Thursday, July 17, 2008

AA to USB - It Works!

The pretty green light says so.

Ok, caveat time: it works in the sense that it takes the power from two AA batteries (or any number of other low voltage sources) and boosts it up to a nice regulated 5 V to power a USB port. However, one reason for doing this project was so that I could have a supplemental battery for my iPhone. By that measure, this device doesn't work, yet. I can connect the iPhone to this thing, but it will not draw power from the 5 V line.

To answer the question of why that's not working, I'd have to get a bit into USB enumeration and details of the USB specifications. For the curious, this guy has a decent explanation on his blog. Wikipedia also has some info on Power in the USB spec. The short answer is this: the iPhone won't blindly draw power from a USB port. Being a good little USB-compliant device with peculiar power needs, it transmits a request to the USB host to draw power, and waits until it gets an acknowledgement. This sort of thing requires a PC, or a least some sort of microcontroller, that is able to implement the USB communications. My simple device can't do that.

That's not to say that it can't still work as a USB charger for other devices. By my reckoning, it should be equivalent to the venerable Minty Boost, which is able to provide at least some power to nearly all of the iPod lineup.

In response to this situation, many have looked to see if there is some way to trick the iPhone into charging off a "dumb" USB port anyway. And, in fact, it appears that at least some have gotten it to work through clever combinations of resistors across the communication lines of the USB connector. I might give that a try.

1 comment:

Allison said...

Alex!! you're such a busy guy! congrats on all of your wonderful projects! We are oh so impressed!! (call us for a french meadow morning next time you're up!!)