High Range FM Transmitter Circuit using 2N3904 Transistors

Contents hide

1 What is a High Range FM Transmitter?

2 Hardware Components

3 2N3904 Pinout

4 FM Transmitter Circuit

5 Working Explanation

6 Applications

What is a High Range FM Transmitter?

An FM Transmitter is an electronic device that can air audio signals from a source on a selected range of frequencies. They are generally used to broadcast stationary audio from devices such as TV & computers, around a room. They are inexpensive & simple to design. So, in today’s article, we will look into a step-by-step tutorial on how to make a High Range FM Transmitter circuit using some NPN transistors & a small no. of other components.

Whatever may be the schematic scope or design, every FM transmitter is based on the same three key stages: Audio pre-amplification, Modulation, and, transmission. The first stage generally consists of amplifying & filtering an input audio signal. In the second stage, the input audio signal is modulated and a carrier signal is generated by a variable frequency oscillator circuit. In the last stage, the modulated audio signal is transmitted using an antenna between the FM frequency range of 88MHz to 108MHz.

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5$ PCB Prototype

Hardware Components

The following components are required to make FM Transmitter Circuit

S.no	Component	Value	Qty
1.	FM Transmitter PCB	–	1
2.	NPN Transistor	2N3904	2
3.	Condenser mic	5mm	1
4.	Variable Capacitor	40pF	1
5.	Inductor	0.1uH	1
6.	Capacitor	0.1uF, 0.01uF, 4.7pF	3
7.	Enameled copper wire	18 AWG	1.5ft
8.	Resistor	1M, 100K, 10K, 1K, 100 Ohm	7
9.	Soldering Iron	45W – 65W	1
10.	Soldering Wire with Flux	–	1
11.	DC Battery	9V	1
12.	Battery Clips	–	1
13.	Jumper Wires	–	As per need

2N3904 Pinout

For a detailed description of pinout, dimension features, and specifications download the datasheet of 2N3904

FM Transmitter Circuit

Working Explanation

The working of this circuit is pretty simple, an input audio signal from a device such as a smartphone is taken through an electret mic. Here, a coupling capacitor of 0.1uF is used to block the DC component of the signal, allowing only AC to flow. This signal acts as a control signal to the base of the 2n3904 transistor (Q1).

The amplified collector output is then sent to the base of the Q2 transistor. The power of the collector output is then modulated & tuned using a trimmer capacitor & is then adjusted to the frequency range of the FM receiver circuit using an LC combination, which is then broadcasted using an 18 AWG coiled antenna.