to plunger 108 from the vehicle 39 s battery and plunger 108 is moved into the body of plug 104 operating against spring 706 12 volts is applied to spring contact 702 to supply operating voltage to the portable device 12 volts is also applied to spring contact 704 which in this embodiment provides detector output p detect one could develop signal p detect directly by detecting 12 volts supplied to the audio circuits but by incorporating switch contact 704 to sense the compression of plunger 108 it becomes more difficult for a user to subvert the rules of allowable power by applying 12 volts to plunger 108 without inserting plug 104 into a mating socket with the aid of fig9 and 10 we will describe mechanisms by which the fm transmitter 326 may radiate the rf out signal in several embodiments in one embodiment shown in fig9 low pass filter 342 is interposed between operating supply voltage terminal 912 and the positive terminal 910 of the automobile dc power source represented as 906 the signal rf out 920 from fm transmitter 326 is then connected by connection 918 to automobile power source positive terminal 910 element z item 902 represents the radiation impedance between the automobile 39 s 12 volt wiring and earth ground 916 if the.