Attempting To Build An AM Radio Receiver With One Transistor And A LM386 Audio Amplifier

One of the best parts for me about maintaining this blog is being able to see my own progression within the amateur radio (and electronics) hobby. About a year and a half ago, I published my first attempt to breadboard a receiver (here). It is rather painful to look at. And it did not work. And now let me cut to the chase with this attempt and tell you it also (mostly) doesn’t work either. But my understanding, while still imperfectly flawed, has grown by leaps and bounds.

The schematic below is an adaptation from one I found on this site. It is in essence a crystal radio, with the piezoelectric high impedance ear piece replaced by a transistor and the LM386 audio amplifier.

It is much easier now for me to “read” a schematic and figure out what the various regions of the circuit are trying to do. In this simple receiver, there are five different fundamental functions: 1) a power supply, 2) the antenna and tuning circuit, 3) AM demodulation, 4) selectivity enhancement, and 5) audio amplification.

I do not know if this circuit would work using a 12V battery which is what I had available. The original circuit called for a 9V source. Since I had a 7809 voltage regulator available, I added the following circuit into the receiver. In essence, this simple circuit would reduce the 12V from my battery down to 9V.

POWER SUPPLY

The actual build is shown below. I added a (pink!) LED with a 1KΩ current-limiting resistor into the circuit.

I also added a simple on-off switch across one of the battery leads so that I would not have to continuously connect and disconnect the powerpole connectors

The entire power supply looks like this;

ANTENNA AND TUNING CIRCUIT

On the left side of the schematic (circled below), an antenna feeds through a small capacitor into a tapped inductor (ignore the fact that the symbol shows a variable inductor — it isn’t variable). The inductor is connected in parallel with a 400pF variable capacitor, forming the tuning circuit of the receiver.

I used a small ferrite rod and more than doubled the 50-60 recommended windings with 26awg magnet wire. I left a long loop of wire at the 8th turn to serve as a tap. My overachieving winding efforts were met with an inductance of 1.9mH. Combined with a variable capacitor with capacitance ranging from 15 to 400pF, the resonant frequency of my LC tuning circuit (according to this calculator) would range between 183KHz and 943KHz. For reference, the AM broadcast band runs from 500KHz to 1600KHz.

I removed about half of my windings, and remeasured the inductance with an LCR meter, finding now 671µH. The resonant frequency now ranged from 300KHz to 1568KHz. Pretty good for the AM band frequencies which in the U.S. range from 540 to 1700kHz.

Here you can see the tuning circuit (with the inductor and variable capacitor) connected to the breadboard. The tapped loop of the inductor is connected to the antenna via a small capacitor. Again I used a power pole connector to facilitate the attachment to the antenna.

The antenna is simply a random length of 18awg speaker wire separated into single strands. I have added another powerpole connection on the end of the antenna to attach it to the tuning circuit.

The remaining portions of the circuit (circled), are shown in the breadboard photo below.

This block of circuitry contains a diode (D1) to demodulate the AM signal. This is done by not allowing the AC current to flow in the reverse direction so that only the forward moving electrical signals are allowed to pass through. The capacitors then “smooth out” the resultant AM signal by now eliminating the medium frequency RF component of the signal, allowing for passage of only the audio component. The audio signal is then passed through a transistor (Q1) which supposedly helps with selectivity of the tuned circuit. I am rather ignorant on this portion of the circuit, so you will have to research it yourself for now. The audio signal then enters the LM386 IC where it is amplified and passed to the 0.5W, 8Ω loudspeaker (marked by a red asterisk above).

Whew, this has been a lot to take in! Best to take a little cat nap!

So, the circuit was easy enough to build. I strung the antenna around my little home laboratory. It was now time to test it! I powered up the circuit and heard a bit of static come from the loudspeaker. I turned the tuning capacitor from its highest position to lowest capacitance. I heard nothing except for a small amount of static. It did not appear to be working. I turned on my oscilloscope hoping to be able to following signals through the circuit and grounded it on one end of the breadboard. After turning the variable capacitor with no luck, I started to turn the potentiometer. That’s when I heard a blast of a very obvious broadcast station crackle through. I went back to turning the variable capacitor, and as far as I could tell, there was no change in the signal as I tuned through the band; the broadcast station continued to have the same quality no matter what i did with the cap. What did make a difference though was whether or not my finger was on the potentiometer. When I touched the pot, the signal boomed in. But when I let go, the signal faded to nearly inaudible.

This led me to believe that the oscilloscope and the probe lead were augmenting the antenna. I turned the o-scope off, and the signal – the same broadcast station – stayed the same. Both with and without my finger.

I unhooked the antenna to see if was actually the source of the signal. Sadly, the antenna (and tuning circuit) appeared to have no impact on what I was receiving. The yellow arrows point to the wires of the variable capacitor now removed from the breadboard. And that was also whether or not the scope probe grounding wire (the alligator clip in the bottom left of the picture) was attached to the circuit or not.

Finally, I unhook all but the wiper which connects to the audio input (pin 3) of the LM386 IC.

Here is a video showing what happened…

So, in putting this all together, my receiver was mostly a failure. Essentially, when I touched the potentiometer, I became an antenna which then fed a signal into the LM386 where it was apparently demodulated and amplified. None of the antenna, tuning circuit, or selectivity portions of the schematic made any difference to my receiver. The only part that “worked” is shown on the right half below.

Regardless, I still can’t believe how far my understanding of electronics and radio has come over the last few years. I have to really look at this and figure out what went wrong when I have a chance. Was it the use of a silicon (versus germanium) diode? Was the connection (made with electrical tape) to the chassis of the variable capacitor too shoddy? Did I triple check the circuit to make sure it was all wired correctly?

Here’s to hoping next year’s attempt at making a receiver works!

Always missing you,

KM1NDY