4 components to make a miniature wireless FM microphone

The above diagram is too simple. The circuit uses very few components, only 4, to form a miniature wireless FM microphone, with a relatively stable operating frequency and a transmission distance of more than 10 meters. When powered by 1.5V, the current is less than 0.5mA, which is rare for such energy-saving microphones. When powered by 3V, the distance can reach 30 meters.

The circuit is shown in the figure. BG, L and the junction capacitance of the transistor form a high-frequency oscillation circuit. The junction capacitance of the transistor is about 2~3P. To make the frequency fall within the FM range, the coil should be wound 7 times on a core with a diameter of 5mm. The vibration of the capacitor microphone when receiving the call modulates the high-frequency signal to produce frequency deviation and realize frequency modulation. Its transmission distance is related to the working current of the transmitting tube. The resistance cannot be too large or too small. It is between 300~500 ohms and the power is less than 1 milliwatt.
When selecting BG, the tube's f _T must be greater than 300MHz. If a 2SC3358 high-frequency tube is used, the frequency will be more stable and the distance will be farther. The inductor L is wound in two coils, but the winding direction must be the same. L1 is wound with 0.5mm diameter enameled wire on a 5mm diameter frame with 4 turns, and L2 is wound with 3 turns. The antenna can use a 10cm long soft wire. When using it, touching the antenna will affect the frequency change. It is very stable when used in a fixed location.

This circuit can be installed in a small ink bottle cap or a pen cover. The battery used should be A13 battery or smaller. But please note that a switch should be added when using small capacity battery.