High sensitivity wireless microphone

lorant

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Here is a highly sensitive wireless microphone that can pick up weak sounds within a range of 5m, with a transmission distance of about 500m. Its operating frequency is within the range of 88-108MHz, and its transmission signal can be received by an FM radio. Another feature of this wireless microphone is that its operating frequency is very stable, and even if you touch the transmitting antenna with your hand, it will not cause the transmission frequency to change. It has a wide range of uses.

Circuit Principle

The circuit diagram of this microphone is shown in the figure.

The circuit consists of sound-to-electricity conversion, pre-emphasis circuit, audio amplifier, ultra-high frequency oscillation modulator and high frequency amplifier power device.

The sound-to-electric converter is performed by the electret condenser microphone B, which outputs the corresponding electrical signal after picking up the ambient sound wave signal, and is sent to the pre-emphasis circuit composed of R2 and C2 through C1 for bandwidth compression to increase the modulation volume of the voice. VT1 is an audio amplifier, which amplifies the pre-emphasized audio signal and sends it to the base of VT2 through C3 for frequency modulation. The voltage between the base and the collector of the common base ultra-high frequency oscillator composed of VT2 changes with the audio signal input to the base, thereby changing the junction capacitance between the base and the collector, and the frequency of the high-frequency oscillator also changes accordingly, thereby realizing frequency modulation.

The emitter output Class C high frequency power amplifier composed of VT3 has two functions: one is to increase the transmission power and expand the transmission distance; the other is to isolate the antenna and the oscillator to reduce the influence of the antenna on the oscillator frequency. The signal after the high frequency power amplifier is output by the emitter of VT3 and sent to the antenna for transmission through C10 and L3. L3 is the antenna inductance coil, which is used when the antenna length is less than a quarter of the wavelength to improve the antenna transmission efficiency. The capacity of C8 and C10 cannot be greater than 20pF, otherwise the change of the antenna will affect the stability of the frequency.

Component selection and fabrication

VT1 can use 9014 or 3DG12 silicon NPN low-noise transistors; VT2 can use 9018 or 3DG80 silicon NPN ultra-high frequency transistors; VT3 can use 8050C or 3DG12C NPN high-frequency medium-power transistors.

C1, C3 and C11 can use ordinary ceramic capacitors; all resistors are 1/8W carbon film resistors; L1~L3 are wound on the ballpoint pen core with φ0.4mm enameled wire for 6 turns, and then removed; L3 should be tapped at 3 turns; L2 and L3 should be arranged perpendicular to each other on the printed circuit board; it is best to use a pull-rod antenna for the antenna, which can also be replaced by an 800mm long plastic flexible wire; B can use a CRZ2-113F electret condenser microphone; the power supply can be four No. 5 batteries or a 4F22 6V stacked battery; S is a small switch.

Circuit debugging

1. Adjust the working points of each level: adjust the resistor R3 to make the collector voltage of VT1 1.5V; adjust the resistor R7 to make the collector current of VT2 4-6mA. At this time, touch the collector of VT2 with tweezers. The current should change significantly, indicating that the high-frequency oscillator is working normally; the working point of VT3 does not need to be debugged.

2. Frequency adjustment: Turn on the indoor radio and search for the local signal in the range of 88-108MHz. If the two frequencies are aligned, a strong audio whistling sound will be generated in the radio. At this point, the frequency of the local FM radio station should be avoided. The way to avoid this is to use a small screwdriver to adjust the turn distance of coil L.

3. Launch site strength test: First make a simple field strength meter, as shown in the figure.

Connect points a and b of the field strength meter to the antenna and ground of the wireless microphone respectively, and fine-tune the turns of L1 and L3 of the wireless microphone to maximize the multimeter reading of the field strength meter. The field strength meter multimeter should be set to the DC 10V or 50V position.

The adjusted circuit can be put into use. For example, if it is used for entertainment, the shell can be made into the shape of a general microphone, and the power supply can be a laminated battery.