diff --git a/controller/Scheduler.py b/controller/Scheduler.py
index b75ae7e..ddeb50b 100644
--- a/controller/Scheduler.py
+++ b/controller/Scheduler.py
@@ -1,5 +1,5 @@
 from time import time, sleep
-from threading import Thread, Event
+from threading import Thread, Semaphore
 
 
 class Scheduler( object ):
@@ -14,10 +14,11 @@ class Scheduler( object ):
     self.__messages = {} # map of thread to list of its incoming messages
     self.__thread = None # currently executing microthread (if any)
     self.__done = False  # whether it's time to exit
-    self.__no_longer_idle = Event()
+    self.__idle = Semaphore( 0 )
     self.__last_error = None # used for unit tests
 
     self.add( self.__idle_thread() )
+    self.__idle.acquire() # don't count the idle thread
 
     self.__scheduler_thread = Thread( target = self.run )
     self.__scheduler_thread.setDaemon( True )
@@ -72,16 +73,15 @@ class Scheduler( object ):
           self.add( result )
         
       except StopIteration:
+        self.__idle.acquire( blocking = False )
         self.__running.remove( thread )
         self.__messages.pop( thread, None )
 
   def __idle_thread( self ):
     while not self.__done:
       # if the idle thread is the only one running, block until there's another running thread
-      if len( self.__running ) == 1:
-        self.__no_longer_idle.wait()
-        self.__no_longer_idle.clear()
-
+      self.__idle.acquire( blocking = True )
+      self.__idle.release()
       yield None
 
   # used for unit tests
@@ -98,11 +98,12 @@ class Scheduler( object ):
       sleep( self.IDLE_SLEEP_SECONDS )
 
   def sleep( self, thread ):
+    self.__idle.acquire( blocking = False )
     self.__sleeping.append( thread )
     self.__running.remove( thread )
 
   def add( self, thread, *args ):
-    self.__no_longer_idle.set()
+    self.__idle.release()
 
     if thread in self.__sleeping:
       self.__sleeping.remove( thread )
@@ -116,7 +117,7 @@ class Scheduler( object ):
 
   def shutdown( self ):
     self.__done = True
-    self.__no_longer_idle.set()
+    self.__idle.release()
     self.__scheduler_thread.join()
 
   # currently executing microthread (if any)