iPhone external mic without headphones?

[InvisiBill]

There are some apps/devices out there that let you use your iPhone as an amp/effects pedal with your electric guitar. You get a cable with a 1/4" jack for your guitar on one end, and a 3.5mm plug and jack on the other end. It plugs into your iPhone to get the guitar input into the phone, and you plug your headphones in the 3.5mm jack to actually hear it after the app processes it. http://www.riotmode.com/cable.html is another example, with simply using a 3.5mm-to-RCAs AV cable and adapting the RCA plugs to fit your phone and amp (video at http://www.youtube.com/watch?v=T6s_31kbD6w).

These things do almost what I want, but not quite. I'd like to use the speaker of the iPhone as the actual output. It's no problem to get/make the guitar-to-iPhone cable as seen above, but I need it to not register with the iPhone that there are headphones plugged in, so the iPhone can play the sound over its builtin speaker.

Does anyone know what signals to the iPhone (specifically a 4, maybe a 4S or an old 3G) that headphones are plugged in? Is there something mechanical with the catch in the bottom of the jack? Is it just electrical conductivity across the headphone plug's conductors? Will it still detect and use the mic input if it doesn't detect the headphone connection? I haven't been able to find anyone else online doing this or started tearing apart old headphones yet. Hopefully I can figure it out, but any insight is appreciated.

 

[Dew]

I am pretty sure that it is all electrical. So, in theory, if you could insulate the top portion of the plug so that all that was conducting is the MIC, it would work. But, that assumes it wont shut off the speakers on a detection of a MIC input, which I am guessing it will.