FM Transmitter Circuit

The FM transmitter circuit comprises several components, including resistors, capacitors, a trimmer or variable capacitor, an inductor (coil), a transmitter, a microphone, a 9V power supply (or a 7809 voltage regulator if the input voltage exceeds 9V), and an antenna.  
 

Components and Functionality  

• Microphone: The microphone captures sound signals and incorporates a sensor with a capacitance element. Variations in air pressure or AC signals alter the capacitance, enabling sound signal detection. 
• Oscillation Circuit: The oscillation circuit is built using a 2N3904 transistor, an inductor, and a variable capacitor. The 2N3904 is an NPN transistor commonly used for signal and voltage amplification. When current flows through the inductor (L1) and the variable capacitor, the circuit oscillates at a resonant frequency matching the carrier signal's frequency.
• Capacitor (C2): Negative feedback through the C2 capacitor stabilizes the circuit. The oscillator generates the radio frequency carrier waves necessary for FM transmission.  
• LC Circuit: The inductor and capacitor form an LC circuit that stores energy for oscillation. The audio signal from the microphone is fed into the transistor's base, modulating the LC circuit's output into a frequency-modulated (FM) wave.  

 

Purpose of the Variable Capacitor  
The variable capacitor adjusts the resonant frequency, allowing the circuit to tune into the optimal FM signal frequency band.  

Transmission and Antenna  
The modulated FM signal is transmitted as a radio wave within the FM frequency range. The antenna, a good conductor (e.g., a 30cm long, 26-gauge copper wire), radiates the signal. Antenna length is crucial for effective transmission and can range from 25 to 27 inches.  

This simple FM transmitter effectively demonstrates the principles of frequency modulation and radio wave transmission.