The present invention discloses an input device for transmitting an input signal to a computer through a game port of the computer. The game port comprises a capacitor and a detecting circuit for detecting a time period needed for charging the capacitor to a threshold voltage, discharging the capacitor when the capacitor is charged to the threshold voltage, and transmitting the detected time period as an input to the computer. The input device comprises a first charging circuit wired to the capacitor for slowly charging the capacitor, a second charging circuit wired to the capacitor for quickly charging the capacitor, and a control circuit wired to the capacitor used for activating the second charging circuit according to the input signal. The control circuit activates the second charging circuit according to the voltage of the capacitor and the input signal so that the capacitor can be quickly charged to the threshold voltage.