This circuit will allow you to control a servo with any analog signal. As long as the input (green node) is between 0.25 and 0.5 volts(using the 5v battery as reference, for the virtual ground configuration between -2.25 and -2volts), the servo will move.
I haven't tried the circuit myself, but once I have some time to spare, I'll build it.
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INSTRUCTIONS
Shake your phone to make the circuit work.
