Open-sourced some code

Scripts/Game Objects/Spawners/ProximitySpawner.cs

@@ -0,0 +1,36 @@
+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+public class ProximitySpawner : MonoBehaviour {
+	//public access members
+	public GameObject prefab;
+	public string targetTag;
+	public float minDistance = 4f;
+	public float maxDistance = 10f;
+	public bool destroyIfTooFar = false;
+
+	//private members
+	Transform targetTransform;
+	GameObject spawnedObject;
+
+	void Start() {
+		targetTransform = GameObject.FindWithTag(targetTag).transform;
+	}
+
+	void Update() {
+		//get the distance
+		float distance = Vector3.Distance(transform.position, targetTransform.position);
+
+		//destroy if the player is too far away
+		if (destroyIfTooFar && distance > maxDistance) {
+			Destroy(spawnedObject);
+			return; //skip out on the next check
+		}
+
+		//check the distance, check the spawned object
+		if (distance > minDistance && distance < maxDistance && spawnedObject == null) {
+			spawnedObject = Instantiate(prefab);
+		}
+	}
+}

Scripts/Game Objects/Spawners/TimedSpawner.cs

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+using System.Collections;
+using System.Collections.Generic;
+using UnityEngine;
+
+public class TimedSpawner : MonoBehaviour {
+	//inspector access elements
+	public GameObject prefab;
+	public float spawnDelay = 10f;
+	public int maxSpawns = 3;
+
+	//private members
+	float lastSpawnTime = float.NegativeInfinity;
+	List<GameObject> spawnList = new List<GameObject>();
+
+	//TODO: maybe all spawners should shut off (and monsters become inactive) when the player is too far away.
+	void Update() {
+		//prune the list
+		spawnList.RemoveAll(spawn => spawn == null);
+
+		//spawn if enough time has passed
+		if (Time.time - lastSpawnTime > spawnDelay && spawnList.Count < maxSpawns) {
+			lastSpawnTime = Time.time;
+
+			spawnList.Add(Instantiate(prefab, transform.position, Quaternion.identity, transform));
+		}
+	}
+}