and a contactor 9 through a terminal 20d to an impedance discriminator 20 this discriminator comprises a level shift circuit 130 which is powered by a car battery 1 through terminal 20b a microcomputer 110 and a buffer circuit 120 which are powered by a voltage stabilizer 100 which is connected to car battery 1 through terminal 20a automatic drive main switch 4 fuse 3 and ignition switch 2 the level shift circuit 130 includes a resistor 131 which is connected from terminal 20b to terminal 20d this resistor serves as a second impedance element and is connected in series with the first impedance elements 15 17 through the slip ring and contactor for feeding current from the battery to develop a potential at terminal 20d the resistor 131 is paralleled with a series combination of resistors 139 and 140 which is coupled at point c to a series combination of resistors 141 and 142 and thence to ground through terminal 20e the junction between resistors 139 and 140 is connected to the base of a pnp transistor 135 which forms a series circuit with resistors 136 and 138 potential at point a between resistors 136 and 138 is coupled to the base of an npn transistor 133 whose emitter is connected through a diode 134 to the circuit node c and whose collector is coupled through terminal 20c and through the relay winding 7b of a horn relay 7 and.