I went through all of this with my Captivate a year or 2 ago, and figured it might be nice for Galaxy S 3 owners as well. You can see the history at http://forum.xda-developers.com/show...php?p=12169590
Samsung phones will trip their dock mode when they sense a resistor of a particular value between the ID (pin 4) & Ground (pin 5) pins on the micro USB connector
- Car mode - 619k ohm (Validate on my Galaxy S 1 & Galaxy S 3)
- Desk mode - 365k ohm (confirmed finally thanks to mikejr83 cracking open the SGS3 dock his wife got him. 1k ohm for SGS1 & 1k or 365k for the SGS2)
- The only appropriate solution I was able to find (a year or 2 ago messing with my old Captivate) was a USB MicroB Plug Breakout Board which was pretty decent at $4 + shipping.
- mikejr83 found some slightly different micro USB connectors at Digi-Key ($0.66 each + reasonable shipping, $2 is the minimum Digi-Key order) and on E-bay (10 for $4 + $1 shipping) as well.
- The Zenis SGS2 dock fits the SGS3 reasonably well & runs about $10 on Ebay (free shipping, but it takes about 2 weeks to come over from asia), It does not trip dock mode & just has micro USB passthrough connectors. According to this XDA thread it's easy to pull appart & looks to be very hackable.
- Chopping up a micro USB connector will not work, they only have 4 wires & the pin #4 you need access to is buried inside the connector at the end so far you have to destroy the whole connector to get to it
- I have a roll of maybe a couple hundred 619k resistors I picked up from a local electronics recycler in my Captivate days. I might start packaging em up and selling them off after I get back from DEFcon if anybody is interested & having a hard time finding them.
For just dock mode & no other usefulness (like actually charging), you can see what doing a poor job of jamming the resistor in there on the Spark Fun breakout board for testing gets us (photo of my old Galaxy S1 because I'm to lazy to get dig it up and transfer the photos if I take any of my Galaxy S 3 showing the same thing).
In order to get "AC charging" which goes up to 750ma instead of "USB charging" which only goes up to 500ma, you merely need to short out the 2 data pins (pin 2/white & pin 3/green) on a normal USB cable. Most people find it easy to short them (the center 2) on the big flat rectangular end (Type A) of a normal USB cable. Given my approach with the breakout board I just soldered a jumper across the 2 data pins on the board.
For my first (and only properly documented) dock build I used Friendly Plastic. Technically called Polycaprolactone it's really easy to work with and melts at around 60 degrees Celsius (140 F), also known as almost the temperature of the dash of my grey truck on a 70-80 degree day.... Oops. It would make a good desk dock, but not a car dock.
Commercial docks & potential mods for them
Given the dirt cheap Infuse dock I picked up I probably won't be working on another car dock unless I decide to put a case on my phone making it not fit in the dock. I have some oven bake modeling clay I'll probably try for that.
At $10-15 it's hard to beat the Zenis dock. On the 2nd page somebody cracked it open and the micro-USB plug looks very similar to the SparkFun one. I don't believe it trips any dock mode, but soldering in the appropriate resistor would be pretty easy. I hope to find the value to trip Desk or Media center dock mode & use Tasker to turn off E-mail and other non-essential noise during the night so as to not roust the wife.
Further information needed
If anybody has any more resistor values for various dock modes or other information to add, let me know.