FM transmitter (FM wireless microphone)
In this project, you are going to make an FM transmitter using a simple circuit. When have a microphone in your hand, you should be tempted to make a speech!
After you finish wiring, set the radio to the FM frequency band and turn the tuning dial to the lowest frequency. Also, set the radio speaker to a suitable volume. Initially, position the radio very close to this circuit.
Turn power ON, and speak to the mic as you slowly rotate the radio's tuning dial toward higher frequencies. You should be hearing your voice from the radio. Because several frequencies are received, turn the tuning dial up to the maximum frequency and then down to a level you can hear best.
Put the radio away from the circuit and you have the wireless microphone set completed.
This circuit is oscillating at high frequency. As you can guess, the frequency will change if you bring your hand close to a bar antenna or wires.
The transistor Q3 has a small junction capacitance "C" inside, which varies with the amount of current flowing into the transistor. With this change of "C", the oscillating frequency of the LC oscillating circuit is modulated so that FM transmission is performed. It oscillates in an FM band of approximately 88 to 108 MHz. The transistors Q1 and Q2 form a circuit that amplifies the voice signal from the earphone.