I'm a retard when it comes to interfacing. I'm pretty much on top of things when it comes to coding.
I need to connect 100 RGB LED´s to my arduino.
The good thing is that they shall show same color. What will I need in order to do this? Some power transistor I guess.
I have not ordered the LED´s yet. So common anode/Cathode is not an issue yet.
I'm thinking of powering the whole thing from a 12V switch mode power supply since that is what I already have.
You just want all 100 to turn on/off together?
Wire them up in series in strings of 5 with a current limit resistor (~150 ohm), from +12 V to the drain of an N-channel mosfet . You maybe be able to get away with 6 in series, depends on the forward voltage drop of the LED. Adjust the current limit R accordingly.
The gate will be connected to an arduino output pin thru a 100 ohm resister, and a 1K resistor to ground if you find it is not turning off satisfactorily.
The source will be connected to ground.
You will have 20 strings of 5+resister wired in parallel.
Choose a logic level MOSFET with low Rds. It will have about 400ma going thru it.
Here is an example but with a 20V source
You could also use a part like ULN2803 if you are not comfortable with discrete transistors.