For example, my favorite hardware add-on for the iPad is the iRig audio interface adapter from IK Multimedia. The adapter — available for about $40 from Amazon — plugs into a 3.5 mm audio jack. Simply plug in your electric guitar in one port, your headphones in another, and use the selection of virtual amplifiers and effects pedals to change the tone and volume of your electric guitar signal. Instead of lugging around a practice amplifier and pedals, you can simply slip the finger-sized adapter and your iPhone/iPad in your pocket, and practice wherever you like.
Compared with the cost of a single effects pedal (typically $50-$150), the iRig is a bargain. There are competing products such as the Peavey AmpKit Link that are more affordable ($30, Amazon). Even so, $30 is a lot to pay for a simple audio interface.
While the software of Android-based systems is open by design, getting into the hardware of any phone or tablet is often problematic. For example, if you want full access to an iPhone’s power and digital input/output, there’s the proprietary adapter. My favorite source is SparkFun (www.sparkfun.com). They sell a basic connector for $5, and a more useable and less delicate breakout connector for $15. The basic connectors are a pain to deal with because of their low profile which means you’ll have to work with a fine tip soldering iron and even finer wire.
Although there are a few alternatives to working with the Apple software development tools, at present, it’s the environment with the most access to the internal hardware and control of anything you connect to the proprietary port. You’ll have to pay Apple a modest fee for the privilege of working with their software development kit ($99, developer.apple.com). If you’re working with the Android operating system, tools are available online for free (developer.android.com), but you’ll have to buy a license if you want to sell your application to the world.
Returning to the iRig audio interface adapter, notice that it plugs into the audio I/O port and not the proprietary Apple connector. The four-conductor, 3.5 mm audio jack is standard on most smart phones and tablets.
As such, if you develop, say, an oscilloscope interface that plugs into the audio jack on the Apple iPad, the same hardware should work on a phone running the Android operating system. By the way, a great source for four-conductor jacks and cable is to repurpose a 3.5 mm to three RCA AV camcorder video cables ($2 for a six foot cable, Amazon).
If interfacing hardware to your smart phone or tablet intrigues you, a great resource is Project HiJack (http://code.google.com/p/hijack-main) from the University of Michigan. HiJack is an open source hardware and software platform for creating peripherals for mobile phones.
Like the iRig, HiJack devices harvest power and use bandwidth from the mobile phone’s headset interface. Check out the link for example source code and schematics for a variety of projects ranging from an EKG monitor to a temperature and humidity sensor.
Even if you’re not interested in creating hardware for your smart phone or tablet, the project is worth exploring for the method used to harvest power from the audio interface. The designers manage to rectify an audio signal to create a 3V DC power supply capable of powering a microcontroller and sensors. Operating at nearly 50% efficiency, the energy harvesting circuitry is worth studying.
Good luck on your app development. NV