Simple output drive NPN, PNP to sink, source

A good explanation for sink and source drive circuits is indicated below:

Output interfacing to microcontrollers: sourcing, sinking and higher voltages than the computer

Nigel's PIC Tutorial Hardware Extras



low and high-current current sink driver circuit

http://i180.photobucket.com/albums/x26/Bucket_PhotoZ/ConstantCurrentLEDdrivers.gif