Controlling a servo with the phone's headset jack
Untested by me, but could potentially cut down on the number of components.
Controlling a hobby servomotor is relatively easy. It requires a single digital pulse-width modulation (PWM) signal at 50Hz, which means one pulse every 20ms. The width of the “high” pulse sets the absolute rotation angle of the servo.
Read more: Link - Smartphone Servo | Make: