FM Radio circuit is the simple circuit that can be tuned to the required frequency locally. This article describes the circuit of the FM radio circuit. This is a pocket-sized radio circuit.
FM Radio Circuit Principle:
Radio is the reception of electromagnetic wave through the air. The main principle of this circuit is to tune the circuit to the nearest frequency using the tank circuit. Data to be transmitted is frequency modulated at the transmission and is demodulated at the receiver side. Modulation is nothing but changing the property of the message signal with respect to the carrier frequency. The frequency range of the FM signal is 87.5MHz to 108.0MHz. The output can be heard using the speaker.
FM Radio Circuit Diagram:
- LM 386 IC.
- BF 494 transistor T1, T2.
- Variable resistor.
- Variable capacitor.
- Inductor coil.
FM Radio Circuit Design:
The FM Radio circuit mainly consists of LM386 IC. This is a low voltage audio power amplifier. It has 8 pins. It operates at a supply voltage of 4-12 volts. It has an op-amp internally, which acts as an amplifier. The non-inverting pin is connected to the variable resistor of 10KOhms. Inverting pin of the LM386 IC is connected to the ground. The sixth pin is connected to the VCC. The fourth pin is connected to the ground. The fifth pin is output and is connected to the capacitor which is connected to the speaker or microphone. Another capacitor is connected to ground pin. The sixth pin is the supply pin connected to the supply voltage. This amplifies the incoming frequency modulated signal.
BF494 is an NPN RF transistor. Initially, it is open circuited. It starts conducting only when the base gets the required cut off voltage. The base of the transistor is connected to the base of the variable resistor through a capacitor of 0.22uF. Emitter pin is connected to the ground. The collector is connected to the tank circuit. The base of transistor Q2 is connected to the tank circuit. Emitter pin is connected to the ground and collector is connected to the supply through a resistor of 22K ohms. The variable resistor controls the volume to the input amplifier. These transistors are used for detecting the frequency modulated signals.
The output of the IC is connected to the headphones or Mylar speaker through a capacitor of 220uf 25v rated. The headphone or speaker will have two wires out. One is connected to the output of the capacitor and the other pin is connected to the ground pin.
Tank circuit consists of a coil and a variable capacitor.T his is connected to the antenna. This is the main part of the circuit as it tunes the radio to the required local frequency. In this tank circuit coil plays the main role. The coil is a copper wire wind into a fixed 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 the incoming signal is matched sound can be heard through the headphone or speaker.
- Now tune the circuit to another frequency using the 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.