FM radio microphone on two transistors, 65,8-74 MHz. Scheme, description

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FM radio microphone on two transistors, 65,8-74 MHz. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Audio equipment

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The radio microphone operates in the FM range - 65,8-74 MHz. Signal reception is carried out using any VHF radio receiver at a distance of up to 25 m. Its schematic diagram is shown in the figure.

FM radio microphone on two transistors, 65,8-74 MHz

The design of the radio microphone is shown in the figure.

FM radio microphone on two transistors, 65,8-74 MHz

The source of the electrical signal is the BM1 microphone. Through the decoupling capacitor C1, the signal is fed to the base of the transistor VT1 for audio frequency amplification. Resistor R2 determines the DC mode of the transistor. From the collector load R3, the amplified signal is fed through the capacitor C2 to the base of the transistor VT2, which operates in the high-frequency generator circuit, assembled according to the capacitive three-point circuit.

This circuit uses capacitor C5 and the capacitance of the emitter junction of the transistor, which changes under the influence of an audio frequency signal entering the base of the transistor. Thus, the frequency modulation of the high frequency signal is carried out. Capacitor C3 grounds the base of transistor VT2 at a high frequency, without affecting the sound signal due to its small capacitance. Resistors R4 and R5 provide DC transistor mode. Semi-variable capacitor C5 is designed to set the carrier frequency of the generator to a section of the FM band free from radio stations.

The output oscillatory circuit L1, C4 is tuned to the generation frequency by the capacitor C4. The loop coil is a signal emitter instead of a commonly used antenna.

The circuit is powered by a GB1 battery of the "Krona" type or similar with a voltage of 9 V.

All parts of the radio microphone, with the exception of the battery and microphone, are located on the printed circuit board shown in the figure.

FM radio microphone on two transistors, 65,8-74 MHz

The loop coil L1 is made without a frame and is wound on a mandrel with a diameter of 8 mm with a silver-plated wire with a diameter of 1 mm and contains 6 turns. The device can use dynamic microphones MD-52A, MD-66 or MD78.

To shield the circuit, an aerosol can was chosen as a housing, the lower part of which is closed with its plastic cap. Thus, the emitting coil L1 is not shielded by the metal case.

See other articles Section Audio equipment.

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