USB chargers are one of the most popular things to build here on Instructables. However countless people have been disappointed to find out that after all of their hard work, their iPods just sit there and do nothing, despite receiving 5V.
Recently my USB car charger died. I thought it'd be great if I could just install USB ports into the center console, and I totally did too. But the iPod wouldn't charge, so I scoured the internet looking for an answer. I found out what I needed to do was put resistors onto the data lines, but nobody agreed on what resistors or even how to hook them up. Finally I just ripped apart my broken USB charger to see how they did it.
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Step 1: Schematics
First off I should mention that this is supplemental Instructable to the countless others that show you how to build a USB charger. I am assuming you've already made something that can supply 5 volts DC. I assume no responsibility for how this jives with your 5V power supply or the USB devices you connect to it. All I really know for certain is it works perfectly with my 4G iPod, my Energizer USB-powered battery charger, and my cellphone.
OK, here we go.
The values of the resistors in the schematic below can be approximate. Those are just the values I measured from the resistors on my dead car charger. You could also hook up different valued resistors in series to dial in the exact ohms (22K + 22K = 44K, perfect for R2.) Also, 1/8W resistors will work just fine.
Once you build the circuit, test it. You should get around 2.7 volts out of D- and 2V out of D+. This may seem scary high but it's in the USB spec for signaling a USB 2.0 connection. This is how the iPod knows it's OK to draw the power it needs to charge.
Step 2: Conclusion
Well, that's pretty much it. It seems simple but I had to go to a lot of trouble to figure this all out.
Feel free to use any images or designs in your own Instructable or whatever. Just give me credit. Also feel free to criticize any aspect of my circuit designs. The configuration and values of the resistors came from a Griffin iPod USB charger that I bought from the Apple store, and I lifted a good deal of the power supply schematics from ranjeevm in this Instructable.
Step 3: Updates
June 18th, 2009: This project will work for the 2nd Generation iPod Touch or any version of the iPhone 3G. However those devices need a very precise 5 volts to sucessfully charge. You may find that simply buying an official charger or battery from ebay or something may save you a lot of frustration.