diff --git a/sim.py b/sim.py
index ff510c2..32d8ae5 100644
--- a/sim.py
+++ b/sim.py
@@ -51,7 +51,7 @@ def step(self, t):
 
         self.error = self.setpoint - self.z # update error
         self.interror += self.error
-        self.deriverror = self.lasterror - self.error
+        self.deriverror = self.error - self.lasterror
         self.lasterror = self.error
 
         noise = np.random.normal(scale=self.sensor_noise) if self.sensor_noise > 0 else 0
@@ -168,4 +168,4 @@ def main():
 
 
 if __name__ == "__main__":
-    main()
\ No newline at end of file
+    main()