Here’s an interesting fun project for the ATtiny45 which lets you wake up your PC by just knocking. Joonas Pihlajamaa posted the detailed project log at CodeAndLife. He describes, “Hardware-wise it’s a simple thing – ATtiny45 emulating a PS/2 device, sending a keypress when three knocks are detected in the attached piezoelectric sensor (or piezo buzzer as they are also called). But if your computer can boot on PS/2 keyboard input and you have your computer stowed somewhere hard to reach (or just want to impress your friends), it’s a pretty neat little gadget!”