From this page:
http://srimech.blogspot.co.uk/2012/08/controlling-servos-from-headphone.html?m=1
Make a 500Hz sound with varying pwm, and play it out of the audio jack (green). The two transistors amplify and invert the signal on the 6V DC power source (blue). This is the same pwm signal to drive three-pin digital servo motors from almost any R/C cars and planes.
