Simple FM Transmitter Circuit (100 meters range)

Want to make your own mini radio station?

This post shows you how to build a simple FM transmitter that works between 87.5 and 108 MHz on the FM dial.

This transmitter uses a simple circuit with a special transistor and an antenna to broadcast your signal.

It is different from other small transmitters because it does not need a tuning knob.

Instead it uses a special part to change the frequency a little bit.

This makes it easier to tune in your radio to hear what you are broadcasting.

WARNING: The laws governing broadcasting are complicated and might differ from nation to nation.

Building and using this transmitter without a license might be illegal.

What is a FM Transmitter Circuit:

A Modulation of Frequency An electronic device known as an FM transmitter circuit uses frequency modulation to create and broadcast radio frequency signals.

With FM modulation the carrier signals frequency is adjusted in proportion to the input signals amplitude.

In contrast the carrier signals amplitude changes in amplitude modulation (AM).

Circuit Working:

BF970 Pinout Diagram:

Parts List:

The transistor acting as an oscillator generates a continuous radio frequency signal.

The frequency is determined by the resonance of the LC circuit formed by inductor L1 and the varicap diode.

The varicap diode introduces frequency modulation to the signal.

As the reverse DC voltage applied to the varicap changes controlled by trimmer P1, the capacitance of the varicap varies leading to changes in the frequency of the transmitted signal.

During the tuning process the initial adjustment involves manipulating the turns in the coil L1 to get close to the desired operating frequency.

Trimmer P1 is then used for fine tuning allowing precise adjustment of the frequency for optimal transmission.

An audio source such as a cassette player or MP3 player is connected to the input of the transmitter.

The audio signal modulates the frequency of the radio signal enabling the transmission of audio content.

The antenna a 7cm insulated copper wire is directly connected to the oscillator radiating the modulated signal.

The simplicity of the circuit design allow for a range of approximately 100 meters.

The circuit can be powered from a 5V USB port a 5 to 12V power supply or a battery.

How to Build:

Below mentioned are the steps for constricting the circuit:

• Place the silicon planar RF PNP transistor on the breadboard.
• Connect the collector, base and emitter pins of the transistor to the appropriate locations on the breadboard.
• Integrate the varicap diode BB105 and trimmer P1 into the circuit connecting them as per the circuit diagram.
• Attach the varicap diode to the circuit ensuring proper connections.
• Develop a connection between the varicap and trimmer P1 allowing for voltage tuning.
• Wind six turns of 0.5mm diameter wire around a 3mm diameter core to create inductor L1.
• Connect the inductor to the circuit ensuring a secure and stable connection.
• Use a 7cm insulated copper wire 1mm diameter as the antenna.
• Connect the antenna directly to the oscillator circuit.
• Choose a power source such as a 5V USB port a 5 to 12V power supply or a battery.
• Connect the chosen power source to the circuit.
• Set trimmer P1 to the center position.
• Turn on an FM radio and select an unoccupied frequency within the 87.5 to 108 MHz range.
• Power on the transmitter and roughly adjust the operating frequency by manipulating the turns in coil L1.
• Fine tune the frequency using trimmer P1 until the radio becomes silent.
• Connect an audio source eg cassette player, CD/MP3 player, record player, PC/laptop audio output to the input of the transmitter.

Testing:

• Ensure all connections are secure and double check the circuit.
• Power on the transmitter and the audio source.
• Tune the FM radio to the set frequency to hear the broadcasted audio.

Considerations:

Be careful of supply voltage changes affecting varicap voltage and consequently the operating frequency.

The antennas direct connection to the oscillator makes the circuit sensitive to touch and nearby conductive objects.

Another 100 meter Transmitter Circuit using BC547 Transistor:

Parts List:

Introducing an innovative wireless FM transmitter circuit which is basically a small RF transmitter that is carefully built around a single transistor.

This circuit functions similarly to a Colpitts oscillator incorporating a tank circuit that is essential for producing the necessary oscillations.

The position and values given to the inductor, C1, C2 and C3 components are closely related to the frequency output.

By carefully adjusting the coil turn, the distance and diameter of the FM receivers response should be adjusted.

A tiny antenna in the form of a 3 inch wire is connected exactly at the desired location to improve responsiveness and ensure the generation of distortion free transmissions.

By enhancing overall performance this addition turns the gadget into a trusted and extremely effective tool.

Formulas:

Follow the below mentioned formulas for designing a Simple FM Transmitter Circuit (100 meters range):

Assemble the required components such as transistor, inductor and capacitors C1, C2, C3.

Place carefully the inductor and by modifying the values of C1, C2 and C3 one can design the tank circuit.

Following formulas are used to calculate the frequency

f = 1 / 2π√LC

where:

• f is the frequency
• π is a mathematical constant which is about 3.14159)
• L is the inductance
• C is the capacitance.

Adjust the coils diameter and turn distance to improve the response over the FM receiver.

Changes are made using the formula below:

f_adjusted = f_initial × (d_adjusted / d_initial) × (D_adjusted / D_initial)

where:

• f_adjusted is the adjusted frequency
• f_initial is the initial frequency
• d_adjusted​ is the adjusted coil turn distance
• d_initial is the initial coil turn distance
• D_adjusted​ is the adjusted coil diameter
• D_initial is the initial coil diameter.

Installing a 3-inch wire antenna at the chosen place will improve alertness and signal quality.

The transmitters performance improves significantly by this improvement.

To achieve the best results in producing distortion-free signals one can develop an entirely new wireless FM transmitter circuit.

How to Build:

The steps to construct the wireless FM transmitter circuit are as follows:

• On the PCB place the transistor T1.
• Solder the transistors emitter, base and collector terminals to the right places on the printed circuit board.
• Make sure the inductor L1 is connected correctly before adding it to the circuit.
• Connect capacitor C1 between the collector and base of the transistor.
• Wire capacitor C2 between the base and ground.
• Connect capacitor C3 between the collector and emitter of the transistor.
• The inductor L1 and capacitors C1, C2, C3 together form the tank circuit are responsible for generating oscillations.
• To get the required frequency change the values and position.
• To improve the FM transmitters quickness adjust the coils diameter and turn distance.
• Use the given formulas to calculate modified frequencies in light of the adjustments.
• At the chosen point in the circuit connect a 3-inch wire to act as the antenna.
• This increases signal quality and responsiveness.
• Add a suitable power source or a 3V battery to the circuit.

Testing

• Test the wireless FM transmitter and adjust an FM receiver to the desired frequency.
• Make sure there is no distortion and that the signals are sent clearly.
• Experiments and a desired operating frequency are used to make improvements.

Conclusion

Its important to remember the rules for controlling the operation of FM transmitters.

One should follow Simple FM Transmitter Circuit carefully and respect the local laws for governing the FM broadcasting.

References:

DESIGN AND CONSTRUCTION OF FM TRANMITTER

Radio transmitter design