You need one more n-channel mosfet or BJT transistor to drive p-channel mosfet:http://ruggedcircuits.com/html/circuit__11.htmlHow much current your led's consume? Probably it would be easier to use just BJT or ULN2003 driver.
You are right, it's sufficient to turn it on. But the problem is to turn it off...When arduino send "high" - 5V, there would be still voltage between source-gate equal :Vsg = Vleds - VhighCircuit #16 is o'k till Vpower = 5V. If it goes up (12V or higher for a few leds) the would be an issue
Vsg = Vleds - Vhigh
...and what making mosfet to conduct is difference between Vgate and Vsource.
How much current your led's consume?
I tried Circuit16 (the first one) but the result is the same
Is it possible that my Led power consumption is to low to get the MOSFET triggering off again
"direct way" and NOTHING HAPPENS?
why it's always depressing when it starts to be interesting
often the problem is not actually where you are looking