Reply to rotray knob as modifier switch

Hello erverybody,
i am looking for a way to turn a rotry knob into a switch for a modifier. So if i turn the knob to the left side the modifier value should be "0" an in the right half it should be "1".
Does anyone know a solution for this problem? I've tried so much

p.s. sorry for my bad english, i'm not native.

greetings Matze