HELP! How to "block: a not binary signal? Helped

Hey guys i want to create a servo wich stays in an inicial position and if you activate its rc the servo should do an 180° turn. Then you can move the servo but if you stop touching the cpad the servo should move back to the position of the 180 turn. Ive tried subtract gates too but it wont work :/ could SB please help me out? :)