sorry guys can somebody let me know if this is ok?
cheers.
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:
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).
davivid:
It will be a ~3 - 5w led dependent on tests
You can drive a 5W LED straight through a MOSFET transistor, no relay needed.