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.
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