[Photo] Another FM microphone

When watching TV late at night, you usually have to lower the volume so as not to affect the rest of others, which may make it impossible to hear the TV sound. If there is a circuit that can transmit TV sound signals to the surrounding space, and then use FM radio to receive it can solve this problem. The circuit is shown in Figure 1.



In the circuit of Figure 1, VT1 and its peripheral circuits form an oscillation circuit with an oscillation frequency of approximately 98 MHz. R1 and Cx are audio pre-emphasis circuits that are used to improve the frequency response of audio signals and improve sound quality.
Both L1 and L2 are made of 1mm enameled wire wound around 10 turns on a 5mm skeleton, and the length can be elongated to about 11mm, as shown in Figure 2.



If the circuit does not work properly, it is necessary to check whether the contact between the component pins and the wiring is good, especially the pins of the transistor must not have virtual soldering. Use tweezers to short-circuit the two ends of L1. If the current of the whole machine changes, it means that the circuit has started. If the current does not change, check whether the resonant capacitor C3 has been connected and adjust its capacitance until the circuit starts to oscillate.
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