Implemeting climbing ladder in unity 2d

I am trying to implement ladder climbing functionality in unity and i have achieved it to some degree.I have put circular collider on the top of the characters head so it detects when the ladder is above his head. what i want to do is that if i press the upward key it only climbs one step of the ladder and stays there..But instead the character falls as soon as i stop pressing the upward key and keeps going up if i press it one time..I have messed with the gravity of the rigidbody but no luck.

void Update() { if((isgrounded || !doublejump) && Input.GetKey(KeyCode.Space) ) { anim.SetBool( "Ground",false); rigidbody2D.AddForce(new Vector2(0,jumpforce)); if(!doublejump && !isgrounded) doublejump = true; } if(Ladder && Input.GetAxis("Vertical")>0) { rigidbody2D.gravityScale =0; rigidbody2D.velocity = new Vector2(rigidbody2D.velocity.x,Input.GetAxis("Vertical")*maxspeed); } if(!Ladder) { rigidbody2D.gravityScale =1; } }