This Clock Knock Block has a bit of everything; milling, Arduino, sensing, solenoids, and plenty of dirty puns. Just knock on the box or on the table right next to it and the time of day will be played back to you in a series of crisp, clean knocks. A big part of the fun here is that the box is a musical instrument.
If you take a look inside you’ll find an Arduino, a piezo sensor, a solenoid, and a nine-volt battery. The piezo sensor detects your knocking as an input. It can even listen to and repeat back a series of your knocks. The Arduino actuates the solenoid, which strikes the wooden enclosure, producing the knocking sound.
We’ve embedded a video of this useless machine after the break (that’s where all the puns are). One note for your own build; this box is made out of mahogany and because it is used as a resonance chamber, this may not work as well if it isn’t milled from a piece of quality lumber.