About: This circuit is a simple two transistor (2N2222) FM transmitter. Work well with a 9 volt battery and an antenna of 10 inches. Please note, according to Indian Wireless rules, transmitter circuit require license. Request you to use the circuit for lab/educational purpose only.
Circuit Diagram
Working: The microphone is amplified by Q1 and the modulating signal is fed to Q2. Q2, C5, and L1 form an oscillator that operates in the 85 to 108 MHz range(The commercial FM Broadcasting frequency). The oscillator is voltage controlled, so it is modulated by the audio signal that is applied to the base of Q2. R6 limits the input to the RF section, and it's value can be adjusted as necessary to limit the volume of the input. L1 and C6 can be made with wire and a pencil. The inductor (L1) is made by winding two pieces of 24 gauge insulated wire, laid side by side, around a pencil six times. Remove the coil you have formed and unscrew the two coils apart from each other. One of these coils (the better looking of the two) will be used in the tank circuit, and the other can be used in the next one you build. The antenna (24 gauge wire) should be soldered to the coil you made, about 2 turns up from the bottom, on the transistor side, and should be 8-12 inches long. To make C6, take a 4 inch piece of 24 gauge insulated wire, bend it over double and, beginning 1/2" from the open end, twist the wire as if you were forming a rope. When you have about 1" of twisted wire, stop and cut the looped end off, leaving about 1/2" of twisted wire (this forms the capacitor) and 1/2" of untwisted wire for leads.
Components:
If 2N2222 is available try with it alternatively you may use SL100.
Also check out
Another Simple FM Transmitter Circuit Using BC 547