the old circuit, as the pass through seems to do something, also the 3.5mm oudio out is nice. I intend to use the 3.5mm out as a source for the BT module. I want to also put a small micro USB break out inside the case, which I think will just barely fit. I would like to use just one USB port to both access the USB sound card function of the BT chip, as well as be able to charge the GBA SP which charges on a 5V input.
On my adapter the two ports have no continuity between the ground pins of EXT.1 & EXT.2. All 4 shields have continuity. It has to be powered on, and paired to a device before connecting the USB. With the USB ground wire connected to the circuit ground, the USB would be charging through EXT.2, while transmitting data from the BT chip that is powered by EXT.1, which is the negative tied to ground. At that point there would be 3 voltage sources and circuits tied to a ground. If I were to plug in an accessory to the EXT.1 port passthrough, such as a link cable or light, then there are quit a few circuits tied to the same ground. I expect I shall need some blocking diodes to keep voltages from going through the wrong circuits, but I'm not sure how many. Bridging isolated grounds is probably not ideal as well. I thought the sheilds were grounds, but they are not tied to ground pins. Which type of ground am I looking at in the BT chips provided diagrams. Should I link the usb 5V to the 5V input of the BT chip? Seems like that would require blocking diodes, and might create ground loop hiss. What is your advice?
Pinnout diagrams are from Google. The triple wiring diagram is from the listing.
Also, out of curiosity, does anyone know if the EXT.2 charging port of the GBA SP can be used to draw 5v from the battery, or is trying to do so likely to burn the board by running a charge circuit in reverse, like those iPhone lightning connector fans and early iPhones.
Thank you for your time.
ps: rereading the listing instructions, the "USB audio out" is described being used as an USB audio in. If it is the latter, and just miss labeled, then it serves little purpose in this project and will probably be used for charging only, but the initial question is still quite interesting to me.