diff --git a/DHT.h b/DHT.h
index d6c2412..d261194 100644
--- a/DHT.h
+++ b/DHT.h
@@ -1,36 +1,39 @@
-#if ARDUINO >= 100
- #include "Arduino.h"
-#else
- #include "WProgram.h"
-#endif
-
-/* DHT library 
-
-MIT license
-written by Adafruit Industries
-*/
-
-// how many timing transitions we need to keep track of. 2 * number bits + extra
-#define MAXTIMINGS 85
-
-#define DHT11 11
-#define DHT22 22
-#define DHT21 21
-#define AM2301 21
-
-class DHT {
- private:
-  uint8_t data[6];
-  uint8_t _pin, _type;
-  boolean read(void);
-  unsigned long _lastreadtime;
-  boolean firstreading;
-
- public:
-  DHT(uint8_t pin, uint8_t type);
-  void begin(void);
-  float readTemperature(bool S=false);
-  float convertCtoF(float);
-  float readHumidity(void);
-
-};
+#ifndef DHT_H
+#define DHT_H
+#if ARDUINO >= 100
+ #include "Arduino.h"
+#else
+ #include "WProgram.h"
+#endif
+
+/* DHT library 
+
+MIT license
+written by Adafruit Industries
+*/
+
+// how many timing transitions we need to keep track of. 2 * number bits + extra
+#define MAXTIMINGS 85
+
+#define DHT11 11
+#define DHT22 22
+#define DHT21 21
+#define AM2301 21
+
+class DHT {
+ private:
+  uint8_t data[6];
+  uint8_t _pin, _type;
+  boolean read(void);
+  unsigned long _lastreadtime;
+  boolean firstreading;
+
+ public:
+  DHT(uint8_t pin, uint8_t type);
+  void begin(void);
+  float readTemperature(bool S=false);
+  float convertCtoF(float);
+  float readHumidity(void);
+
+};
+#endif