Although the first doorbells were mechanical, activated by pulling a cord connected to a bell, modern doorbells are electric, operated by a pushbutton switch.[from wikipedia]

A doorbell is a signaling device typically placed near a door to a building's entrance. When a visitor presses a button, the bell rings inside the building, alerting the occupant to the presence of the visitor.[from wikipedia]

In most wired systems, a button on the outside next to the door, located around the height of the doorknob, activates a signaling device (usually a chime, bell, or buzzer) inside the building. Pressing the doorbell button, a single-pole, single-throw (SPST) pushbutton switch momentarily closes the doorbell circuit.[from wikipedia]