2. Referring to FIG. 10C, in some implementations, the doorbell camera 106 includes a voltage monitoring unit 1022 configured to generate the supply monitoring signal based on the input AC voltage. The LED indicator of the doorbell camera 106 is configured to, based on a value of the supply monitoring signal, display on a surface of the device housing 702 one of a set of predetermined visual patterns, thereby sending a visual message to a user of the doorbell camera 106 to indicate whether the input AC voltage is low with respect to the supply threshold. For example, the supply threshold may be 8V. When the input AC voltage is greater than 8V, the supply monitoring signal may control the LED indicator to display a green color along the peripheral edge 1040 of the doorbell button 1002, indicating that the input AC voltage is sufficient to drive the doorbell camera 106. In contrast, when the input AC voltage is not greater than 8V, the supply monitoring signal may control the LED indicator to display a red color along the peripheral edge 1040 of the doorbell button 1002, indicating that the input AC voltage is insufficient to drive the doorbell camera 106. Referring to FIG. 10D, in some implementations, the bypass unit 1012 may also function as a voltage meter to determine whether the transformer 1010 can provide a supply voltage sufficient to drive the doorbell camera 106 which normally consumes more power than a conventional doorbell. The bypass unit 1012 includes a voltage monitoring unit 1012A, an LED indicator 1012B, and bypass circuit 1012C. The supply monitoring signal is generated by the voltage monitoring unit 1012A of the bypass unit 1012, and the LED indicator 1012B is located in the bypass unit 1012.