The alarm mechanism was designed to be triggered once per countdown. A steel ball sits in an upper compartment and falls down as soon as the slider is moved by the servo. The ball hits an old phon bell in the middle of the mechanism and falls down into the lower compartment. The compartments are now closed again. When the clock is flipped for restarting the countdown, the ball is also flipped and now the lower compartment is the upper compartment. So flipping the timer is also rearming the timer, and no additional energy is needed to move the ball, just gravity.

The servo is triggerd at startup and 1 minute before the countdown ends. All parts are custom designed and 3D printed.