Science Fair Project Encyclopedia
Doorbell
A doorbell is a signaling device commonly found near a door.
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How doorbells work
Wired system
In most doorbell wired systems, a button, 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 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 AC mains electrical power to a lower voltage, typically 3-30 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. If there is another button (typically near a back door) it is connected between the transformer and the third terminal on the signaling device.
Most signaling devices consist of two solenoids and two flat bars. The flat bars are tuned to two pleasing notes. The flat bars are mounted loosely above and below the solenoids. When the doorbell button is depressed the first solenoid's plunger strikes the first bar. When the button is released a spring on the solenoid pulls the plunger down, causing the plunger to strike the other bar. Another spring returns the plunger to its neutral (centered) position. If the other bell is used, it will activate the other solenoid, which will strike only one bar. More elaborate signaling devices play a short musical tune, such as Westminster Quarters.
Wireless system
A button near the door activates a built in transmitter. The transmitter sends a radio signal to the doorbell radio receiver inside the building. When the radio signal is detected by the receiver, it activates a buzzer, speaker or light.
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