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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. Although the first doorbells were mechanical, activated by pulling a cord connected to a bell, modern doorbells are electric, operated by a pushbutton switch. Modern doorbells often incorporate and miniature to increase security.
osti.gov The tradeoff is that the wiring to the button carries only safe, low-voltage power isolated from earth ground.
A common signaling device is a chime unit consisting of two flat metal bar resonators, which are struck by a plunger operated by a solenoid. The flat bars are tuned to two pleasing notes. When the doorbell button is pressed, the solenoid's plunger strikes one bar, and when the button is released, a spring on the plunger pushes the plunger back, causing it to strike the other bar, creating a two-tone sound (" ding-dong"). If a second doorbell button is used, it might be wired to a second solenoid, which strikes only one of the bars, to create a single-tone sound (" ding"). Alternatively, the second button might feed the single solenoid via an oscillating switch (often a mercury switch), to give a "warbling" sound (" ding-dong-ding-dong-ding-dong"). The Edwards Sylvan C-26 had both additional features, suiting three doors. Some chimes have tubular bells instead of bars.
More elaborate doorbell chimes play a short musical tune, such as the Westminster Quarters.
Doorbells for hearing loss people use visual signaling devices — typically electric light — rather than audible signaling devices.
Fully battery-powered wired models are also common, either using a two-bar design or an electric bell. These do not consume standby power, but require the user to change the batteries, which are usually large primary cells located in the bell box.
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 . In many cities throughout the world, this is the predominant form of doorbell signalling.
For simple single-pole, single-throw 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 allows the current to flow in only 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 input.
Popular systems include the Ring doorbell, Vivint Home Security doorbell, and the Nest Hello. These consist of a single unit which is located in place of the traditional push-button, and in addition to a physical button, contains a high-definition camera, passive infrared sensor and Wi-Fi capability. The device is connected to the home Wi-Fi network, and notifications of a button-press or detected movement are pushed to a paired smartphone or other electronic device such as a tablet computer. When a notification is received, the user will typically see a live video stream from the smart doorbell, showing who is at the door and potentially allowing a 2-way audio conversation.
The devices can be powered by an internal battery, or they may use the existing bell wiring for continuous power.
The video is typically recorded via Wi-Fi to a cloud internet service, meaning that if the unit is tampered with, damaged or stolen, then this recording will still be captured and can be analyzed to determine the identity of the responsible party.
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