FM Radio Circuit

FM Radio Circuit is an electric brain for the machine.

FM radio circuit is small electric system.

It catches FM radio waves and changes to sound we can hear.

This circuit use parts like antenna, mic, speaker, and some wires which is used in many radios to listen music or voice.

Below are few type of radios with their qualities:

Radio:

• Radio is an electric device which makes sound using radio waves.
• Radio uses antenna to catch some signals.
• It find which signal is needed it leave other signals.
• Then it makes signal strong and changes to voice or music again.

FM Radio:

• FM radio is very common way to send sound by waves.
• Many other tools also use FM like hospital machines, radar, walkie talkie.
• FM waves are better than AM waves because it is with less noise and problem.
• On radio FM work between 88 to 108 MHz.
• FM give better sound than AM and catches FM stations and play good sound.

AM vs FM:

• FM send more sound types than AM.
• AM send around 15 kHz and FM send around 4.5 kHz.
• FM radio sound is more clear and nice while FM has bigger range for sound.

Circuit Working:

Parts List:

This is FM radio circuit and it uses few parts to catch local FM stations.

Main parts are transistor BF495 T2, 10k resistor R1, coil L, small variable capacitor VC 22pF and transistor BF495 T1.

These parts make Colpitts oscillator.

Trimmer VC helps to set the station between 88 to 108 MHz.

Capacitor C1 220nF take sound signal from resistor R1 and send it to sound amplifier.

Trimmer 22pF is good because we can easily change its value from few pF to 20pF.

Coil L is small air coil with 4 mm wide and is made with 4 turns of 22 SWG copper wire.

Capacitors C3 100nF, C6 100µF 25V and resistor R3 1k makes band pass filter.

This helps to remove high signal and keep only low sound signal.

IC LM386 is used for sound power and it make 1 to 2 watts sound which are good for small speaker.

Pin 5 of LM386 give sound output.

Volume is changed using 22k VR which is the volume knob at pin 3.

Circuit works with 6V to 9V battery.

How good radio works depends on coil L, antenna type and how far FM station is.

Formulas:

This formula show how LM386 amplifier make signal louder (gain).

It discuses about what happen when we add or do not add capacitor between pin 1 and pin 8.

Formula (No Capacitor):

Gv = 2 × 15K / (150 + 1350)

• Gv mean voltage gain for how much amplifier increase the input sound signal.
• 15K is inside resistance of IC.
• 150 and 1350 are resistance impedance between pin 1 and pin 5/8.

When no capacitor is used then gain is around 20 times.

In decibels: Gain (dB) = 20 × log(Gv) = 26 dB

With Capacitor:

New formula:

Gv = 2 × 15K / 150

Now we put capacitor between pin 1 and 8.

Capacitor act like shortcut for low resistance so 1350 ohm is removed.

Total resistance becomes smaller with 150 ohm only.

This make gain go up to 200 times.

In decibels: Gain = 46 dB

Important to Know:

Adding capacitor between pin 1 and 8 makes gain much higher.

Good for weak sound and makes it louder.

But high gain can also bring more noise and distortion.

For correct capacitor value check LM386 datasheet.

It tell what value gives what gain.

How to Build:

To build a FM Radio Circuit follow the below mentioned steps for connections:

Colpitts Oscillator:

• Connect coil L, 10k resistor R1 and 22pF variable capacitor VC to transistor BF495 T1.
• Make coil L with 4 turns of 22 SWG enameled copper wire and the coil should be 4 mm inside.
• Use 22pF trimmer as VC which will helps to tune station between 88 to 108 MHz.
• Turn the trimmer slowly to catch correct radio station.

Modulation and Signal:

• Use capacitor C1 220nF to take signal from resistor R1 and send it to audio amplifier.
• Connect pin 3 of IC LM386 to a 22k potentiometer VR to control volume.
• Pin 5 of IC LM386 gives louder sound output.

Band Pass Filter:

• Use capacitor C3 100nF, C6 100µF 25V, and resistor R3 1k to make band pass filter.
• This filter remove high frequency and keep low sound frequency and it helps with sound to come clear.

Connect Antenna:

• Connect antenna to the circuit.
• We can use 60 cm copper wire or a telescopic antenna.
• Try different wire length to see what gives better signal.

Power Supply:

• Use 6V to 9V battery to give power to the circuit.

Testing Circuit:

• After building the circuit turn ON the battery.
• Turn the VC trimmer slowly to tune FM station between 88 to 108 MHz.
• Use VR (volume control) to check sound volume.

Important Notes:

• Check all soldering and wire connections are correct.
• If circuit is not working well try small changes or use other parts.
• Do not use too high voltage or it may break the parts.
• This small FM radio circuit is good for learning and for local FM stations.

Conclusion:

In this FM Radio Circuit the modern FM radios use ICs that do many jobs in one chip.

Some circuits also use AFC (automatic frequency control) to keep signal stable when station is weak or there is noise.

References:

Radio receiver