Fm transmitter is always an interesting circuit idea .This circuit provides the fm frequency signal about 80 to 115 mhz. You can receive the signal of this transmitter on receiver at distance around about 200 meters. This is a small range portable fm transmitter circuit which requires very few components.,this makes FM transmitter circuit is very simple and portable. This can be operate at 9v DC supply voltage.
This circuit requires two npn BC 546 transistors and some other components. This is low power and so simple circuit of FM transmitter. BC 546 transistor is connected with the microphone , the signal from mic is fed to base of transistor T1 . And this signal to be amplified and then it is passed to base of T2 , that is connected with collector of T1 through capacitor and emitter of T2 is connected with ground through resistor. The Transistor T1 is connected as the preamplifier which is preamplified the signal which is given by mic. And Transistor T2 portion operates as the power amplifier of signal and inductor and capacitor modulated the signal which is amplified by transistor. Use a proper antenna to maintain the maximum capacity of range og transmitting the signal. signal can be set by tunning of variable capacitor (trimmer) C7. A non metal screw is suitable to tunning the trimmer because it does not affects the frequency by touch the terminals.
This small FM transmitter circuit is very easy to construct on a general purpose pcb board . connect the all components as shown in circuit diagram and give a supply of 9v regulated DC.
Test on any FM receiver on fm tunning by the adjusting of frequency by trimmer.
Although it is low-power, it might be illegal to regular transmit. Always use battery for power this ckt because ac-dc adaptor can make noise like humm. 9v battery is better or 9 v voltage regulator use for 9v at 12 v main input before 9v.