How to smoothly change a value up and down based on key press?

Hi,

I was wondering if anyone could help me out here. Basically, I have a point light, and I want it to slowly increase in intensity while a button is held down, but then slowly decrease back to 0 once the button is released. I've fiddled around with the Mathf.Lerp function in C#, but I can't seem to get it to work. Anyone have any ideas? Thank you so much to anyone who takes the time to answer. :)

EDIT: I got it to work, but I still have a problem. It smoothly increases while the key is held, and smoothly decreases once it's released, but my problem is that there is a delay of a few seconds between when the key is held down and when the light begins to increase in intensity. Any ideas? Here's my current code.

 using UnityEngine;
 using System.Collections;
 
 public class LightController : MonoBehaviour {
 
     public Light thrustLight;
 
     public float tParam = 0;
     public float lightSpeed = 5.0f;
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
     
         if (Input.GetKey ("up")) {
 
             if (tParam < 1) {
 
                 tParam += Time.deltaTime * lightSpeed;
 
                 thrustLight.GetComponent<Light> ().intensity = Mathf.Lerp (0.1f, 1.8f, tParam);
 
             }
         } 
 
         else {
 
                 tParam -= Time.deltaTime * lightSpeed;
 
                 thrustLight.GetComponent<Light> ().intensity = Mathf.Lerp (0.1f, 1.8f, tParam);
 
         }

EDIT AGAIN: I solved it! Because I didn't have an " if (tParam > 0) { }" in my else, the value was going backwards with time, making it delay. Putting that in there makes sure it doesn't go backwards and stays within 0 to 1. Here's my working code for anyone currently trying to google solve this issue:

 using UnityEngine;
 using System.Collections;
 
 public class LightController : MonoBehaviour {
 
     public int thrustForce = 5;
     public Light thrustLight;
 
     public float tParam = 0;
     public float lightSpeed = 5.0f;
 
     void Update () {
     
         if (Input.GetKey ("up")) {
 
             if (tParam < 1) {
 
                 tParam += Time.deltaTime * lightSpeed;
 
                 thrustLight.GetComponent<Light> ().intensity = Mathf.Lerp (0, 1.8f, tParam);
 
             }
         } 
 
         else {
 
             if (tParam > 0){
 
                     tParam -= Time.deltaTime * lightSpeed;
 
                 thrustLight.GetComponent<Light> ().intensity = Mathf.Lerp (0, 1.8f, tParam);
 
             }
         }