How to Build FM Transmitter Circuit

This two-transistor FM transmitter circuit operates by converting audio input signals into radio frequency electromagnetic waves that can be picked up by a standard FM radio receiver tuned between 88 and 108 MHz. The core of the circuit utilizes two 2N2222 NPN transistors working together to amplify the incoming audio signal and oscillate at VHF carrier frequencies. Audio enters the circuit via an audio jack or a condenser microphone, where it modulates the base of the first transistor. This audio stage provides signal amplification before passing the modulation to the oscillator stage built around the second transistor. An LC tank circuit—formed by an inductor made from Gauge 18-22 copper wire and a 20pF variable capacitor, alongside a 4pF ceramic capacitor—sets the operating radio frequency. The various resistors (ranging from 100 ohms to 1 megaohm) and ceramic capacitors (including 10nF and 100nF values) establish proper DC biasing operating points for the transistors and provide essential power supply decoupling and high-frequency filtering. Powered by a 9V battery and connected via a 9V battery cap, the circuit radiates the modulated RF signal through the copper wire antenna, allowing wireless audio transmission over a short distance.