Electronic and Arduino circuits for beginners with Arduino sketches.
Also see the circuits in the beginner series of electronics and Arduino tutorials.
This article shows how to connect a buzzer to an Arduino when the buzzer operates at a different voltage to the Arduino such as 9V, 12V or some other voltage.
Build a Knight Rider LED chaser display using an Arduino Mega 2560. This breadboard circuit uses 24 LEDs in for the display.