1.5V mini wireless microphone

The microphone uses a single tube, has a simple circuit, and requires only 1.5V operating voltage. If powered by a button battery, the volume can be very small. The frequency is tuned to the 88~108MHz FM band, and 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 via the high- and low-frequency resistor-capacitor filters formed by R1, C1; R2, and C2. Due to the positive feedback amplification effect of the transistor, the high-frequency signal from the high-frequency oscillator formed by L1 and C3 is effectively fed back to the base of the transistor via C4. These two signals are mixed by the transistor to form a modulated high-frequency carrier, which is transmitted to the antenna via C6, radiating the signal into the surrounding space.

Fine-tuning the gap in coil L1 changes the FM frequency. The FM reception frequency can be selected between 88 and 108 MHz. For oscillator coil L1, use 0.6mm diameter enameled wire wound four times around the core of a standard ballpoint pen. The triode is a 9018 high-frequency, low-power transistor, though an ultra-high-frequency, low-noise transistor like the C3358 is even better.

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.