Doorbell
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. The electric doorbell was invented by Joseph Henry in 1831.[1]
Most people use their index finger to push a door bell, but young people who have grown up with text messaging are more likely to use their thumb.[2]
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[edit] 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. This 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;[3][4] 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.
Doorbells for hearing-impaired people use visual signaling devices — typically light bulbs — rather than audible signaling devices.[5][6]
[edit] 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. 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.
[edit] See also
[edit] References
- ^ Bachman, Frank P. (2006). Great Inventors and Their Inventions. Yesterday's Classics. p. 78. ISBN 1-599-15066-2. http://books.google.com/books?id=rBDTCtKSWfkC&pg=PA78., Extract of page 78
- ^ "Why your thumbs are getting bigger!". CBBC Newsround. 25 March 2002. http://news.bbc.co.uk/cbbcnews/hi/sci_tech/newsid_1892000/1892881.stm. Retrieved 2010-07-29.
- ^ "Why Did President Bush Suddenly Start Talking about Standby Power?", presentation by Alan Meier, this is a cool device. http://www.energystar.gov/ia/partners/prod_development/downloads/power_supplies/PSPresent2.ppt
- ^ Miscellaneous Electricity Use In U.S. Homes Marla C. Sanchez, Jonathan G. Koomey, Mithra M. Moezzi, Alan Meier and Wolfgang Huber, LBNL Berkeley CA, http://www.osti.gov/bridge/servlets/purl/795945-9c8LM1/native/795945.pdf
- ^ "Alerting and Communicating Devices for Deaf and Hard of Hearing People — What's Available Now". Clerccenter.gallaudet.edu. http://clerccenter.gallaudet.edu/InfoToGo/418.html. Retrieved 2011-09-27.
- ^ "NPR — At Gallaudet, a Turn Inward Opens New Worlds". Npr.org. http://www.npr.org/templates/story/story.php?storyId=5369960. Retrieved 2011-09-27.
[edit] External links
- HowStuffWorks: How a Doorbell Works
- How to Repair a Doorbell
- Deaf doorbell circuit
- Comparison between wired and wireless doorbells
