Automatic switch of TV sound transmitter

As shown in the figure, the main circuit of the whole machine is the common single-chip FM stereo broadcast transmitter integrated circuit BA1404, and its circuit is very mature. L1 and L2 are both enameled wires with a wire diameter of 0.3mm. Make 5 turns. The inner diameter of the coil is 3mm and the length is 8mm. Coil wound according to this data. The transmitting frequency is approximately 100MHz. Within the reception range of ordinary FM radios. The automatic switch part is on the upper left side of the picture. The principle is that after the TV is turned on, a strong electromagnetic field is generated around the TV. This electromagnetic field is mainly generated by the line scanning system of the TV's picture tube. The line scanning frequency of TV sets is 15.625kHz in my country. Therefore, compared with AC mains power, it is still higher, as long as this electromagnetic field can be detected. And turn on the power switch of the broadcast system to complete the task.