user1111: I've found a lot of tutorials about how to use an Arduino to control power supplies to lamps and things like that using a relay. However, I also need to power the Arduino with the same power that is going through the relay. 
In other words, I'd like to be able to plug my device into the wall. The device would use the 110VAC to power the Arduino. My device would also have some outlets that I could plug something into. The Arduino would be connected to a relay and would decide when to enable electricity to the outlet. The important part being that I would like everything to be on one power source. 
Do I need to have a step down converter to transform the power, in addition to the relay? Is there some sort of break out board that has both of these functionalities in one board? What is the best way to do this?

user7856: In these cases where power must come from mains, my preferred solution is to buy a cheap phone charger+usb cable from ebay. They are sealed and safer than do-it-yourself solutions. The power adapter is barely larger than a normal plug for mains.
If you absolutely need to reach the arduino with only one cable, you can still use the solution i gave, only use a power cord to reach the contraption, then use a socket expander for mains and very short usb cable.

1111: <p>I've found a lot of tutorials about how to use an Arduino to control power supplies to lamps and things like that using a relay. However, I also need to power the Arduino with the same power that is going through the relay. </p>

<p>In other words, I'd like to be able to plug my device into the wall. The device would use the 110VAC to power the Arduino. My device would also have some outlets that I could plug something into. The Arduino would be connected to a relay and would decide when to enable electricity to the outlet. The important part being that I would like everything to be on one power source. </p>

<p>Do I need to have a step down converter to transform the power, in addition to the relay? Is there some sort of break out board that has both of these functionalities in one board? What is the best way to do this?</p>
 7856: <p>In these cases where power must come from mains, my preferred solution is to buy a cheap phone charger+usb cable from ebay. They are sealed and safer than do-it-yourself solutions. The power adapter is barely larger than a normal plug for mains.</p>

<p>If you absolutely need to reach the arduino with only one cable, you can still use the solution i gave, only use a power cord to reach the contraption, then use a socket expander for mains and very short usb cable.</p>
