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Comment in question
Posted by Elec, on Feb-2009,
1. Alternate drive
By putting the resistor in serie with the transistor base, there is no consumption when the relay is deactivated. It also prevents a short circuit case if the microcontroller pin is set to output high. 
It can also prevents relay activation while the processor is being reset. 
For small relays, direct microcontroller drive is possible using 2, 3 or 4 pins from the same port so that they can be set/reset together and share the load. The pins must not be programmed as open collector, as this would stress the micro's protection diodes. In this case, there is no need for external components.
 
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