1) High-frequency transistors V1 and capacitor C3, C5, C6 to form a three-point oscillator capacitor
2) C4, L form a resonator: resonant frequency is the frequency of the FM microphone launch, according to the parameters of the diagram components transmitting frequency can be between 88 ~ 108MHZ, just covering FM radio reception frequency, by adjusting the L value (La stretch or compression coil L) can easily change the transmission frequency to avoid the FM radio. Transmit signal coupled to the antenna through C4 to launch out.
3) R4 is V1 of the base bias resistor to provide a transistor's base current, to work in the enlarged area V1.
4) R5 is a direct current feedback resistor, a stabilizing effect transistor operating point.
5) MIC microphone voice signal is collected.
6) The resistance of R3 for the MIC to provide a DC bias, R3 the resistance the greater the sensitivity of the microphone capture sound weaker, the higher the resistance the smaller the microphone sensitivity.
7) microphone collected audio signals AC coupled through C2 and R2 after the match sent to transistor base.
8) The circuit of two diodes D1 and D2 reverse parallel, since the main function of a two-way limiting, the diode conduction voltage of only 0.7V, if the signal voltage exceeds 0.7V diode conduction will be streaming, so you can ensure that the voices signal amplitude can be limited to between plus or minus 0.7V, the voice signal is too strong over modulation transistor will generate sound distortion can not even work properly.
9) CK is an external signal output socket, headphone socket can be TV or music players headphone jack and other external audio source through a dedicated cable into the FM transmitter, external sound signal attenuation through R1 and D1, D2, after clipping sent to transistor base for frequency modulation.
10) D3 LED circuit used to indicate working status, when the FM microphone was electrical work and would light up, R6 is a light-emitting diode current limiting resistor. C8, C9 is the power supply filter capacitor, because the big capacitor normally produced by the winding process, so the larger the equivalent inductance, a small shunt capacitor C8 can power high-frequency resistance.
11) K1 and K2 circuit is a switch, it has three different locations, disconnect the power when the income into the far left, far right is the K1, K2 FM microphone connected to make use of, the middle is connected K1, K2 off , do use the wireless transponder, because it uses a wireless transponder microphone does not work, but the microphone will consume a certain quiescent current, so disconnect the K2 can reduce power consumption, extending battery life.
By changing the transistor's base and emitter capacitance to achieve between the FM, when the voice of the voltage signal added to the base of the transistor, the transistor's base and emitter capacitance between the size of the place as the voice of the voltage signal synchronization change, while enabling the transmitting power transistor changes, to achieve frequency modulation.
Transmitter finished PCB diagram
Transmitter circuit
Transmitter finished map
Note (pcb diagram line part of the antenna within the system, after the completion of the design do not need to match the antenna. Simple debugging they can go to work! If felt appropriate emission increases from the poor to the supply voltage. The Circuit 9018 can withstand the voltage within 12V; also can add a modulation circuit in the amplification effect on the increase in firing range!!! FM radio receiver can be replaced!!!)
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