Arduino Uno - relay with separate power
This relay is designed to operate on 5V.  

Most commonly this will come from the Uno's 5v pin. 

If you want to externally power the relay itself then make sure to connect the +5 to the Vcc on the relay board and the - lead to the GND on the relay board.

Uno's GND must also be connected to the GND on the relay so the Digital outputs from the Uno will properly control the IN1 and IN2 inputs to the relay board.



The Uno has multiple GND connections so it makes the most sense to connect the - to one of them then another to the relay.  



Note that the Uno's 9V Vcc external power comes in through the barrel connector on the Uno itself. The Uno will pass power through its internal regulator to provide 5v at the appropriate connection.



If you are switching external devices that run on 120V it would be on the screw-terminal side of the board only.




  To prevent accidental activation of the relays, note that this particular board is ACTIVE-LOW which means that as you initialize the output pins you should first set them HIGH so setting the pinMode will not default to LOW which would trigger the relay.


In the setup() section, using pins D5 and D6 as an example:

digitalWrite(5,HIGH); pinMode(5,OUTPUT);
digitalWrite(6,HIGH); pinmode(6,OUTPUT);