The red board atop the Pi is the Pi Crust breakout board by Pumping Station: One member, Joe Walnes. It’s a compact and low-profile way to interface with the GPIO header. Alternatively, the Pi Cobbler breakout board is shown in the Adafruit tutorial. Note: the current Raspbian image doesn’t have the Adafruit’s PWM kernel module, so it is necessary to run Adafruit’s Occidentalis image for the servo tutorial to work.
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.
Adafruit’s Raspberry Pi Lesson 8. Using a Servo Motor: This lesson describes how to control a single servo motor using Python. Servo motors are controlled by pulses of varying lengths. This requires fairly accurate timing. The Raspberry Pi has one pin that generates pulses in hardware, without having to rely on the operating system. Occidentalis includes an interface to make use of this pin for controlling a servo motor. (read more)
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