1.5V wireless microphone

The microphone uses a single tube and the operating voltage is only 1.5V. The frequency is tuned to the FM band of 88~108MHz. The microphone signal can be clearly received within a range of 30 meters using an ordinary FM radio. The circuit diagram is as follows

　　Brief Introduction of Circuit Working Principle

　　The sound signal received by the microphone is coupled to the base of the transistor through the high and low frequency resistance and capacitance filters composed of R1, C1; R2, C2. Due to the positive feedback amplification of the transistor, the high frequency signal of the high frequency oscillator composed of L1 and C3 is equivalently fed back to the base of the transistor through C4. The two signals are mixed by the transistor to form a modulated high frequency carrier, which is transmitted to the antenna through C6, and the antenna transmits the signal to the surrounding space.

　　Fine-tuning the gap of the L1 coil can change the frequency value of the FM frequency modulation wave. When in use, the FM receiving frequency can be arbitrarily selected between 88~108MHz. The oscillation coil L1 uses 0.6mm diameter enameled wire to wind 4 turns on the core of an ordinary ballpoint pen. The triode uses a 9018 high-frequency low-power tube. Of course, it is better to use an ultra-high frequency low-noise tube such as C3358.

　　Tips for beginners: The MIC (microphone) in this circuit does not provide bias current. If you use an electret microphone head, don't forget to provide it with bias current.