Thank you for your replay!
It was very good read, indeed.
I have another question:
Is it possible to wire guitar effect pedal to Arduino UNO in way that I use it like input signal?
When I step on the pedal, led on it turns ON and another light is being switched ON (on another location) via Aurdiono UNO.
I am intending to soldier wire to pedal's led terminal. (I never measured this value; I guess it's at least 3V and very little current.)
It looks too simple and I wonder if I'm missing something.
Standard guitar effect pedal is using 9V DC power supply totally separated from Aurdino's power supply.
This is where I start to doubt my plan 
sikter