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A simple and easy to make FM transmitter

Published on Jul 19 2010 // FM Transmitter Circuits


A simple and easy to make FM transmitter

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 , so that 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.