Fellow hackerspace member, Jay Hopkins, had the great idea to wheel over the oscilloscope to demonstrate how the pulse width of the signal on the Pi's PWM pin corresponds to the degree to which the servo rotates:
(The spining red board on top of the servo motor is just a spare Pi Crust. It's only function was to make it easier to see the servo was moving.)
The position of the servo motor is set by the length of a pulse. The servo expects to receive a pulse roughly every 20 milliseconds. If that pulse is high for 1 millisecond, then the servo angle will be zero, if it is 1.5 milliseconds, then it will be at its centre position and if it is 2 milliseconds it will be at 180 degrees.
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