Do you want to make a simplest and cheapest fm radio transmitter yourself ? Here is the article to teach amateurs step by step do it yourself DIY fm transmitter kit. With this DIY kit, you can broadcast your audio over an ordinary FM radio within the FM broadcast band. Below are the steps, follow me now.
1) high-frequency transistor V1 and capacitor C3, C5, C6 form a three-point capacitance oscillator
2) C4, L form a resonator: the resonant frequency is the transmission frequency of the FM microphone, according to the parameter transmitting frequency of the element can be in the range of 88 ~ 108MHZ, just covering the reception frequency of the FM radio, by adjusting the value of L (pull stretched or compressed coil L) can easily change the transmitter frequency to avoid the FM radio. Through C4, the transmit signal coupled to the antenna and then emitted.
3) R4 is a base bias resistor and to the transistor V1 is to provide a certain amount of base current, so that V1 in the enlarged area.
4) R5 is a DC feedback resistor, to play a role in stabilizing the transistor operating point.
5) microphone MIC collecting outside sound signal.
6) resistor R3 provides DC bias for the MIC, the greater the resistance of R3, the sensitivity of the microphone sound collecting weaker, the higher the smaller the resistance sensitivity of the microphone.
7) exchange of voice signals collected by the microphone by the the C2 coupling and R2 after the match to the transistor base.
8) circuits D1 and D2 are two diode reverse parallel, mainly from a bidirectional limiter, the diode conduction voltage of only 0.7V, if the signal voltage exceeds 0.7V will shunt diode conduction, this can ensure the sound the amplitude of the signal can be limited to plus or minus 0.7V too strong voice signal causes the transistor modulation, sound distortion can not even work properly.
9) CK external signal output socket, headphone jack of the TV or Walkman headphone jack, external sound source through the introduction of a dedicated cable FM transmitter, external sound signal attenuation through R1 and D1, D2 limiter sent The transistor base is frequency-modulated.
10) circuit in the light-emitting diode D3 is used to indicate the working status will light up when the FM microphone was electrical work, R6 is a light-emitting diode current limiting resistor. C8, C9 is the power supply filter capacitor, because the large capacitor winding process commonly used to produce the equivalent inductance parallel with a small capacitor C8 can make power high-frequency resistance.
11) circuit in the K1 and K2 is a switch, which has three different positions, disconnect the power when appropriated leftmost and rightmost K1, K2 turned do FM microphone, an intermediate position is turned K1, K2 off do wireless repeater, because it is wireless transponder use the microphone does not work, but the microphone will consume a quiescent current, so disconnected K2 can reduce power consumption and extend battery life.
By changing the base of the transistor and the capacitance between the emitter to achieve FM, when the the sound voltage signal applied to the base of the transistor on, the base of the transistor and the capacitance between the emitter with the size of the sound voltage signal occurs synchronized change, while the emission frequency of the transistor to change to achieve frequency modulation.
Transmitter finished PCB diagram
Note (the pcb diagram polyline part within the system antenna, after the completion of the design do not need to match the antenna. Simple debugging after working sleep unsatisfactory transmitting distance may be appropriate to increase the power supply voltage of the circuit 9018 can withstand a voltage of 12V or less; You can also add a modulation amplifier circuit can effectively increase the transmission distance!!! available FM radio receiver instead!!!)
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