Components

• An electromagnet is a coil of wire wrapped around a piece of magnetic metal. Its coiling is meant to amplify the magnetic field that surrounds it, and consequently pose a greater attraction to the metal around it. Many doorbells have a solenoid, which is an electromagnet consisting of a coiled wire wrapped around a spring-equipped metal piston. Flanking the piston is a pair of tone bars.
  Working with the electromagnet is the transformer, which is meant to convert the standard 120-volt household current into a lower voltage, usually between 10 and 14 volts. The transformer connects to the electromagnet using small-gauge wires.
  
  

"Ding!"

• When you press the door bell button, the transformer delivers the low-voltage electricity to the bell unit, charging the piston and causing it to pull against its spring and hit the tone bar in front of it. This action creates the "ding" sound.
  
  

"Dong!"

• When you release your finger from the door bell button, the charge stops. The piston then pulls back to its previous position, hitting the other tone bar to create the "dong" sound.
  
  

Other Configurations

• Some door bells make two sounds, one for the front door and one for the back door. Solenoids on the bell unit are marked "front" and "back" and correspond with a front door bell and a back door bell, each of which has its own circuit. In this kind of design, the "back" solenoid hits one tone bar instead of two. Therefore, while the "front" terminal can produce a "ding-dong" sound, the "back" terminal just generates a "ding" sound.
  
  

Electronic Door Bells

• Electronic door bells have grown in popularity. These door bells don't use electromagnets or tone bars, but instead rely on an integrated circuit. This system registers when the door bell button is pressed as a digitally recorded song or message. Most electronic door bells have a wireless control system that has no need for the wiring of regular door bells. Nevertheless, conventional chime door bells are still the premier choice for households.