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.
- 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 process of capturing electromagnetic waves through the air is commonly referred to as radio reception. The fundamental principle of this circuit involves utilizing a tank circuit to finely tune the receiver to the desired frequency. During the transmission phase, the data is modulated onto the carrier frequency and subsequently demodulated at the receiving end. Modulation entails adjusting the characteristics of the message signal in relation to the carrier frequency. In the case of this circuit, it operates within the frequency modulation (FM) signal range of 87.5MHz to 108.0MHz, and the output can be audibly perceived through speakers.
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 makes extensive use of the LM386 IC, which serves as a low-voltage power amplifier for audio. This IC comprises eight pins and requires a supply voltage within the range of 4 to 12 volts to function. Internally, it houses an operational amplifier (op-amp) that acts as an amplifier. The non-inverting pin is connected to a 10KOhm variable resistor, while the inverting pin of the LM386 IC is grounded. VCC is linked to the sixth pin, while the ground pin connects to the fourth pin. The fifth pin serves as the output and is connected to a capacitor that is further linked to either a speaker or microphone. Another capacitor is connected to the ground pin, and the supply pin, connected to the supply voltage, is the sixth pin. This arrangement amplifies the incoming frequency-modulated signal.
The BF494, an NPN RF transistor, remains open-circuited initially, conducting only when the base reaches the required cutoff voltage. A 0.22uF capacitor connects the transistor’s base to the variable resistor’s base. The emitter pin is connected to ground, and the collector pin is linked to the supply via a 22K ohm resistor. The variable resistor controls the input amplifier’s loudness, while these transistors detect the frequency-modulated signals.
The IC’s output is connected to headphones or a Mylar speaker via a 220uF 25V-rated capacitor. Two leads emerge from the headphone or speaker, with one connecting to the capacitor’s output, and the other connecting to the ground pin.
The tank circuit comprises a coil and a variable capacitor, with the antenna connected to it. This component is of utmost importance as it fine-tunes the radio to the desired local frequency. The coil within the circuit consists of copper wire 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.