A simple FM tuner microphone circuit diagram

The circuit (see figure) uses a capacitive feedback oscillator with a stable and adjustable frequency. Its feedback signal is in the form of a capacitor divider that feeds back the output signal of the oscillating tube to the input. Where Re is a DC negative feedback resistor, C3 is a DC blocking capacitor, and Ce is an emitter bypass capacitor. L, C1, C2, and C form a resonant circuit. Since C2 is connected between the base and the emitter of the transistor BG, it constitutes a feedback circuit divided by C1 and C2, and the feedback signal is taken from the voltage on C2. The oscillation frequency of this circuit is f = 1/2Ï€, where C = C1 * C2 / C1 + C2.

The experimental evaluation of the FM microphone is simple and easy to work, more suitable for beginners imitation. In the open area, the transmission distance of this circuit is 20 to 30 meters. The long working frequency has a large offset. The harmonic content of the signal is high, which will cause interference to the adjacent frequency.
In the specific production, the MIC is best not to use a flexible wire to pull out, but to solder it to the board. The inductor L can be wound around 5-7 Φ on a Φ0.3mm round bar. After adjusting the pitch, it is fixed with high frequency wax. When judging whether the circuit is oscillating, the following simple method can be used. Use the ordinary pointer multimeter AC2V block, any test pen is suspended, and the other pen touches the antenna. If the pointer is found to swing, it indicates that the circuit has started to oscillate, and the pull distance can be debugged.