A method for protecting an electronic apparatus driven by a DC motor and a detection circuit for detecting positioning signals thereof. The electronic device includes an optical encoder, a code strip, and a DC motor. While moves along the code strip, the optical encoder outputs a first positioning signal and a second positioning signal for the control of the DC motor. The method includes the steps described below. First, states of the first and second positioning signals are detected. If the states of the first and second positioning signals are normal, the DC motor is controlled according to the first and second positioning signals. If the first positioning signal or the second positioning signal is abnormal, a preventive method is performed.