For a small and free festival in Ghent, BE I designed the electronics for an “electricity timer”. This device was made to encourage people to donate to the event, and it’s basically a big timer that you can recharge by inserting coins. When the timer get’s at 10 minutes a blue flashing light goes on, and when it’s at 0 a loud siren will go off and the music is turned of.
The machine was built around an Arduino that controls a servo that the clock-hand is attached to. When you drop coins into the machine they roll down a shaft, and are detected by an optical interrupt before they drop in a bucket. Every time a coin is detected two minutes are added to the timer.