Lerp a light intensity gradually to 0

I used this code until now to make a light dim to zero and back to 3 on the same button press. Now it does not stay at 0 but repeats the coroutine on every press. I know there is a better way to do this but haven't figured it out yet.

 public Light light;
     bool y;
     IEnumerator ScaleX1(float start, float end, float time)
     {
         float lastTime = Time.realtimeSinceStartup;
         float timer = 0.0f;
 
         while (timer < time)
         {
             light.intensity = Mathf.Lerp(start, end, timer / time);
             timer += (Time.realtimeSinceStartup - lastTime);
             lastTime = Time.realtimeSinceStartup;
             yield return null;
         }
         light.intensity = end;
     }
 
     void Start()            // =====START====
     {
         y = true;
     }
 
     void Update()            // ====UPDATE=====
     {
         if (Input.GetKeyDown(KeyCode.Q) && y == true)
         {
             StartCoroutine(ScaleX1(3.0f, 0.0f, 0.25f));
             y = false;
         }
         if (Input.GetKeyDown(KeyCode.Q) && y == false)
         {
             StartCoroutine(ScaleX1(0.0f, 3.0f, 0.25f));
             y = true;
         }
         print(y);
     }
 }