With a booster, this simple radio circuit functions similarly to a crystal radio.
With a 10 foot antenna it can detect a lot of closest powerful radio stations.
More stations can be picked up with a longer antenna but unwanted channels can also be picked up.
While trying to listen to a lesser station these undesired stations might become audible in the background.
Try connecting a long wire antenna to the coil rather than the junction of the coil and capacitor to improve the capacity to select the exact station one need.
A strong ground connection is also necessary.
Touching the headphone ends to a concrete surface outside can be OK if one is just listening to local stations.
Circuit Working:
Parts List:
| Component | Description | Quantity |
|---|---|---|
| Resistors | 100k, 1M, 10K (1/4 watt) | 2 each |
| 470k, 470Ω (1/4 watt) | 1 each | |
| Capacitors | ||
| Ceramic 100nF | 1 | |
| Ceramic 330pF | 1 | |
| Electrolytic100uF 25V | 1 | |
| Gang 365pF | 1 | |
| Semiconductors | ||
| Transistor BC547 | 1 | |
| IC LM358 | 1 | |
| Diode OA91 | 1 | |
| Inductor 220uH | 1 | |
| Antenna | 1 | |
| Headphone | 1 | |
| Battery 9V | 1 |
To build an inductor coil 200 turns of 28SWG enameled copper wire over a 4 inch PVC pipe that is 7 to 8 inches in diameter.
This will create an inductor that has a value of about 220uH.
The best connections between the diode and antenna are made possible by adding terminals to the inductor every 20 turns with the last 60 turns going to the antenna and finishing with the diode.
Silicon diodes can work well with powerful signals but a germanium diode is advised for best results.
A 0.1uF capacitor directs the audio frequency into the non-inverting input of the proper operational amplifier which acts as a high impedance buffer stage while a 300pF capacitor draws the carrier frequency from the rectified signal at the diode cathode.
The next op amp stage which connects to the previous stage by a 10k resistor increases the voltage level by about 50 times.
To keep the transistor emitters DC voltage between 3 and 6 volts it is essential to either use more closely matched values or connect a capacitor in line with the 10k resistor if the values of the 100k and 1Meg resistors differ by 1%.
Another technique is to use a lower feedback resistor (470k) to restrict total gain.
While stereo headphones can also be used high impedance headphones give a better sound quality.
A 9V battery provides around 10mA of power to the designed radio circuit.
Formula:
The parts required for a simple radio circuit with headphones are as follows:
The concept of amplitude modulation or AM provides the base for information transmission through AM radios.
The necessary formula and its functions are shown below:
Wave Carriers:
The carrier wave is a high frequency electromagnetic wave that carries the information flow.
The formula for a carrier wave is:
Ac * sin(2πfc * t + φ) = v(t)
where
- v(t) represents the carrier waves actual voltage at time t.
- Ac stands for the amplitude of the carrier waves.
- The AM radio broadcast range includes the carrier wave frequency which is fc and is measured in hertz Hz (535 kHz to 1605 kHz on average).
In simple words the carrier waves phase angle or φ is sometimes set to zero.
Information Signal:
On the radio the information signal carries the real audio recordings such as music or voice.
Low frequency voltage waveforms usually serve to illustrate audio changes.
Amplitude Modulation:
The amplitude of the carrier waves in AM changes in direct proportion to the stream of information.
There are other methods to mathematically represent the modulated AM wave however the following is a common form:
vAM(t) = Ac * [1 + m * sin(2πfm * t)] * sin(2πfc * t + φ)
where,
- vAM(t) represents the instant voltage of AM waves.
- How much the carrier amplitude has changed from its unmodulated state is represented by the modulation index or m which is a value between 0 and 1.
- The frequency of the information signal is fm which is measured in hertz Hz.
- This frequency range is often limited for audio transmission to the 20 Hz–20 kHz range that is within the human hearing range.
Sidebands:
By using the AM technique two additional frequency components called sidebands are created around the carrier frequency.
These sidebands carry the information from the original signal.
These frequencies are supplied by:
fc to fm is the lower sideband frequency.
fc + fm is the upper sideband frequency.
Take note:
These calculations can provide a basic understanding of the mathematical concepts behind AM radio.
More complex circuit is really required for AM transmission and receiving.
How to Build:
To build a Simple Radio Circuit with Headphones follow the connections mentioned below:
Putting the Inductor Together:
- Around the 4 inch long, 7 to 8 inch diameter PVC pipe, wind 200 rounds of 28SWG enameled copper wire.
- An inductor with a value of about 220uH will result from this.
Putting the Antenna Together:
- Connect the 10 foot wire antenna to one of the coils center points.
- This improves choice.
Diode Connection:
- Connect the coil to the diode.
- Every 20 turns add terminals to the inductor the final 60 turns connect to the antenna and end with the diode.
Section on Audio Frequencies:
- A 300pF capacitor is used to extract the carrier frequency from the rectified signal at the diode cathode.
- Use a 0.1uF capacitor to direct the audio frequency into the LM1458 op amps non-inverting input which serves as a high impedance buffer stage.
Stage of Amplification:
- To increase the voltage level by about 50 times use an additional op amp stage LM1458.
- Use a 10k resistor to connect the first operational amplifiers output to the second.
Adjusting the resistor:
- Make sure that within 1% the values of the 100k and 1 Meg resistor units are equal.
- If not keep the DC voltage within the transistor emitter between 3V and 6V by using more closely matched values or by connecting a capacitor in series with the 10k resistor.
Adjustment for Gain (Optional):
- If required use a lower feedback resistor (470k) to restrict the total gain.
Connection of Earphones:
- For the best listening experience connect stereo headphones or high impedance earbuds to the output.
Source of Power:
- Use a 9V battery to power the circuit which draws around 10mA.
Take note:
- Note that in audio applications the LM358 may cause severe crossover distortion since it is a class B amplifier without bias.
- Consider replacing it with the LM1458 for improved performance.
Conclusion:
For better performance in audio applications it would be best to use the LM1458 IC instead of the IC LM358 which is an important point to remember.
The proper construction and function of the radio circuit depend on extreme care to every detail and following of safety rules throughout the building process.
Depending on particular needs and requirements, experimenting with various parts and combinations may improve the circuits performance even further.
As one construct the circuit and make sure to take the right safety precautions and verify the connections.