A monitoring system includes a controller, a number of input devices, a number of digital sensors, a number of analog sensors, a number of displays, and a number of alarms. Each input device includes a switching circuit, an input circuit, and a connector. The connector is connected to a digital sensor or an analog sensor. The switching circuit includes an n-channel metal-oxide-semiconductor field effect transistor (MOSFET), a p-channel MOSFET, a first resistor, and a second resistor. The input circuit includes an operational amplifier and a third resistor.