1) High-frequency transistor 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 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 value of L (pull stretched or compressed coil L) can easily change the transmission frequency to avoid FM radio. Transmit signal coupled to the antenna through the C4 to launch out.
3) R4 is V1 of the base bias resistor, to provide a transistor base current, so that work in the enlarged area V1.
4) R5 is the current feedback resistor, a stabilizing effect transistor operating point.
5) external microphone MIC collected sound signal.
6) The resistance of R3 for the MIC to provide a DC bias, R3 the resistance the greater the sensitivity of the microphone sound capture weaker, the higher the resistance the smaller the sensitivity of the microphone.
7) the microphone sound signal collected by the exchange coupling and R2 C2 matched to the transistor’s base.
8) The circuit of two diodes D1 and D2 reverse parallel, since the main function of a two-way limiting, the diode turn-on voltage is only 0.7V, if the signal voltage exceeds 0.7V shunt diode will be so to ensure the sound signal amplitude can be limited to between plus or minus 0.7V, the signal will sound too strong over-modulation transistor, resulting in sound distortion can not even work properly.
9) 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 transistor base for frequency modulation.
10) circuit D3 LED to indicate working status, 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 to the power of high-frequency resistance.
11) circuit is a switch K1 and K2, which 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 , do use the wireless repeater, wireless repeater for use do not work the microphone, but the microphone will be quiescent current consumption, so off K2 can reduce power consumption, extend battery life.
By changing the transistor base and emitter capacitance to achieve FM, when the sound signal is added to the voltage on the base of the transistor, the transistor base and emitter capacitance between the size of the place with the sound synchronized voltage signal changes, while making changes transmission frequency transistors to achieve frequency modulation.
Figure transmitter PCB products
Note (pcb diagram line is inserted in the antenna system, after the completion of the design with the antenna does not need a. Simple debugging to work after it! If the distance is not ideal launch feel may be appropriate to increase the supply voltage. The circuit 9018 can withstand a voltage less than 12V; You can also add a modulation circuit in the amplification, but also can effectively increase the transmission range!!! FM radio receiver can be replaced!!!)