 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 is plugged into a wall outlet inside the building.When the radio signal is detected by the receiver, it activates a sound chip that plays the sound of gongs through a loudspeaker—either a two-note "ding-dong" sound or a longer chime sequence such as "Westminster Quarters".