Simple Radio Circuit with Headphones

This is a basic radio circuit that works like a crystal radio but with a booster.

It can pick up many strong radio stations nearby with a 10 foot antenna.

A longer antenna can help catch more stations, but it might also pick up unwanted ones.

These unwanted stations can sound faint in the background when you are trying to listen to a weaker station.

To improve choosing the exact station you want, try connecting a long wire antenna to the coil instead of where the capacitor and coil connect.

You also need a good ground connection.

If you are just listening to local stations you might be able to get away with touching the headphone ends to a concrete surface outside.

Circuit Working:

Parts List:

To construct the, inductor wind 200 turns of 28SWG enameled copper wire over a 7 to 8 inch diameter, 4 inch long PVC pipe, resulting in an inductor with a value of approximately 220uH.

Adding terminals to the inductor every 20 turns facilitates optimal connections for the diode and antenna, with the last 60 turns connected to the antenna and ending with the diode.

While a germanium diode is recommended for optimal results silicon diodes can also function well with strong signals.

The carrier frequency is extracted from the rectified signal at the diodes cathode through a 300pF capacitor and the audio frequency is directed through a 0.1uF capacitor into the non inverting input of the relevant op amp functioning as a high impedance buffer stage.

The succeeding op amp stage amplifies the voltage level by approximately 50 times connected to the preceding stage via a 10k resistor.

If the 100k and 1Meg resistors have dissimilar values (1%), it is crucial to either use more closely matched values or include a capacitor in series with the 10k resistor to maintain the transistor emitters DC voltage between 3 and 6 volts.

Alternatively, limiting overall gain with a smaller feedback resistor 470k is another strategy.

High impedance headphones provide an enhanced listening experience although stereo type headphones are also compatible.

The suggested radio circuit draws around 10mA from a 9V battery.

Formula:

Information transmission via AM radios is based on the idea of amplitude modulation, or AM.

Below is the relevant formula with its functions:

Wave Carrier:

The information signal is carried by a high frequency electromagnetic wave called the carrier wave.

The carrier wave formula is:

v(t) = Ac * sin(2πfc * t + φ)

where,

• The carrier waves instantaneous voltage at time t is denoted by v(t).
• The carrier waves amplitude is denoted by Ac.
• The carrier wave frequency, expressed in hertz Hz , is fc, the AM radio broadcast spectrum encompasses this frequency (535 kHz to 1605 kHz on average).
• The carrier waves phase angle, or φ, is commonly set to zero for simplicity.

Signal of Information:

The actual audio material you hear on the radio, like as voice or music, is carried by the information signal.

Usually, the waveform of low frequency voltage is used to depict the fluctuations in audio.

Modulation of Amplitude:

In AM, the carrier waves amplitude varies in direct proportion to the information stream.

The modulated AM wave can be represented mathematically in a variety of ways, but the following is one frequent form:

vAM(t) = Ac * [1 + m * sin(2πfm * t)] * sin(2πfc * t + φ)

where,

• The AM waves instantaneous voltage is denoted by vAM(t).
• The modulation index, or m is a number between 0 and 1 that indicates how much the carrier amplitude has changed from its unmodulated condition.
• Stronger modulation is correlated with higher modulation index.
• The information signals frequency, expressed in hertz Hz, is fm.
• For audio transmission, this frequency range is normally restricted to the range of human hearing, which is between 20 Hz and 20 kHz.

Sidebands:

Around the carrier frequency, two extra frequency components known as sidebands are produced by the AM method. The information from the original signal is carried by these sidebands. These frequencies are provided by:

• Lower sideband frequency: fc – fm
• Upper sideband frequency: fc + fm

Note:

A rudimentary knowledge of the mathematical ideas behind AM radio may be gained from these calculations.

AM transmission and reception really entail more intricate circuitry.

How to Build:

To build the simple radio circuit, follow the connections mentioned below:

Constructing the Inductor:

• Wind 200 turns of 28SWG enameled copper wire around the 7 to 8 inch diameter, 4 inch long PVC pipe.
• This will create an inductor with a value of around 220uH.

Connecting the Antenna:

• Attach the 10 foot wire antenna to one of the central points on the coil.
• This helps improve selectivity.

Diode Connection:

• Connect the diode to the coil.
• Add terminals to the inductor every 20 turns, with the last 60 turns connecting to the antenna and ending with the diode.

Audio Frequency Section:

• Extract the carrier frequency from the rectified signal at the diodes cathode using a 300pF capacitor.
• Direct the audio frequency through a 0.1uF capacitor into the non inverting input of the LM1458 op amp which functions as a high impedance buffer stage.

Amplification Stage:

• Use another op amp stage LM1458 to amplify the voltage level by approximately 50 times.
• Connect the output of the first op amp to the second via a 10k resistor.

Resistor Adjustment:

• Ensure that the units of 100k and 1 Meg resistors are similar in value (within 1%).
• If not, use more closely matched values or add a capacitor in series with the 10k resistor to maintain the DC voltage within the transistor emitter between 3V and 6V.

Gain Adjustment (Optional):

• If necessary limit the overall gain by using a smaller feedback resistor 470k.

Earphones Connection:

• Connect high impedance earphones or stereo type headphones to the output for an optimal listening experience.

Power Supply:

• Power the circuit with a 9V battery drawing approximately 10mA.

Note:

• Keep in mind that the LM358 is mentioned as a class B amplifier with no bias and may produce excessive crossover distortion in audio applications.
• For better performance, consider using the LM1458 as a substitute.

Conclusion:

It is important to note the recommendation to use the LM1458 as a substitute for the LM358 for improved performance in audio applications.

Throughout the construction process, attention to detail and adherence to safety precautions are essential for the successful assembly and functionality of the radio circuit.

Experimenting with different components and configurations may further enhance the circuits performance based on specific preferences and requirements.

Ensure that you follow proper safety precautions and double check connections as you build the circuit.

References

The circuit of a simple radio receiver

Radio receiver design