Conecting a relay - General Electronics - Arduino Forum

I've noticed in several pictures that a relay is controlled by the arduino by connecting the 5v arduino pin to the relay and the ground is connected through a transistor, with is controlled by another pin on the arduino. In other pictures I se that a normal output pin from the arduino is connected to the relay, and no transistor is used. What is the difference between these two approaches. Is there an advantage in using the transistor, or are these just two ways of doing the same thing? I put tw

SOLVED: Relay does not work with external Power Supply - General Electronics - Arduino Forum

Connecting Relay and arduino with a load - General Electronics - Arduino Forum

How to hook up my 5V water pump - General Electronics - Arduino Forum

Connecting 8 Channel Relay Module To Arduino - General Electronics - Arduino Forum

220V safety control using relay module - General Electronics - Arduino Forum

How to use this 12V Relay with Optocoupler? - General Electronics - Arduino Forum

Problem with amp draw. Any Ideas. - General Electronics - Arduino Forum

How to connect C14 and UK AC mains to relay? - General Electronics - Arduino Forum

How best I connect floor lamp to relay ? - General Electronics - Arduino Forum