DIY a fm transmitter yourself

Making their own FM transmitter

1 ) High-frequency transistor V1 and capacitor C3, C5, C6 to form a three-point oscillator capacitor
2) C4, L form a resonator : a resonance frequency is the frequency of the FM microphone transmitter , may be between 88 ~ 108MHZ, FM radio reception frequency just cover according to the parameters of the transmit frequency component of the FIG , by adjusting the value of L (La stretch or compress the coil L) can easily change the transmission frequency to avoid the FM radio . C4 is coupled to a transmit signal then emitted by the antenna .
3) R4 is the base bias resistor V1 , to provide a transistor base current , so that V1 in the enlarged area .
4) R5 DC feedback resistors , transistors play the role of a stable operating point .
5 ) the external microphone MIC collecting an audio signal .
6 ) resistors R3 to provide a DC bias MIC , the greater the value of R3 , microphones capture sound weaker sensitivity , the higher the resistance the smaller the microphone sensitivity.
7 ) microphone to capture the sound signal through AC coupling and R2 C2 after the match to the transistor base .
8 ) in the circuit two diodes D1 and D2 reverse parallel , mainly from a bidirectional limiter function diode conduction voltage of only 0.7V, if the signal exceeds the voltage will be 0.7V shunt diode conduction , which can ensure sound You can limit the amplitude of the signal between positive and negative 0.7V, the sound signal is too strong over-modulation transistor will generate sound distortion can not even work properly.
9) CK is an external signal output jack , TV headphone jack or Walkman can be an external audio source , such as the introduction of FM transmitter headphone jack via a dedicated cable, external sound signal attenuation through R1 and D1, D2 after clipping to frequency modulation transistor base .
10 ) to light emitting diode D3 is used to indicate the working state , when the FM microphone electrical work will be lit , R6 is a light-emitting diode limiting resistor. C8, C9 is the power supply filter capacitor, because generally use a large capacitor winding production process , it is the equivalent inductance is relatively large , in parallel with a small capacitor C8 can make the power of the high frequency resistance.
11 ) circuits K1 and K2 is a switch, which has three different positions , when the switch off the power when the leftmost , rightmost is K1, K2 FM microphone is turned on to make use of the intermediate position is turned K1 , K2 off do use a wireless repeater , because it uses a wireless repeater microphone does not work, but the microphone will consume some quiescent current, so disconnected K2 can reduce power consumption to extend battery life.
The capacitance between the emitter electrode and the FM achieved by changing the base of the transistor when the voltage signal applied to the voice of the base of the transistor when the transistor between the base and the emitter capacitance will occur as the size of the sound signal is synchronized with the voltage change the transmit frequency while the transistor is changed to achieve frequency modulation.

Note (pcb polyline figure is a built-in antenna part , after the design is completed with the antenna does not require a simple debugging after work it! Transmitting distance is not ideal if feel may be appropriate to increase the supply voltage The circuit 9018 within the withstand voltage of 12V ; circuit can also zoom in one level modulation , the transmitter can increase the effective distance of FM radio receivers can be used instead) ! ! ! ! ! !