Most simple FM transmitter circuit diagram

This is the most simplest and single transistor FM wireless transmitter circuit that ever posted in CircuitsGallery. In telecommunication field, frequency modulation (FM) transmits information by changing the frequency of a carrier wave according to message signal. FM uses VHF radio frequencies usually 87.5 – 108.0 MHz to transmit and receive the FM signals.

Here I will give you the explanations about FM transmitter, how to make a FM transmitter and how a simple FM transmitter is designed and assembled etc. The performance and working of a wireless audio transmitter circuit mainly depends on inductor coil specification and the value of variable capacitor. Even a slight change of inductance or capacitor can shift the harmonic frequency from the VHF (88-108 MHz) band.  This article will enable you to answer “how to build an FM radio transmitter?”. Also you will get the confidence to build your own FM transmitter.

FM transmitter Circuit diagram

Components required

1. Power supply (9V)
2. Resistors ¼ watt (4.7k Ω; 470Ω)
3. Capacitors (0.001µF; 4.7µF; 0.022µF)
4. Trimmer capacitor (10-100pF)
5. Inductor (0.1µH) (6-7 turns using 26 guage copper wire)
6. Transistor (2N3904)
7. Condenser Mic
8. Antenna (Copper wire of 20 cm long and 24 gauge)

Working of FM transmitter schematic

• A condenser microphone is used to accept the sound signals. Inside the mic, a capacitive sensor diaphragm is present. It vibrates according to the air pressure changes and generates AC signals.

• The inductor L1 and variable capacitor (trimmer) forms an oscillating tank circuit along with the transistor 2N3904. It is the common NPN transistor used for general purpose amplifications.

• As long as the current exists across the inductor coil L1 and the variable capacitor, the tank circuit will oscillate at the resonant carrier frequency for FM modulation. Capacitor C2 acts as a negative feedback to the oscillating tank circuit.

• Every FM transmitter circuit requires an oscillator part to generate the radio Frequency (RF) carrier waves. The name ‘Tank’ circuit is derived from the capacity of the LC circuit to store energy for oscillations.

• The input audio signal from the mic is fed to the base of transistor which modulates the LC tank circuit carrier frequency in FM format.

• The variable capacitor is used to change the resonant frequency for fine adjustment to the FM frequency band.

• The modulated signal from the antenna is radiated as radio waves at FM frequency band. Antenna is nothing but a simple copper wire of 20 cm long and 24 gauge.

• The length of the antenna is very significant in the FM transmitter circuit. Here you can use a 25-27 inches long copper wire as antenna.

Design of Inductor for FM transmitter

Our aim is to resonate inductor at 88-108MHz band FM frequency.

The length, inner diameter, number of turns etc. are the important factors to be considered while inductor designing. You can design inductor using the following formula,

where,

L - Inductance of the coil in µH
d - Coil diameter in Inches
S - Coil length in Inches and
n - Number of turns

[Solved] FM Transmitter is not working! Trouble shooting of FM transmitter circuit

• Use a battery operated FM pocket radio to receive the radio signal transmitted via the FM transmitter circuit.

• Place the FM radio half meter away from the transmitter and tune to 80- 108MHz range.

• Produce a sound near to the mic, if the tank circuit is properly tuned the sound will be heard in the radio receiver.

• Adjust the shaft of the trimmer and check again whether the sound is clear than previous case.

• If the sound clarity is fine, stick the inductor coil with glue to avoid further frequency variations.

• Now your best FM transmitter is ready to use.

Related radio circuits

• Short range portable FM transmitter circuit
• FM Transmitter using VMR6512 Hi-Fi Audio FM transmitter module

Pinout of 2N3904 & other components

• 2N3904 transistor pinout
• Variable capacitor trimmer
• Condenser Mic
• Inductor

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