Help! How to build a bypass mechanism for a headphone jack

[CrashGawker]

I bought a bay device that gives me front audio USB and firewire ports, but unfortunately I wasn't aware that the companies making these aren't smart enough to make them compatible with motherboard headers. Instead the use pass through cables that are supposed to run out an empty PCI slot in your case and plug into the back of your computer. Normally that would irk me a little bit just long enough for me to figure out how to rewire the cables from their initial state to hook up to the motherboard headers...unfortunately only the USB and 1394 cables lent themselves easily to that process.

The audio cable on the other hand, since the thing was designed to plug into the back of the computer not a header, doesn't have the required audio return lines that allow the audio signal to return to the motherboard from the bay device when the headphone jack is not in use. That means if I hook the device up I would have to plug my speakers into the bay device instead of the back of the computer and play cable swap everytime I wanted to go with headphones (which pretty much negates the entire purpose of having front audio ports to begin with).

SO...on to the purpose of this thread. I need some way to modify this damn thing to recognize when there is a plug jacked into the audio port and if there IS NOT have it send the audio signal back to the appropriate header pins. The simplest way to accomplish the SECOND half of that goal would simply be to hard wire the audio lines to both the front port and the return pins...but then the speakers won't cut off when I plug my headphones in. Yeah it would work...but only so long as I always have a power switch or volume knob on my speakers and keep it within easy reach...

So what I need to know is how can I determine, electronically, that there is a plug present in the jack and cut the signal off from the return lines depending on that status? I'm not an electrical wizard, which is why I'm asking you guys, but I'm pretty good with a soldering iron so as long as someone can help me figure out what I need to wire up and how to wire it up I'll be able to put it together.

One small stipulation...the bay device is built to house a hard drive. I'm not CURRENTLY using that feature...but I'd prefer to keep it so any electrical add ons I have to put in would need to be small enough to fit into a very small amount of empty space in the device. If that's just simply not an option (IE going to require larger area to build new circuits) I'll be happy to trade the hard drive option for a working front audio port.

 

[gee]

Can you change out the audio jack easily?

Lots of audio jacks are "bypass switched"... without anything plugged into the jack, the "bypass" outputs are connected to the inputs, but plug in a jack and the bypass outputs aren't connected to anything.

 

[CrashGawker]

yes I think I could swap out the jack relatively easily...it's hard soldered into the PCB but I could cut the traces if I had to. Is there an easy way to tell if the exisiting plugs are bypass capable without that function being hooked up?

The PCB has a set of 6 contact holes for each of the jacks but only has connections in 5 of the six holes and I wondered if there was a chance that the extra hole would be the bypass line back so soldering a line to those holes would activate the currently inactive feature and run it back to the mobo. How would I check for that?

 

[gee]

If there's 6 pins on the jack, it's probably 2 grounds + L, R, L bypass and R bypass.

- does the jack have a clear cover? lots of the 1/8" ones do, if there's a bypass mech you can probably see it in action when you plug in a cable.
- if not, remove the jack and test with an ohmmeter. Look for two pairs of pins with continuity to each other, and see if it goes open circuit when you plug in a jack.
- Or if there's a part # written on the jack, you might be able to find a datasheet for the jack which will explain this sort of functionality.