Making accessories that tie into an iOS device's Dock connector is an expensive proposition: it requires getting certain components from Apple and applying for a costly "Made for iPhone" (or iPod or iPad) license. However, it is possible to use the headphone jack for two-way data communication with an iPhone and also to power small electronic circuits. A group of students and faculty from the University of Michigan's Electrical Engineering and Computer Science Department have developed a small device it calls the "HiJack" to make sensing peripherals easily accessible to those on a tight budget.

Project HiJack is a hardware and software platform for enabling communication between a small, low-power peripheral and an iDevice. The system uses a 22kHz audio signal, which is converted into 7.4mW of power at 47 percent efficiency. That power runs a TI MSP430 microcontroller as well as any attached electronics, and allows the HiJack to communicate with an iOS application. The components to build a HiJack cost as little as $2.34 in significant quantities.

Other peripherals have used a similar technique to draw power and communicate with mobile devices. Mobile payment processing startup Square uses a small device to read the magnetic stripes on credit cards—it is powered by the headphone jack on an iPhone or other mobile device. There is also a low-power FM transmitter for some iPod models that is powered by the headphone jack.

The team behind Project HiJack envisions users building low-cost sensing and data acquisition systems for student and laboratory use. So far, it has built an EKG interface, soil moisture sensor, an integrated prototype with temperature/humidity sensors, PIR motion sensor, and potentiometer, and a version with a breadboard for prototyping new sensor applications.

Schematics for the HiJack board, as well as source code to enable communication via the audio port, are available on Google Code so that anyone with some soldering skills and the wherewithal can build a HiJack for his or her own use. Currently, software exists to work on iOS, but the hardware design should work with nearly any mobile device that has a combination headphone/microphone jack. The team plans to build APIs to enable the HiJack to work on Android and Windows Phone 7 in the future.

There is a way to get a HiJack for an iOS device without making one yourself, though: the team is putting 20 prebuilt and assembled HiJack boards up for grabs to those who submit a proposal for how they would use it. If your proposal is selected, you have to agree to two conditions: release any code for your project as open source, and let the team document your project on its website.