diff --git a/src/main.cpp b/src/main.cpp
index ccf874b..5aedadf 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -1,5 +1,30 @@
 /******************************************************
  *  (C) 2020 Dirk Jahnke (dirk.jahnke@mailbox.org)
+ * Uses Arduino with Ethernet Shield.
+ * Thus the GPIO usage is limited, because the shield
+ * needs the SPI pns: 10 (SS), 11 (MOSI), 12 (MISO), 13 (SCK).
+ * 
+ * If the SD card is to be used, then pin 4 is used as SS
+ * for the SD card.
+ * 
+ * We use 2 outputs to control Relays for
+ * - switching LED at the mailbox (light on/off) and
+ * - trigger the internal bell (gong).
+ * Following pins can be used as I/O:
+ * - 1
+ * - 2
+ * - 3
+ * - 5
+ * - 6
+ * - 7
+ * - 8
+ * - 9
+ * - 14 (A0)
+ * - 15 (A1)
+ * - 16 (A2)
+ * - 17 (A3)
+ * - 18 (A4)
+ * - 19 (A5)
  ******************************************************/
 
 #include <Arduino.h>
@@ -15,24 +40,24 @@ IPAddress mqttServerIP(192, 168, 89, 95);
 const char *mqttUser = "default";
 const char *mqttPassword = "12345678";
 
-const char *doorBellTopic = "doorBell";
-const char *postboxFlapTopic = "postboxFlap";
-const char *postboxDoorTopic = "postboxDoor";
-const char *doorBellTriggerSoundTopic = "doorBellTrigger";
-const char *postboxLightTopic = "postboxLight";
-const char *doorLightRequestTopic = "doorLightRequest";
+const char *doorBellTopic = "doorBellGW/doorbell";
+const char *postboxFlapTopic = "doorBellGW/mailboxFlap";
+const char *postboxDoorTopic = "doorBellGW/mailboxDoor";
+const char *doorBellTriggerSoundTopic = "doorBellGW/doorbellTrigger";
+const char *postboxLightTopic = "doorBellGW/light";
+const char *doorLightRequestTopic = "doorBellGW/lightRequest";
 
 EthernetClient ethClient;
 EthernetServer server(80);
 PubSubClient mqttClient(ethClient);
-#define DOOR_BELL_BUTTON_PIN 7
-#define POSTBOX_FLAP_PIN 5
-#define POSTBOX_DOOR_PIN 3
-#define LIGHT_REQUEST_BUTTON_PIN 2
+#define DOOR_BELL_BUTTON_PIN 9
+#define LIGHT_REQUEST_BUTTON_PIN 8
+#define POSTBOX_FLAP_PIN 7
+#define POSTBOX_DOOR_PIN 6
 // Output pins
 #define LED_PIN 13
-#define DOOR_BELL_BUZZER_PIN 6
-#define POSTBOX_LIGHT_PIN 4
+#define DOOR_BELL_BUZZER_PIN 14
+#define POSTBOX_LIGHT_PIN 15
 
 #define OUTPUT_NONE 0xff