Apple's proprietary dock connector may keep would-be peripheral manufacturers at bay, but if you want to build your own iOS companion devices, there's another way. You might have noticed that the Square credit card reader uses Apple's 3.5mm headset jack to transmit power and data at once, and gadgeteers at the University of Michigan are busy open-sourcing the same technique for all the DIY contraptions you can dream of. Project HiJack has already figured out how to pull 7.4 milliwatts out of a 22kHz audio tone, and built a series of prototype boards (including working EKG, temperature, humidity and motion sensors) that transmit data to and from an iOS app at up to 8.82 kbaud -- using just $2.34 worth of electronic components. See the basic principles at work in the video above, and -- as soon as the team updates Google Code -- find out how to build your own at the links below.

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iPhone headset socket hijacked to power DIY peripherals (video)