doorbell

A doorbell is a signaling device commonly found near a door. In most 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 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 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 pressed, the first solenoid's plunger strikes the bottom bar. When the button is released, a spring on the plunger pushes the plunger up, causing it to strike the other bar. If the other bell is used, it will activate the other solenoid, which will strike only one bar — typically the bottom bar. China is the leading producer of doorbell.