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    FM microphone with a 9018 production tutorial

    Principle of the wireless microphone article:

    The following is the FM wireless microphone circuit, the circuit is very simple, no extra device. High-frequency transistor V1 and capacitor C3, C5, C6 to form a three-point oscillator capacitor, for beginners we do not pondering the specific capacitance of three-point principle, we only know that this circuit is a high-frequency oscillator can be. Transistor collector load C4, L form a resonator, the resonant frequency is the frequency of FM transmitter microphone, according to the parameters of the diagram elements in the transmission frequency between 88 ~ 108MHZ, just cover the FM radio reception frequency, by adjusting the L, value (tension or compression coil L) can easily change the transmission frequency to avoid FM radio. Transmit signal coupled to the antenna through the C4 to launch out.



    V1 R4 is the base bias resistor, to provide a transistor base current, so that work in the enlarged area V1, R5 is the current feedback resistor, a stabilizing effect transistor operating point.

    FM FM microphone that works by changing the transistor base and emitter capacitance to achieve frequency modulation, when the sound signal is added to the voltage on the base of the transistor, the transistor base and emitter capacitance with voltage signal with the size of the sound changes occurred simultaneously, while making changes transmission frequency transistors to achieve frequency modulation.

    MIC can be collected outside the microphone sound signal, here we are using a small electret microphone, the sensitivity is very high, weak voice can be collected, while the microphone work this must be the work of the DC bias voltage to the resistor R3 to provide a certain DC bias, R3 the resistance the greater the sensitivity of the microphone capture sound weaker. The higher the resistance the smaller the sensitivity of the microphone, the microphone sound signal collected by the exchange coupling and R2 C2 matched to the transistor's base, the circuit of two diodes D1 and D2 reverse parallel, the main effect limiting the function of a two-way , the diode turn-on voltage is only 0.7V, if the signal voltage exceeds 0.7V shunt diode will be so you can ensure that the voice signal amplitude can be limited to between plus or minus 0.7V, the signal will sound too strong over the transistor modulation, resulting in sound distortion can not even work properly.

    CK is an external signal output jack, headphone jack to the TV Walkman headphone jack or an external audio source such as a dedicated cable through the introduction of FM transmitter, external sound signal attenuation through R1 and D1, D2, after clipping to the transistor base pole for frequency modulation. So this package can not only make a wireless microphone, but can also be a TV wireless headset use.

    D3 LED circuit used to indicate the working state, when the FM microphone was electric lights will work, R6 is a light-emitting diode current limiting resistor. C8, C9 is the power supply filter capacitor, because the big capacitor generally produced by winding process, so the equivalent inductance is relatively large, a small capacitor C8 in parallel can reduce the power of high-frequency resistance, this circuit is very common.

    K1 and K2 in the circuit is actually a switch, it has three different positions, appropriated disconnect the power when the far left, far right is K1, K2 FM microphone connected to make use of the middle position is connected to K1, K2 off, make use of the wireless repeater, wireless repeater for use do not work the microphone, but the microphone will consume a certain amount of quiescent current, so you can disconnect the K2 reducing power consumption, extend battery life.

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