Relay Control not Working External Power Supply!

I'm building an arduino project to control my garage door.
On it I've an Esp8266, an arduino uno and a relay!
I want the system components to be powered by external supply but the relay only works when powered by arduino 5V.
As you can see in the attached image (dont work.png), I'm dropping the voltage from almost 8.5V to aproximately 4.8V, but that way the relay doesn't work.
If I connect it directly to arduino (work.png), it works perfectly, but I want to work with it conected to the external supply!
I'm am a noob in electronics so I would apreciate some help with it!
Thank you!

You need to use a DC-DC converter.. or a regulator like 7805 etc.

You can't use a potential divider to drop the supply voltage to the relay because when it tries to power the coil the voltage will drop right down and it will fail to come on.
If you don't want to power the relay from the Arduino 5V output which is fair enough if it draws a bit of current then you will need to get a dc-dc buck converter to drop the input voltage down to 5V.

So, if I use a step down like the one in the image, it will work fine?

Yes that one should work fine.