This is a simple 2N2222 transistor based radio FM transmitter circuit within the FCC (Federal Communications Commission) limit, that is no license is necessary for using this stable FM transmitter circuit. What is an FM transmitter? FM stands for Frequency Modulation and FM transmitter is a radio frequency (RF) transmitter which uses frequency modulation of audio signals. Frequency modulation means that the frequency of a carrier signal is changed according to the input audio signal.The portable FM transmitter is able to transmit sound or music to any standard radio FM receiver. The range of this transmitter is near to 120 meters. Audio signal is applied via a condenser micro phone, which is a capacitive sound sensor. Simplicity and portability makes this transmitter circuit superior to other FM transmitters. The transmission frequency of this FM transmitter can be adjusted using the variable capacitor C7. The main applications of this circuit includes FM transmitter for car, walky talky, wireless audio transmitter etc.
FM transmitter circuit schematic
- Resistors ¼ watt (220Ω; 1kΩ x 2; 2.2kΩ; 10kΩ; 15kΩ; 6.8kΩ; 4.7kΩ x 2)
- Electrolytic capacitors 16V (2.2µF; 10µF x 2)
- Capacitors ceramic (0.1µF x 2; 4.7pF)
- Variable capacitor (100pF)
- Inductor (0.1µH)
- Transistors (2N2222 x 2)
- Condenser mic
Working of radio transmitter
- The signal from the condenser microphone is passed through a coupling capacitor C1 and amplified by transistor Q1, which acts as a pre amplifier. The amplified signal is applied to the base of transistor Q2.
- R2 and R6 provide necessary biasing to the amplifier transistor Q1 (2N2222). Amplified sound signal will be available at the collector of Q1 and it coupled to the base terminal of transistor Q2 via the capacitor C3 and R4.
- Inductor L2 and variable capacitor C7 forms a tank circuit required for making oscillations. Capacitor C6 is the feedback capacitor.
- Q2, C7 and the inductor forms a RF oscillator circuit that operates in the 80 to 130 MHz range, which is voltage controlled oscillator so its frequency can be modulated by the input audio signal.
- Resistor R4 confines the input to the RF oscillatory section, you can probably use a potentiometer instead of R4 to regulate the volume.
- The modulated FM wave will be obtainable at the collector of Q2 and it is transmitted using the antenna (20 cm long, 24 gauge copper wire).
Inductor value design for FM transmitters
You can design inductor using the following formula,
L – Inductance of the coil in µH
d – Coil diameter in Inches
S – Coil length in Inches and
n – Number of turns