Simple FM transmitter Circuit

 Simple FM transmitter Circuit

This is a circuit diagram of the FM transmitter which constructed very easily. This FM Transmitter circuit needs very less components. This circuit is based on two NPN transistors of 2N2222 which is a low-power amplifier. At the first transistor stage, the input signal is preamplified and then the transistor T2 will Amplify the signal as a power amplifier and then amplified the signal to be modulated and transmitted through output as an antenna. Use 9 or 12 v DC regulated power supply, Regulated power supply needed because oscillation affects the frequency at the output as power variation occurs in supply.
This circuit construction is so easy and very portable to operate. The use of this transmitter is only for educational purposes.

Circuit diagram of small FM Transmitter

FM transmitter circuit diagram

components

  • Resistors ¼ watt (220Ω-1;  1kΩ -2; 2.2kΩ; 10kΩ-1;  15kΩ; 6.8kΩ;  4.7kΩ -2)
  • Electrolytic capacitors 16V (2.2µF; 10µF x 2)
  • Capacitors ceramic (0.1µF x 2; 4.7pF)
  • Variable capacitor (50pF)
  • Inductor (0.1µH)
  • Transistors (2N2222 x 2)
  • Condenser mic

Working

The signal is amplified by transistor Q1 which is come from the condenser mic. Here the signal is preamplified. , Transistor 1 acts as a preamplifier. The sound pass-through mic is amplified by transistor 1. The amplified signal is then sent to the base point of Q2 through the capacitor and resistor. And collector is connected by coil and capacitor with an antenna.

Two resistors connected as biasing providing the base of transistor1. An 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.

An oscillator circuit built using Q2, C7, and the inductor that operates in the 80 to 130 MHz range, which is basically a voltage controlled oscillator and that frequency can be modulated by the input audio signal. Inductor L2 and variable capacitor C7 forms a tank circuit required for making oscillations. Capacitor C6 is the feedback capacitor for providing the required output power signal.

 Resistor R4 connected to control the input RF signal to the oscillatory section. you can use a variable resistor for this.

value of inductor design for FM transmitters

By using the following formula inductor can be designed
L = (d²n²) / (18d+40l)
where,
l – Inductance of the coil in µH
d – Coil diameter in Inches
S – Coil length in Inches and
n – Number of turns
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