It is only when the correct voltages are applied to the A/C amplifier from the key inputs that the primary output (the A/C compressor relay) will be energised.
只有A/C扩大器上的电压是来自输入端的正确电压时，初级输出（即A/C压缩机继电器）才会被激活开启。
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This AC electronic music door bell is composed of general electronic door bell, rectified power supply, relay and power amplifier and does not need battery to power it.