Doorbells

Although almost every house has a doorbell, we rarely stop and think about how they work. At the basic level, a doorbell consists of an electromagnet that triggers some kind of noise (a buzzer, bell, or chime). When you press the button of a doorbell, you are completing the bell's circuit, activating the electromagnet.
In this page, I will outline how certain kinds of doorbells work.

Buzzer and Bell Doorbells

A buzzer doorbell revolves around a circuit consisting of an electromagnet, a metal arm, and a small metal piece. When you press the button, the circuit is closed, and the electromagnet turns on. This causes the arm to be attracted upward to the electromagnet. However, as soon as the arm moves, the circuit breaks, and it falls back down again (due to gravity), only to be attracted once more by the electromagnet. The sound of the arm hitting the metal piece intermittently causes the buzzing noise.

Below is an interactive diagram of the doorbell (you can press the button to see it in action):

Diagram from HowstuffWorks.com

A bell doorbell works in the same way, except instead of the arm hitting a metal plate, it hits a bell.