LED stripes with common cathodes
To switch 12V (standard voltage for RGB stripes) with N-Channel MOSFETs, we need a Gate-Source voltage that is higher than the Threshold voltage, meaning that the Gate voltage has to be significant higher than 12V, but the Uno can only source 5V. So we have to use P-Channel MOSFETs. This type of MOSFET starts conducting when the Gate-Source voltage is negative enough (that means U_GS < U_th (U_th < 0)) and it has a very high impedance if U_GS is zero. With one additional N-Channel MOSFET (enhancement mode) per channel and a few resistor, we can achieve the described conditions for the output stage (see below).
For using the LED stripes with common cathode you can use the following circuit:
