The following wireless microphone circuit schematic diagram is very sensitive, it can pick up every sound in 20 meters radius. It also transmits the radio signal up to 2 kilometres in open air. Therefore, you should pay attention that operating this circuit might violate your country regulation. This FM transmitter radiate strong radio frequency to the environment. Following is the schematic diagram :
The pre-amplifier for the microphone is the first transistor (Q1). When you do not want to transmit the sound picked up by the microphone, you can omit this circuit. You can connect your mp3 player directly to C1 for example. Q2 is the core of this circuit. It is a modified Collpits oscillator which frequency is determined by L1,C4,C6, and the transistor’s internal base-emitter capacitance. To compromize between the efficiency and the size, the antenna use 1/16 wave length. The R1 can be replaced by the one with higher value if you want the microphone to be less sensitive (for example 10k or 22k). This might overcome the feedback problem when this wireless microphone FM transmitter is used for a public address system.