Simple FM Transmitter Circuit (100 meters range)

Is anyone Interested in starting its own little radio station?

This post shows 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 the radio to hear what is 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 regards to the input signals amplitude.

On the other hand amplitude modulation (AM) modifies the carrier signals amplitude.

Circuit Working:

BF970 Pinout Diagram:

Parts List:

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

The frequency is measured 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 differ 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 better 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 around 100 meters.

The circuit is 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 Simple FM Transmitter Circuit (100 meters range):

• On the breadboard place the silicon planar RF PNP transistor.
• Connect the transistors emitter, base and collector pins to the correct spots on the breadboard.
• As shown in the circuit design add the trimmer P1 and varicap diode BB105 into the circuit.
• Be sure all the connections are correct when one attaches the varicap diode to the circuit.
• Create a voltage tuning connection between the trimmer P1 and the varicap.
• Inductor L1 is made by winding six rounds of 0.5mm wire around a 3mm core.
• Ensure the connection between the inductor and the circuit is strong and secure.
• For the antenna use a 7 cm length of 1 mm diameter insulated copper wire.
• Connect the oscillator circuit directly to the antenna.
• Select a power supply such as a battery, a 5 to 12V power supply or a 5V USB port.
• Connect the circuit to the chosen power source.
• Place trimmer P1 in the middle.
• Switch up an FM radio and pick an open frequency between 87.5 and 108 MHz.
• Turn on the transmitter and use coil L1s turns to roughly change the operational frequency.
• Use trimmer P1 to fine tune the frequency until the radio stops playing.
• Connect an audio source such as a record player, CD/MP3 player, cassette player or PC/laptop audio output to the transmitters input.

Testing:

• Ensure all connections are safe and verify the ones again the circuit.
• Power on the transmitter and the audio source.
• Tune the FM radio to the set frequency to hear the broadcasted audio.

Considerations:

Keep in mind that changes in the supply voltage may affect the varicap voltage and also the operating frequency.

Because of the antennas direct connection to the oscillator the circuit is sensitive to touch and conductive objects nearby.

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 including 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 reaction and ensure the generation of distortion free transmissions.

By improving overall performance this changes makes the device a reliable and very powerful instrument.

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:

• The modified frequency is called fadjusted.
• The starting frequency is called the finitial.
• The modified coil turn distance is called dadjusted.
• The initial coil turn distance is called the dinitial.
• The modified coil diameter is called dadjusted.
• The initial coil diameter is called dinitial.

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

The transmitters performance improves significantly by this changes.

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 response.
• 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.
• Be 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