A signal transmitting circuit is provided for cutting off or outputting at least one driving signal for driving a designated light source. The signal transmitting circuit includes an input differential signaling driver, an output differential amplifier, and a fixed-voltage-level-difference supply device. The input differential signaling driver is for receiving a display signal, and outputting an inverting signal and a non-inverting signal according to the display signal. The output differential amplifier is for receiving the inverting signal and non-inverting signal and outputting or cutting off the driving signal according to the voltage-level difference between the inverting signal and the non-inverting signal. The fixed-voltage-level-difference supply device is for supplying a fixed-voltage-level difference to replace the inverting signal and the non-inverting signal received by the output differential amplifier; therefore the output of the output differential amplifier remains low voltage-level to cut off the output of light with a designated color.