Many users, while enjoying the SpikerBox demo’s we have done, have also expressed excited curiosity that the iPhone can be used as a portable data recorder / oscilloscope. To truly take advantage of your iPhone though, you want your signal to go directly to the line input. Though you can buy one of these cables, in the open-source spirit of Backyard Brains, here is the schematic to build your own. You need: one 4.7 kOhm resistor, one 10 uF capacitor, one 3.5 mm audio three conductor cable you cut in half, and one 3.5 mm audio four conductor cable you cut in half. Bring out your soldering iron, your wire-stripper, and your favorite beverage!
Of course, its only AC coupled, probably +-2V max and 20KHz but still handy for audio techs!