The Arduino Uno is one of the most common and widely used Arduino
processor boards. There are a wide variety of shields (plug in boards
adding functionality). It is relatively inexpensive (about $25 - $35).
The latest version as of this writing (3/2014) is Revision 3 (r3):
· Revision 2 added a pull-down resistor to the 8U2 HWB line,
making it easier to put into DFU (Device Firmware Update)
mode
· Revision 3 added
o SDA and SCL pins are now brought out to the header
near the AREF pin (upper left on picture). SDA and SCL
are for the I2C interface
o IOREF pin (middle lower on picture that allows shields
to adapt to the voltage provided
o Another pin not connected reserved for future use
The board can be powered from the USB connector (usually up to
500ma for all electronics including shield), or from the2.1mm barrel
jack using a separate power supply when you cannot connect the
board to the PC’s USB port.