Simple FM Transmitter Circuit (100 meters range)

Want to make your own mini radio station?

This guide 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 basic 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: Broadcasting regulations are complex and vary by country.

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

What is a FM Transmitter Circuit:

A Frequency Modulation FM transmitter circuit is an electronic device that generates and broadcasts radio frequency signals using Frequency Modulation.

FM is a type of modulation where the frequency of the carrier signal is varied in proportion to the amplitude of the input signal.

This is in contrast to Amplitude Modulation AM, where the amplitude of the carrier signal is varied.

Working of the Circuit:

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.

Construction Steps:

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.
• Establish 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 chosen frequency, and you should hear the broadcasted audio.

Considerations:

Be cautious 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:

Unveiling an innovative wireless FM transmitter circuit this design is fundamentally a compact RF transmitter meticulously constructed around a singular transistor.

This circuit operates akin to a colpitts oscillator seamlessly integrating a tank circuit crucial for generating the requisite oscillations.

The frequency output is intricately tied to the positioning and values assigned to the inductor, C1, C2 and C3 components.

The optimization of the FM receivers response is achievable by fine tuning the coil turn distance and diameter.

To elevate the responsiveness and ensure the production of distortion free signals a diminutive antenna manifested as a 3 inch wire can be affixed precisely at the designated point.

This augmentation enhances the overall performance transforming the device into a highly efficient and reliable bug.

Formulas:

Assemble the components including the transistor, inductor and capacitors C1, C2, C3.

Configure the tank circuit by strategically positioning the inductor and adjusting the values of C1, C2 and C3.

Utilize the following formula to calculate the frequency:

f = 1 / 2π√LC

where:

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

Fine tune the coil turn distance and diameter to optimize the response over the FM receiver.

Adjustments can be made based on the following formula:

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.

Enhance responsiveness and signal quality by attaching a 3 inch wire antenna at the specified point.

This addition significantly improves the performance of the transmitter.

You can create a unique wireless FM transmitter circuit tailored to deliver optimal results in generating distortion free signals.

How to Build:

Below is a guide on how to build the wireless FM transmitter circuit:

• Place the transistor T1 on the PCB.
• Connect the collector, base, and emitter terminals of the transistor to the corresponding points on the PCB.
• Integrate the inductor L1 into the circuit, ensuring proper connection.
• Connect capacitor C1 between the collector and base of the transistor.
• Connect 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 collectively form the tank circuit responsible for generating oscillations.
• Adjust the values and positioning to achieve the desired frequency.
• Experiment with the coil turn distance and diameter to optimize the FM transmitters response.
• Use the provided formulas to calculate adjusted frequencies based on changes.
• Affix a 3 inch wire as the antenna at the specified point in the circuit.
• This enhances responsiveness and signal quality.
• Connect a 3V battery or an appropriate power supply to the circuit.

Testing

• Tune an FM receiver to the desired frequency and test the wireless FM transmitter.
• Ensure the signals are transmitted clearly and without distortion.
• Adjustments can be made based on experimentation and the desired frequency of operation.

Conclusion

It is important to note that the use of FM transmitters is subject to regulations and users should remember to comply with local regulations regarding FM broadcasting and use this simple FM transmitter circuit responsibly.

References

DESIGN AND CONSTRUCTION OF FM TRANMITTER

Radio transmitter design