off and the upper diode 7 is turned on and a lower voltage v cc from the battery is supplied to the tuning voltage generating circuit 3 in this case it is possible to design the maximum value of the voltage v t to be for example about 8 volts and therefore the source voltage of 9 volts from the battery is sufficient for the operation of the tuning voltage generating circuit 3 although the zener diode 6 is connected between the resistor 5 and the lower diode 8 in the embodiment of fig1 it may be interposed between the diode 8 and the tuning voltage generating circuit 3 further the lower diode 8 may be used also as a rectifying diode in the periphery of the fly back transformer 2 yet further if a zener diode is used as the upper diode 7 assuming that v cc 9 volts a 24 volt zener diode is used the upper diode 7 may be used also as the zener diode 6 so that it is possible reduce the number of parts in place of using such diodes 7 and 8 as in the first embodiment of fig4 the change over circuit may be constructed of a relay circuit as shown in a second embodiment of fig2 alternatively the change over circuit may be constructed by such an alternatively activated two transistor circuit as shown in a third embodiment of fig3 the voltage drop caused in the change over circuit in these embodiments is smaller than that in the first embodiment using the diodes and therefore the second and third embodiments are advantageous in the case where the source voltage from the battery is low as described above according to the present invention the tuning voltage.