The growing prevalence of smartphones, even in developing regions of the world, means that using an audio jack for power could meaningfully impact individuals the world over by providing a small source of DC power.
Step 1: What You Need
Oscilloscope & lead wires
Device that plays audio and takes microphone over a 3.5mm audio jack (we are using a Samsung Galaxy Vibrant).
3.5mm TRRS Audio Cable (1)
Varied values of resistors, between 1Ω and 15 kΩ
1.5 mH inductor (1)
Schottky diodes (4)
You can use a breadboard. If you’ve never used one before, see http://www.instructables.com/id/Breadboard-How-To/.
You can use an oscilloscope. If you’ve never used one before, see http://www.instructables.com/id/Oscilloscope-How-To/.