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How do electromagnet doorbells work

The pushing of the doorbell closes an electric circuit. Then electric current flows through an electromagnet, and the doorbell rings.

in other words
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

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