12v / Common Ground

sorry guys can somebody let me know if this is ok?

The pin on the arduino can not provide enough current to drive a relay.

You need a transistor connected to either 5 or 12v, depending on what kind of relay you are using.

You will also need a diode so the relay won't "kick" the arduino's pin every time it releases again.

This is however not common ground.

Am I correct in thinking it would be enough to drive a solid state relay / Opto Isolator such as: http://www.farnell.com/datasheets/40301.pdf

as I would like to keep the part count down to a minimum.

Also can I confirm that the Arduino ground is ok to be connected to the 12v ground?

'Yes', to the solid-state relay

'Yes', to the common ground

I want to add that the circuit/load getting switched may or may not share the ground (IOW, it could be "isolated").

Why are you using a relay? The load is a single LED - is that the final intention or do you have a larger load in mind?

Most LEDs melt/explode if you put 12V across them - did you mean to have a current-limiting resistor?

It will be a ~3 - 5w led dependent on tests

If you use a MOSFET you can use PWM...

You will need series resistor (and a heatsink for the LED).

It will be a ~3 - 5w led dependent on tests

You can drive a 5W LED straight through a MOSFET transistor, no relay needed.