diff --git a/ampel-firmware/ampel-firmware.ino b/ampel-firmware/ampel-firmware.ino
index 39f6b0c3b9ede61f313a190e4c292eb6c9bde408..3fdc1d1c5f6f9b290c6e00edc6dc8b897c57b8fb 100644
--- a/ampel-firmware/ampel-firmware.ino
+++ b/ampel-firmware/ampel-firmware.ino
@@ -189,7 +189,7 @@ 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/led_effects.cpp b/ampel-firmware/led_effects.cpp
index f0b672c9d0c76e1cac6916d94783e85577f387fd..84cd7aadbd93d009fea1fd470b71d36ccaa63b57 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 or after every LED has been turned off.
    */
-  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 countdown < 0;
   }
 }
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);