Spawn OnTriggerEnter

using UnityEngine;

using System.Collections;

public class SpawnItem : MonoBehaviour {

     public Transform[] spawnpoints;

 public float spawntime = 1.5f;

 public GameObject[] en2;

void Start ()

 {

     InvokeRepeating ("SpawnEnemy" , spawntime, spawntime);

 }

void SpawnEnemy ()

 {
     int spawnIndex = Random.Range (0,spawnpoints.Length);

     int objIndex = Random.Range (0, en2.Length);

     Instantiate (en2[objIndex], spawnpoints [spawnIndex].position, spawnpoints 
             [spawnIndex].rotation);
 }

}

with this script every 1.5 seconds my empty object( to which I attached this scipt and contains other objects empty) spawn random objects that I entered in the insperctor. I don't want that an object spawn every 1.5 seconds, but spawn only when a previously spawn object enter into a trigger.

What should I change?