diff --git a/wolke/fan1/fan1.ino b/wolke/fan1/fan1.ino
index beea458c..c70c8d10 100644
--- a/wolke/fan1/fan1.ino
+++ b/wolke/fan1/fan1.ino
@@ -1,3 +1,5 @@
+// Frequency Measurement on multiple PINs using counting
+
 #define FAN_COUNT 1
 #define ITER_DELAY 200
 #define REPORT_EVERY_ITERS 8000
diff --git a/wolke/fan2/fan2.ino b/wolke/fan2/fan2.ino
new file mode 100644
index 00000000..9502cfbc
--- /dev/null
+++ b/wolke/fan2/fan2.ino
@@ -0,0 +1,41 @@
+// Frequency Measurement on 1 PIN using hardware interrupts
+// and measuring actual duratations (period lengths)
+
+static const int PIN_FAN1 = 2; // D3 (Yun) = Interrupt 0 / O5 (Tinkershield)
+
+unsigned long last_time = 0;
+unsigned long last_period = 0;
+
+void handler () {
+    unsigned long current_time = micros();
+
+    if (last_time == 0) {
+        last_time = micros();
+        return;
+    }
+
+    last_period = current_time - last_time;
+    last_time = current_time;
+}
+
+void setup () {
+    Serial.begin(115200);
+    pinMode(PIN_FAN1, INPUT_PULLUP);
+
+    // https://www.arduino.cc/en/Reference/AttachInterrupt
+    attachInterrupt(0, handler, RISING);
+}
+
+void loop () {
+
+    int rpm = 0;
+
+    if (last_period != 0) {
+        rpm = (int) round(60. * (1000000. / ((double)(last_period))) / 2.);
+    }
+
+    Serial.print(rpm);
+    Serial.println();
+
+    delay(500);
+}