AM Radio Project

This is my custom-built AM radio circuit, designed to pick up local stations. It uses:

• 50-foot 24 AWG wire antenna to receive signal.
• A germanium diode: "a half-wave rectifier, which passes mostly the positive half-cycles of the modulated carrier wave".
• A custom LC circuit for tuning to the resonant frequency (radio station).
• A Texas Instument LM386 amplifier for audio output.
• A 9V battery for powering the audio amplifier.

Circuit Diagrams

This project came from Chapter 1 of The Art of Electronics by Horowitz and Hill. On page 56, they introduced the circuit diagram that explains the LC circuit.

In searching for an appropriate audio amplifier, I found the LM386 Low Voltage Audio Amplifier and its recommended circuit diagram for an AM radio.

My project followed both designs. However, I did not have a capacitor small enough (C1 (in LM386 specs) or C2 in The Art of Electronics) or the ferrite bead in parallel with the RC circuit, so I skipped both of those parts in my project.

Step 1: Calculating the Inductor

From section 1.7.14 on Resonant Circuits in The Art of Electronics, I learned the formula for the resonant frequency:

f₀ = 1 / (2π√(LC))

Given that my variable capacitor is approximately 365 pF (spec sheet), I calculated that my inductor needed to be about 150 µH to access frequencies along the AM radio spectrum.

To simplify the process of designing the inductor, I used an online calculator to determine the number of turns needed. Using a 3-inch empty plastic bottle as the air core and 18 AWG enamel copper wire, the result was approximately 46-47 turns to generate 150 µH of inductance.

Here's a closer look at my coil:

Step 2: Connecting the Antenna

I strung up my antenna and connected it to my breadboard. Here's how it looks:

Step 3: Completing the Circuit

To complete the AM radio circuit, I followed these steps:

1. Connecting the LC Circuit:

   • As per the circuit diagram in The Art of Electronics, I connected the LC circuit to ground and in parallel with the rest of the circuit (before the germanium diode).
   

2. Adding the Germanium Diode:

   • I added the germanium diode in series with the resistors leading to pin 2 of the LM386. The variable resistor shown is a 10K-ohm resistor which helps control the volume.
   

3. Adding Remaining Parts:

   • I added the remaining components to the breadboard, including capacitors in parallel to achieve ~250 µF. All of this connects to the 8-ohm speaker.
   

4. Adding 8 ohm Speaker and Battery Connection:

   • The battery is not yet connected, but the battery connector is prepared to receive a 9V battery.
   

5. Establishing Grounding:

   • Without a grounding rod, I ran a wire to an outdoor copper water pipe to serve as the ground connection.
   

Step 4: Testing the Circuit + Results

I was only able to hear two stations, with 860 kHz coming in clearest (see video). Even still there was quite a bit of background static / noise. My LC circuit didn't get me the range of stations I was hoping for. Also, being indoor without access to a window for my antenna might also be causing limitaitons. I left out a few parts designed to block HF noise. Nonetheless, I could listen to an interview with Liz Truss, former British PM, pretty cool.

My Understanding of How This Works

The antenna picks up a variety of signals across different frequencies. When these signals enter the circuit, they encounter a "fork in the road": they can either pass through to the speaker or enter the LC circuit. The job of the LC circuit is to resonate at the desired frequency (as determined by the resonant frequency formula) and divert all other frequencies. This allows only the frequency of interest to continue through the circuit.

The selected signal then passes through the germanium diode, which rectifies the signal by converting the modulated radiofrequency carrier wave into an audiofrequency waveform. The diode accomplishes this by only allowing current to flow in one direction, effectively preserving the envelope of the AM signal.

The 10K variable resistor and 2200 pF capacitor (from the circuit design/not in my project) are intended to form a low-pass filter, which smooths the rectified signal by removing high-frequency remnants of the carrier. Since I did not include the capacitor, my circuit allows some high-frequency carrier components to pass through, resulting in a less smooth audio signal.

Finally, the LM386 audio amplifier takes the processed signal and amplifies it to drive the 8-ohm speaker. While I don't fully understand all the inner workings of the LM386 yet, it serves as a crucial "black box" in this project, and I look forward to learning more about its operation in the future.

Please share feedback on ways I can improve this project and or better understand how this works!