A signal detecting device and a light-emitting apparatus using the same are provided. The signal detecting device includes an electromagnetic generator and a first switch. The electromagnetic generator includes a first receiving terminal and a second receiving terminal to receive an input signal from the first receiving terminal and the second receiving terminal and generate electromagnetic induction according to the input signal. A first terminal of the first switch is coupled to a first signal source. A second terminal of the first switch is coupled to an output terminal of the signal detecting device. The first switch determines whether the two terminals of the first switch are conducted according to the electromagnetic induction caused by the input signal.