A voltage monitoring device includes a comparison voltage generating circuit for providing a comparison voltage, a comparator, an electronic switch, and an LED. The comparator outputs a control signal in response to a comparing result between the comparison voltage and a voltage output by a power supply. The electronic switch includes a first terminal receiving the output of the comparator, a second terminal connected to a reference voltage, and a third terminal connected to ground. The LED includes an anode connected to the reference voltage, and a cathode connected to the second terminal of the first electronic switch.