Charging the 3G iPhone - My Personal Workaround

Hi everyone, here is my own solution for charging the iPhone 3G in the car. This may not be a solution that works for you, but may help you in finding one. I'll summarize the entire project, but if you just want to see the pictures, you can scroll down to the bottom of the post.

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I was one of the many people who were disappointed to find out that the iPhone 3G no longer supports charging via the firewire pins, which means that most car chargers, and many audio docks are no longer able to charge the new iPhone. As a result, my iPhone experiences the most battery drain in the car, while using GPS, 3G, taking calls, and listening to music. And like many iPod/iPhone owners, I have invested a lot of money in an iPod integration kit.

Because the iPhone still of course supports charging via USB, I thought of the idea to somehow take the 12v firewire and step it down to 5v using a voltage regulator, and then route it to the appropriate USB pins. All the other used pins would remain unaffected. Utilizing my high school electronics skills, this seemed like a feasible and inexpensive solution to my problem; I would be able to charge the iPhone while retaining all functionality.

Looking at my USA-Spec PA11 iPod interface, the iPod dock connector is a detachable cable which connects to an 8-pin mini din jack on the interface. I figured that the easiest way to incorporate the regulator was to replace this existing cable with a custom-made one. I would house the PCB and all components in a separate enclosure, drilling two holes on either end. One side would connect to the mini din jack, the other side would connect to the iPhone.

Studying the existing iPod cable, I found that it used 3 surface mount resistors, one surface mount fuse, and the following pins:

2 - AV GND
3 - Line Out (R)
4 - Line Out (L)
12 - Serial Send (330 ohm resistor in series)
13 - Serial Receive
18 - 3.3v (330 ohm resistor)
19/20 - Firewire 12v
21 - Accessory Indicator (connected to Pin 30 via 510k ohm resistor)
29/30 - Firewire GND

With this information, I now know that Pin 19 (Firewire) is where the 12 volts is coming from. I need to step it down to 5 volts using an LM7805, and connect it to Pin 23 (USB). I will use the exact same resistors found on the iPod cable. I won't be needing the fuse, because the LM7805 will provide all the voltage and temperature protection that I will need.

Charging the iPhone isn't simply a matter of applying 5 volts to the USB Pins. In order to initialize charging, the iPhone must "see" a certain voltage on the USB data pins (specifically ~2.8v on Pin 25 and ~2v on Pin 27). Because of this I had to incorporate a voltage divider network to provide the appropriate voltages.

So, the pinout above will remain virtually the same, except Pin 19 will be replaced with Pins 23 25 27, and Pin 29 will be replaced with Pin 15.

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So here is a link to the photos of my 3G charger:

http://s35.photobucket.com/albums/d160/francislamiel/3G%20Charger/?albumview=sli deshow

It works flawlessly, and all the parts/materials cost me under $30. When I bought the reisistors, I bought them in packs of 100 when I only needed 1 of each for the project (for some reason it actually costs more to buy a 3-4 pack).

If anyone is willing to try to make something like this, feel free to ask me any questions. Hopefully this helped a few people, thanks for checking it out!

Resources:
http://pinouts.ru/Devices/ipod_pinout.shtml
http://www.tzywen.com/modules.php?name=News&file=article&sid=696&mode=thread&ord er=0&thold=0