Bring in some muscle to your output pins with 8 mighty Darlingtons! This driver chip contains 8 drivers that can sink 500mA from a selectable 5V or DC input voltage supply and has kickback diodes included inside for driving coils. This will let your little microcontroller or microcomputer power solenoids, DC motors (in one direction) and unipolar stepper motors.
The GPIO Pins are connected to ULN2803 inputs through a DIP switch, with the its outputs going to two array of Wire-to-Board header. Alternatively, these ports can be used to supply +5V or input voltage to other external circuitry, or embedded devices.
Optional driver to install – WiringPi
1. Install GIT
sudo apt-get install git-core
2. Download WiringPi
git clone git://git.drogon.net/wiringPi
3. Enter the directory of wiringPi
4. Install wiringPi
5. Test GPIO port
gpio mode x out (x = 0~7)
gpio write x 1 (x=0~7, 1=On)
gpio write x 0 (x=0~7, 0=off)
For more detail: ULN2803 8-channel RC Servo Port for Raspberry Pi