Abstract: A warning device for logically controlling headlights of a motor vehicle comprising a photocoupler which picks up signal voltage from a horn and triggers an astable multivibrator via a gate-controlled circuit to output a signal voltage in a predetermined time to a monostable multivibrator which outputs a continuous time-sequencing pulses to the encoding input terminal of a decoder via the gate-controlled circuit thereby causing the decoder to produce sequential output, the sequential output being divided by a multi-input gate-controlled circuit into two output voltages which are amplified by transistors to drive two relays connected with high beam and low beam thus making the high beam and low beam to give light alternately.