markotitel:
I guess you understood what I want to do fully?
I think I know what you're trying to do. Let me confirm it. You have an AC source. And whenever the AC source becomes active, you want the arduino to be able to detect it, right?
At the moment, it seems that your aim to detect the AC signal is to use the analog input to constantly sample the voltage. Since the AC signal amplitude will change, and even disappear... this needs consideration. The arduino analog-to-digital converter doesn't convert negative voltages from the negative half of the AC signal. And maybe not a good idea to apply negative voltages to the analog input pin of the arduino. This means you might have to at least use 1 electronic component, like a diode prevent negative voltages from reaching the analog input pin. But if you use 1 cheap component, then might well use 2 more cheap components --- a resistor and a capacitor. The diode, resistor and capacitor can form a 'peak detector' circuit, that basically produces relatively slow-changing voltages. This allows the arduino to detect your AC signal fairly easily. Makes your programming easier to do as well.