and actuating the magnetic valve mv thereby releasing the supply of fuel the driver transistor t3 is also coupled to a vacuum switch s1 or s2 the vacuum switch s1 is normally closed whereas the vacuum switch s2 which may also be used is of the type which is normally open the output circuit 2 is constructed as follows the emitter of the driver transistor t3 is connected through a resistor r20 to the negative supply line 10 and its collector is connected through resistors r18 and r19 to the positive line 11 the junction of these two resistors is connected to the base of the transistor t4 whose emitter is directly coupled to the positive supply line 11 and whose collector is connected to the magnetic valve mv the other electrode of which is grounded or connected to the negative supply line 10 a protective resistor in series with the magnetic valve is not present because the valve is sufficiently protected by a portion of the circuit which includes the resistor r21 which lies between the line 11 and the collector of t4 and the diode d5 which is connected to the base of the transistor t3 thus if the loose valve plug 12 is accidentally grounded the magnetic valve is protected because in that case the diode d5 connects the base of the driver transistor t3 directly to ground which blocks it as well as the subsequent transistor t4 the presence of the resistor r21 insures that the cathode voltage of the diode d5 is normally so high as to block it so that it does not interfere with.