During the last embedded systems meetup at my hackerspace in Chicago, Pumping Station: One, we did hands-on activity where we went through a couple of Adafruit's wonderful tutorials. First, we learned how to read a photoresistor without an ADC using Adafruit's Basic Resistor Sensor Reading on Raspberry Pi. Next, we hooked up typical 5V hobby servo motor to the Pi per Adafruit's Raspberry Pi Lesson 8. Using a Servo Motor (written by element14 member simon.monk!):
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 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 Adafruit servo tutorial has a great explanation of how the position of a servo motor is controlled:
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.
Here's a short clip showing the signal on the scope while the servo rotates:
Here's a longer clip where I explain the setup in further depth:
|Raspberry Pi model B||Raspberry Pi|
|Basic USB optical mouse||IONE / Pro Signal|
|120-240v to 5V power supply with micro USB connector||Raspberry Pi|
|Basic USB keyboard||Gear Head / A4 Tech|
|4GB Class 4 SD card preloaded with Debian 6 Linux||Samsung|
|GPIO breakout kit for Raspberry Pi||Adafruit|
|Basic breadboard with 830 connection points||Twin Industries|
|Jumper wires for use on breadboard||Bud Industries|
|Basic hobby servo motor, 5V||Adafruit|
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