Add example 'Lichtfalle' for LinuxWochen Linz workshop

Lichtfalle/Lichtfalle.ino

+/* Lichtfalle.ino
+ *
+ * Copyright 2015 Roland Richter
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty
+ * of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ * See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+const int BarrierPin = 2;
+const int BuzzPin   = 12;
+
+volatile boolean alarm = false;
+
+// Interrupt service routine
+void triggerAlarm()
+{
+    alarm = true;
+}
+
+
+void setup()
+{
+    pinMode(BarrierPin, INPUT);
+
+    pinMode(LED_BUILTIN, OUTPUT);
+    pinMode(BuzzPin, OUTPUT);
+
+    attachInterrupt(0, triggerAlarm, RISING);   // Zeile 38
+                                                // Interrupt 0 = Pin 2 auf UNO
+}
+
+
+void loop()
+{
+    if (alarm) {
+        tone(BuzzPin, 880);
+        digitalWrite(LED_BUILTIN, HIGH);
+    }
+    else {
+        noTone(BuzzPin);
+        digitalWrite(LED_BUILTIN, LOW);
+    }
+
+    delay(50);
+}
+
+/* IDEEN:
+   1. Warum wird in Zeile 38 der Interrupt mit einer steigenden Flanke ("RISING")
+      verknüpft? Was würde passieren, wenn man hier "FALLING" oder "CHANGE"
+      verwenden würde?
+   2. Erweitere das Programm so, dass der Alarm beendet wird, sobald die
+      Lichtschranke nicht mehr unterbrochen ist.
+   3. Ändere den Alarm: so lange die Lichtschranke unterbrochen ist, soll
+      die LED blinken und den Ton parallel dazu ein- und ausgeschaltet werden.
+*/