A doorbell is a signaling device typically placed near an entry door to a building. When a visitor presses a button the bell rings inside the building, alerting the occupant to the presence of the visitor. Although the first doorbells were mechanical, activated by pulling a cord, modern doorbells are electric — they are actuated by an electric switch. A precursor to the electric doorbell, specifically a bell that could be rung at a distance via an electric wire was invented by Joseph Henry around 1831. An early patent for an electric door bell, patent 237,455, was obtained in 1881 by Edwin Swan of Indiana for an "illuminated electric push button".
Wired doorbells 
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) switch. momentarily closes the doorbell circuit. One terminal of this button is wired to a terminal on a transformer. A doorbell transformer steps down the 120–240-volt AC electrical power to a lower voltage, typically 10–20 volts. The transformer's other terminal connects to one of three terminals on the signaling device. Another terminal is connected to a wire that travels to the other terminal on the button. Some signaling devices have a third terminal, which produces a different sound. If there is another doorbell button (typically near a back door), it is connected between the transformer and the third terminal. The transformer primary winding, being energized constantly, does consume a small amount (about 1 to 2 W) of standby power constantly; the tradeoff is that the wiring to the button carries only safe, low voltage isolated from earth ground.
A common signaling device is a chime unit consisting of two flat metal bar resonators, which are struck by plungers operated by two solenoids. The flat bars are tuned to two pleasing notes. When the doorbell button is pressed, the first solenoid's plunger strikes one bar, and when the button is released, a spring on the plunger pushes the plunger up, causing it to strike the other bar, creating a two-tone sound ("ding-dong"). If a second doorbell is used, it is wired to the other solenoid, which strikes only one of the bars, to create a different sound.
More elaborate doorbell chimes play a short musical tune, such as Westminster Quarters.
Wireless doorbells 
In recent decades, wireless doorbell systems have become popular, which don't require wiring in the walls. The doorbell button contains a built-in radio transmitter powered by a battery. When the button is pushed, the transmitter sends a radio signal to the receiver unit, which plugs into a wall outlet inside the building. When the radio signal is detected by the receiver, it activates a music synthesizer, which plays the sound of gongs through a loudspeaker, usually a short tune, such as Westminster Quarters. These doorbells make a "DING-DONG" noise. To avoid the possibility of nearby wireless doorbells on the same radio frequency interfering with each other, the units can usually be set by the owner to different radio channels.
In larger metropolitan cities, a trend has developed over the past decade that uses telephone technology to wirelessly signal doorbells as well as to answer the doors and remotely release electric strikes. In many cities throughout the world, this is the predominant form of doorbell signalling.
Musical and continuous power doorbells 
As with wireless doorbells, musical doorbells have also become more common. Musical and continuous power doorbells serve as an attempt to bridge the gap between newer digital circuitry and older doorbell wiring schemes. A major difference between the standard setup of a wired doorbell and a musical doorbell is that the musical doorbell must maintain power after the doorbell button is released to continue playing the doorbell song. This can be achieved in one of two ways.
For simple SPST doorbell buttons, the chime device employs a rectifier diode and ballast capacitor at the voltage input stage of the circuit. Upon pressing the doorbell button, power is connected through the rectifier diode or series of rectifier diodes called a full wave rectifier, which only allows the current to flow in one direction, into the ballast capacitor. The ballast capacitor charges at a rate far greater than the rest of the circuit needs to complete a given song. Once the button is released, the capacitor retains the charge and maintains power for a short duration to the rest of the circuit.
For mixed, wireless and wired, input doorbells, a special doorbell button is needed to maintain power continuously to the doorbell chime. The circuit is similar to the one above, except that the rectifier diode is now moved into the doorbell button housing. Pressing the doorbell button allows both negative and positive sides of the AC power signal to flow into the circuit, while releasing the button only allows either the positive or negative side to flow into the circuit. By differentiating the full and half wave signals, the doorbell is able to function as it does in the previous wired case, while also providing continuous power to the doorbell for other purposes such as receiving wireless doorbell button inputs.
See also 
- Scientific writings of Joseph Henry, Volume 30, Issue 2. Smithsonian Institution. 1886. p. 434.
- "Why Did President Bush Suddenly Start Talking about Standby Power?", presentation by Alan Meier, this is a cool device. energystar.gov
- Miscellaneous Electricity Use In U.S. Homes Marla C. Sanchez, Jonathan G. Koomey, Mithra M. Moezzi, Alan Meier and Wolfgang Huber, LBNL Berkeley CA, osti.gov
- "Alerting and Communicating Devices for Deaf and Hard of Hearing People — What's Available Now". Clerccenter.gallaudet.edu. Retrieved 2011-09-27.
- "NPR — At Gallaudet, a Turn Inward Opens New Worlds". Npr.org. Retrieved 2011-09-27.