A fault detector circuit is provided for detecting or controlling any signal related to a laser emitter in an electronic device. The fault detector circuit includes a first NMOS transistor and a comparator. A drain of the first NMOS transistor is coupled to a first supply voltage through the laser emitter. A gate of the first NMOS transistor is coupled to a control voltage. A source of the first NMOS transistor is grounded. The comparator is coupled to the drain of the first NMOS transistor and a reference voltage. The comparator compares a voltage of the drain of the first NMOS transistor with the reference voltage to generate a first detecting signal for determining whether or not the first supply voltage of the laser emitter needs to be cut off.