GoPro Hero 3 & 4 cameras use a special 10 pin mini USB that can enable extra functions. The usb connectors are sold by chargeconverter.com and they’ve conveniently documented the pinout. A resistor between pin 4 and ground tells the camera which accessory has been plugged in.
ID resistor values and corresponding features:
33kOhm – two mic channels in and composite video out
100kOhm – composite video out + audio out
330kOhm – two mic channels in
A 33k resistor allows GoPro Hero 3 cameras to be powered over usb while recording external audio and outputting video, but the Hero 4 switches to the internal microphone if you’re also injecting power over usb. The Hero 4 will only begin outputting video without external power, but once the video feed has started you can then connect power and it keeps working.
I designed this little board to break out the mic channels, video out and power. It is basically just a tiny version of GoPro’s $40 combo cable.
Schematic and layout are super simple:
The connector body has protrusions that are supposed to be through-hole reinforcements, but I made them just overhang the edge of the board to be secured with a bit of epoxy to leave the micro SD card as exposed as possible.
You can download the EagleCAD pcb files here on BitBucket.