The problem is that each led uses 0.06A power, I have 120 led so I need an external power supply.
I would like to turn the led on and off when I turn on and off my PC.I will connect Arduino to the USB of my PC so Arduino should power on and off by itself with the PC but how can I power on and off the led circuit that is connected to the external power supply?
I would like to implement this circuit. Is this circuit good for my purpose or not? Should I add some sort of relays or Arduino is able to "cut off" the ground when its powered off?
That makes no sense whatsoever. What are you thinking here?
It's pretty simple.You connect the red and black wires from the drive power adapter to your LED string. You probably also want to run a pair of wires from that point to provide power to the other end of the (each) LED string.Then you want to run the power wires - 5 V and ground - back from the start of the LED strip together with the data wire to the Arduino. Note that there should be a 470 Ohm resistor in series with the data wire where it connects to the LED strip.
Where is the part where the led strip power on and off with Arduino?
Where is the MOSFET in this example?
Well, the LED strip will power off when you turn off the computer, same as the Arduino since they are both connected to the computer power supply.These LEDs retain the last command you gave them. If their power is turned off, they forget it and when you turn them on, they are dark until you give them a command (and if they are not, you need to put a 47k resistor on the Arduino pin that drives them, to ground).So the Arduino needs to give them their "marching orders" to start with, and if you want them to turn off before the power is turned off, the Arduino needs to tell them to do so.I presume it is sitting in the container on your bench, safely amongst all the other spare components. What on earth does a MOSFET have to do with all this?
LED strip will not be connected to my PC power supply but to an external one.
So how can Arduino says to the led strip to power off?