Simple 3V two tube 100 m FM FM wireless microphone circuit diagram

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The volume is only the size of the matchbox, and its electrical schematic is shown in the figure. The inductor L is wound 5 turns on a 3 mm drill bit with a high-strength enameled wire (0.5-0.7 mm in diameter) at intervals of about 5.5 mm. The resistor uses a 1/16 watt RTX model, except for the electrolytic capacitor, and the other capacitor uses a small CC1 type ceramic capacitor. The transistor can be 3DG8 (or 3DG6), requiring β>100. Micro condenser microphones, changing the resistance R1, can change the sensitivity of the microphone (resistance R1 can be selected in the range of 10-100 kohms, high sensitivity when the resistance is large). The antenna can be replaced by a 10 cm long wire and soldered to the BG2 collector. The power supply uses two No. 5 dry batteries.
Once the components are installed, they can be debugged. The normal value of the BG1 collector voltage is around 1.4 volts. The BG2 collector current should be 4-6 mA. When debugging, do not put your hand too close to the transmitter, and use a non-inductive screwdriver for the screwdriver. The carrier signal center frequency of the transmitter is mainly determined by C4 and the inductance L. The elongated inductor L can increase the carrier frequency of the transmitter. When debugging, keep the transmitter at a distance from the FM radio, not too close, so that the radio does not pick up the carrier's higher harmonics. Tuning the FM radio, generally receiving a carrier signal from the transmitter between 80-108 MHz.
If the receiver only receives the carrier and there is no sound, the fault may be in the audio amplifier and condenser microphone. There should be 0.7-1.5 volts on the condenser microphone during normal operation. When the machine is normal, the output power can reach 5-8 mW and the launch distance reaches 100 meters.