Hello, I need to understand the exact working of an electronic circuit (see picture) but I am very bad at electronic. This is the electronic circuit of a personal alarm that triggers an alarm when you pull out a metal part (like a grenade). All I know is that when you remove the metal part (that looks like a piston) from , the metal disk on the left (which is a piezoelectric disc) produces a loud sound. I would like to know what exactly happens. What is the part that transforms the electricity from the battery to an "alternative" current that makes the piezoelectric vibrate ? Thanks a lot to anyone that could help me.