diff --git a/ampel-firmware/ampel-firmware.ino b/ampel-firmware/ampel-firmware.ino
index 39f6b0c3b9ede61f313a190e4c292eb6c9bde408..bf673706f3acfa69386ed1c48c0e8e81f70a935f 100644
--- a/ampel-firmware/ampel-firmware.ino
+++ b/ampel-firmware/ampel-firmware.ino
@@ -141,7 +141,12 @@ void loop() {
   keepServicesAlive();
 
   // Short press for night mode, Long press for calibration.
-  checkFlashButton();
+  // Inactive during calibration.
+  if (!(sensor::current_state == sensor::PREPARE_CALIBRATION_STABLE ||
+        sensor::current_state == sensor::PREPARE_CALIBRATION_UNSTABLE))
+  {
+    checkFlashButton();
+  }
 
   checkSerialInput();
 
@@ -189,7 +194,7 @@ void checkFlashButton() {
       led_effects::toggleNightMode();
     } else {
       Serial.println(F("Flash has been pressed for a long time. Keep it pressed for calibration."));
-      if (led_effects::countdownToZero() < 0) {
+      if (led_effects::countdownToZero()) {
         sensor::startCalibrationProcess();
       }
     }
diff --git a/ampel-firmware/co2_sensor.cpp b/ampel-firmware/co2_sensor.cpp
index 6e0e01eb6102ccd60586a618fd38d73f726fa213..6fcf2f86863bbf6a726b90fa56aa1f7a1ab4e36b 100644
--- a/ampel-firmware/co2_sensor.cpp
+++ b/ampel-firmware/co2_sensor.cpp
@@ -30,21 +30,6 @@ namespace sensor {
   char timestamp[23];
   int16_t stable_measurements = 0;
 
-  /**
-   * Define sensor states
-   * BOOTUP -> initial state, until first >0 ppm values are returned
-   * READY -> sensor does output valid information (> 0 ppm) and no other condition takes place
-   * NEEDS_CALIBRATION -> sensor measurements are too low (< 250 ppm)
-   * PREPARE_CALIBRATION_UNSTABLE -> forced calibration was initiated, last measurements were too far apart
-   * PREPARE_CALIBRATION_STABLE -> forced calibration was initiated, last measurements were close to each others
-   */
-  enum state {
-    BOOTUP,
-    READY,
-    NEEDS_CALIBRATION,
-    PREPARE_CALIBRATION_UNSTABLE,
-    PREPARE_CALIBRATION_STABLE
-  };
   const char *state_names[] = {
       "BOOTUP",
       "READY",
diff --git a/ampel-firmware/co2_sensor.h b/ampel-firmware/co2_sensor.h
index d9eff8724f018e885f260060aaed58678c814092..eb2612ddefc5898c29c89062a5de812dab43bc6e 100644
--- a/ampel-firmware/co2_sensor.h
+++ b/ampel-firmware/co2_sensor.h
@@ -24,6 +24,24 @@ namespace sensor {
   extern float humidity;
   extern char timestamp[];
 
+  /**
+   * Define sensor states
+   * BOOTUP -> initial state, until first >0 ppm values are returned
+   * READY -> sensor does output valid information (> 0 ppm) and no other condition takes place
+   * NEEDS_CALIBRATION -> sensor measurements are too low (< 250 ppm)
+   * PREPARE_CALIBRATION_UNSTABLE -> forced calibration was initiated, last measurements were too far apart
+   * PREPARE_CALIBRATION_STABLE -> forced calibration was initiated, last measurements were close to each others
+   */
+  enum state {
+    BOOTUP,
+    READY,
+    NEEDS_CALIBRATION,
+    PREPARE_CALIBRATION_UNSTABLE,
+    PREPARE_CALIBRATION_STABLE
+  };
+
+  extern state current_state;
+
   void initialize();
   bool processData();
   void startCalibrationProcess();
diff --git a/ampel-firmware/led_effects.cpp b/ampel-firmware/led_effects.cpp
index f0b672c9d0c76e1cac6916d94783e85577f387fd..2577161130e5fb500c94a591bc6bc8ac3247fe0e 100644
--- a/ampel-firmware/led_effects.cpp
+++ b/ampel-firmware/led_effects.cpp
@@ -211,14 +211,17 @@ namespace led_effects {
   }
 
   /**
-   * Displays a complete blue circle, and starts removing LEDs one by one. Returns the number of remaining LEDs.
-   * Can be used for calibration, e.g. when countdown is 0. Does not work in night mode.
+   * Displays a complete blue circle, and starts removing LEDs one by one.
+   * Does nothing in night mode and returns false then. Returns true if 
+   * the countdown has finished. Can be used for calibration, e.g. when countdown is 0.
+   * NOTE: This function is blocking and returns only after the button has
+   * been released.
    */
-  int countdownToZero() {
+  bool countdownToZero() {
     if (config::night_mode) {
       Serial.println(F("Night mode. Not doing anything."));
       delay(1000); // Wait for a while, to avoid coming back to this function too many times when button is pressed.
-      return 1;
+      return false;
     }
     pixels.fill(color::blue);
     pixels.show();
@@ -229,6 +232,6 @@ namespace led_effects {
       Serial.println(countdown);
       delay(500);
     }
-    return countdown;
+    return true;
   }
 }
diff --git a/ampel-firmware/led_effects.h b/ampel-firmware/led_effects.h
index a910d307f7c6294db889e704f745c3777136d19b..8f04d83e751a9de5f2110653d10a794d5494bd36 100644
--- a/ampel-firmware/led_effects.h
+++ b/ampel-firmware/led_effects.h
@@ -26,7 +26,7 @@ namespace led_effects {
 
   void setupRing();
   void redAlert();
-  int countdownToZero();
+  bool countdownToZero();
   void showWaitingLED(uint32_t color);
   void showKITTWheel(uint32_t color, uint16_t duration_s = 2);
   void showRainbowWheel(uint16_t duration_ms = 1000);