How the Alarm Sound Works on Casio Modules 240, 691 & 901

After reading several forum discussions and studying these vintage modules, I finally reached a simple explanation of how the alarm sound is produced.

Unlike what many people assume, the piezo element is not located inside the module itself. Instead, the crystal contains two piezo elements and a conductive silk-printed circuit on its inner surface.

When the module triggers an alarm, electrical current is sent through the alarm spring, across the conductive print inside the crystal, through both piezo elements, and back to the module via the two metal contact arms.As the current passes through the piezo elements, they vibrate and generate the familiar alarm beep.

It is a clever and elegant design—using the watch crystal itself as an active part of the alarm system rather than just a protective window.