Battery-free audio repeater circuit

Anonymous battery-free audio transponder. This transponder can transfer the audio signal to the frequency modulation band of 88-108MHZ so that it can be received with an FM receiver. It is possible to achieve (1) listening to TV sound with an FM radio. If you listen with headphones at the same time, you can realize the TV wireless headphone function 　 (2) Use a recorder to record TV audio programs; (3) Record with two single-card recorder antennas. This transponder has the characteristics of simple circuit, no need for batteries, and easy installation and adjustment. Especially suitable for beginners to practice. See the attached diagram for the circuit, where C is the ready-made inductor. The audio voltage introduced from the plug is filtered by VD1, VD2 voltage doubler rectifier C2, C3, R2π type, and after the light-emitting diode LED is stabilized, a voltage of about 1.5V is obtained for circuit operation. VT, L, C1, etc. form a three-point high-frequency oscillation circuit. The VT base is also controlled by the audio frequency, so the oscillation frequency reaches FM. The FM wave is radiated by the antenna (0.4m soft wire). After the circuit is installed, first adjust C6 to allow the FM radio to receive the signal, and then adjust BP to make the sound quality clear. Can.