Radio Circuit Diagrams

FM Radio Circuit

The FM Radio circuit is a basic circuit that can be locally tuned to the desired frequency. The FM radio circuit is described in this article. This is a radio circuit that fits in your pocket.

Outline

  • FM Radio Circuit Principle:
  • FM Radio Circuit Diagram:
    • FM Radio Circuit Design:
    • How to Operate FM Radio Circuit?
    • FM Radio Circuit Applications:
      • Limitations of the Circuit:

FM Radio Circuit Principle:

The receiving of electromagnetic waves through air is known as radio. The primary premise of this circuit is to use the tank circuit to tune it to the nearest frequency. The data to be sent is frequency modulated during transmission and demodulated at the receiving end. Modulation is the process of altering the message signal’s properties in relation to the carrier frequency. The FM signal has a frequency range of 87.5MHz to 108.0MHz. Speakers can be used to hear the output.

FM Radio Circuit Diagram:

FM Radio

Circuit Components:

  • LM 386 IC.
  • BF 494 transistor  T1, T2.
  • Variable resistor.
  • Variable capacitor.
  • Inductor coil.

FM Radio Circuit Design:

The LM386 IC is used extensively in the FM Radio circuit. This is a low-voltage power amplifier for audio. It has a total of eight pins. It requires a supply voltage of 4 to 12 volts to work. Internally, it has an op-amp that works as an amplifier. The variable resistor of 10KOhms is attached to the non-inverting pin. The LM386 IC’s inverting pin is wired to ground. The VCC is connected to the sixth pin. The ground pin is attached to the fourth pin. The fifth pin is output, and it’s connected to a capacitor that’s connected to a speaker or microphone. The ground pin is connected to another capacitor. The supply pin, which is coupled to the supply voltage, is the sixth pin. The incoming frequency modulated signal is amplified in this way.

A NPN RF transistor is the BF494. It is initially open circuited. Only when the base reaches the required cut-off voltage does it begin to conduct. A 0.22uF capacitor connects the base of the transistor to the base of the variable resistor. The ground pin is connected to the emitter pin. The tank circuit is connected to the collector. The tank circuit is connected to the base of transistor Q2. The emitter pin is connected to ground, and the collector pin is connected to the supply through a 22K ohm resistor. The loudness of the input amplifier is controlled by the variable resistor. The frequency modulated signals are detected using these transistors.

The IC’s output is connected to the headphones or Mylar speaker through a 220uf 25v rated capacitor. Two wires will come out of the headphone or speaker. One pin is connected to the capacitor’s output, while the other is connected to the ground pin.

A coil and a variable capacitor make up the tank circuit.

The antenna is connected to this. This is the circuit’s most important component, as it adjusts the radio to the required local frequency. The circuit coil is a key component of this tank. A coil is a copper wire that has been wound into a specific number of turns.

How to Operate FM Radio Circuit?

To operate the FM Radio circuit following steps are to be followed:

  • Initially connect the circuit as shown in the figure.
  • Connect the power supply to the circuit.
  • Now press the switch.
  • Now adjust the frequency of the circuit by varying the trim pot (variable capacitor) of the tank circuit.
  • Thus the circuit starts tuning to the nearest frequency.
  • When the frequency of incoming signal is matched sound can be heard through the headphone or speaker.
  • Now tune the circuit to the  another frequency using tank circuit.
  • You can listen another sound incoming at that particular frequency.
  • Adjust the variable resistor to increase or decrease the volume.

FM Radio Circuit Applications:

  • The circuit acts as a pocket sized radio, by tuning to the particular frequency.
  • With a little modification it can be used in applications of voice transmission.

Limitations of the Circuit:

  • This is a theoretical circuit and it requires some changes to implement it practically.

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