[Photo] 4 components make mini wireless FM microphone

Seeing the picture above, it is too simple. The circuit uses very few components, and only four of them form a miniature wireless FM microphone. The working frequency is relatively stable, the transmission distance is more than 10 meters, and the current is less than 0.5mA when 1.5V power is supplied. Such energy-saving microphones are rare. The distance can reach 30 meters with 3V power supply.
The circuit is shown in the figure, BG and L and the junction capacitance of the triode constitute a high-frequency oscillation circuit. The junction capacitance of the triode is about 2 ~ 3P. To make the frequency fall within the FM range, the coil should be wound around a 5mm diameter core for 7 turns. The vibration of the time modulates the high-frequency signal to generate frequency deviation and realize frequency modulation. The launch distance is related to the working current of the launch tube. The resistance can not be too large or too small. Between 300 and 500 ohms, the power is less than 1 milliwatt.
When choosing BG, the fT of the tube must be greater than 300MHz. If the 2SC3358 high-frequency tube is used, the frequency is more stable and the distance will be farther. The inductance L is divided into two coils to be wound, but the winding direction must be the same. L1 uses a 0.5mm diameter enameled wire to wind 4 turns on the 5mm diameter skeleton, and L2 winds 3 turns. The antenna can be used with a 10cm long flexible wire. When you touch the antenna, it will affect the frequency change. It is very stable when used in fixed locations.

This circuit can be installed in the cap of the small ink bottle, or in the pen cover. The battery uses A13 size batteries or smaller, but pay attention to adding a switch when using small-capacity batteries.

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