Here is my project...
I just bought 5m of Rgb led strip and i want to make this sound controlled. I have been looking on youtube and all and everyone seams to be just hooking the neg and pos to the speaker out puts and using it this way. It seams to me that this is not a good idea for many reasons one being depending one the amount of Leds I have may and will change the ohms and not be comparable with the amps ohms along with the fact that if the music is not loud enough then they simply won't light up.
From my research and thoughts on the subject what i would like to do is have an external power source for the Leds this way they get the exact voltage every time.
Could i hook up a mosfet (mossfet?) to the speaker and use it to switch on and off my 12v power lead for the Leds?
Also if I did that how could I get different colors to light up? Would I need to run a second set of wires from another say a tweeter(as a second) (and a sub as one) to make a different color happen?
It depends on what sort of RGB LED strip you have.
The simplest have three wires to ground one for each colour, with another wire to supply the 12V, there is no need for any extra resistors as they are built into the strip. This sort you can control with three FETs.
The other sort need logic signals feeding to them depending on the type of chip on the strip.
Kelly:
Could i hook up a mosfet (mossfet?) to the speaker and use it to switch on and off my 12v power lead for the Leds?
"MOSFET". Metal Oxide Semiconductor Field Effect Transistor, usually written in capital letters even though it's an acronym, not an abbreviation.
Yes, that should work so long as you don't get "logic-level" MOSFETs (which would tend to be 100% on or off, very little in between). Logic-level also have more capacitance which could affect the sound.
You'll also need a diode on the MOSFET gate to prevent negative voltages arriving there (speakers are AC).
Kelly:
Also if I did that how could I get different colors to light up? Would I need to run a second set of wires from another say a tweeter(as a second) (and a sub as one) to make a different color happen?
That's trickier. You can try connecting to different parts of the speaker and see what happens but ideally you need a spectrum analyzer. You're asking an an Arduino forum and Arduino users seem to like the "MSGEQ7". nb. This would connect to a line-out connection, not a speaker.
PS: If you want to drive it with an Arduino then you DO want "logic-level" MOSFETs.