It was not all that long ago that I tried unsuccessfully once again to make an AM radio receiver from discreet components and an LM386 low voltage power amplifier.

Well, apparently the fifth time is a charm…because THIS ONE WORKS!!!

As you can see in the video and pics below, there are only really a handful of components. Two ICs, the LM386 and a LM741 op-amp. That is a 10-365pF capacitor on the left in parallel with a homemade coil. A 10K potentiometer in the middle. An 8-ohm 0.5W loudspeaker. An 1N34A germanium diode. Some electrolytic caps and some resistors. A alligator-clamped a few feet of wire to one end of the inductor as an antenna. The LM741 requires both a positive voltage and a negative voltage, in this case +9V and -9V to function.

I think in making this particular receiver work, it was in part the careful construction of the tuning coil. I have a much better understanding now of LC circuits that I previously did. I knew how to utilize a inductor and LC frequency formulas (and online calculators!) to achieve a coil that along with a variable capacitor could be tapped to achieve resonant frequencies within the AM broadcast band (540kHz to 1700kHz). I used 26 gauge magnet wire wrapped 70 times around a 1 5/16″ PVC pipe. The enamel was scratched off in between the two pieces of electrical tape, trying to expose the outer surface without causing shorts between adjacent coils. A wiper was constructed from a bent piece of 14awg solid wire, and grounded. One end of the coil was also grounded (using the same screw as the 14awg wiper, which was then grounded on the breadboard with the rest of the circuit). The other end of the coil attached to the antenna, and also back into the circuit on the breadboard. The antenna and coil are directly connected to the germanium diode.

The variable capacitor (left) is in parallel with the tuning coil. The leftmost IC is the LM741 which requires the dual positive and negative voltage sources. After crossing the germanium diode, the now rectified signal passes through the LM741 op-amp. This feeds through a 10K potentiometer that can increase the volume of the signal as it enters the LM386 which further amplifies the audio before passing it into the loudspeaker.

Unfortunately, this circuit is copyrighted. It is from one of Forrest M. Mims III’s “Engineer’s Mini Notebook” of prior Radio Shack fame. In particular it is part of Volume II of the four volume set, called “Science and Communication Circuits & Projects”. Volume I, “Timer, Op Amp & Optoelectronic Circuits and Projects” is also handy in getting the dual power supplies correct. I ordered the entire compendium from Solarbotics. As always, I receive absolutely no payment or products for this website whatsoever; this is an entirely non-monetized personal endeavor so don’t think I care if you visit that site or not. These books just seem very hard to find these days, and I was just lucky to stumble upon this company that had them for sale. I may not be the only one that feels that way.

Finally! A working DIY receiver! There is still a lot of experimentation to do with this one. Like will a Schottky diode work instead of germanium unobtanium? I will let you know what I find out!

You are always on my mind.

KM1NDY

Every now and then I decide it’s time to homebrew a receiver. You may remember my attempt back here. Or even way back here. They never work. So this even more complicated, 3 transistor, 2 diodes, and audio amplifier IC definitely did not work. Again. Well sort of. Technically it is actually a receiver. Just not what I was hoping for.

See the electrolytic capacitor I am pointing out down below? And the resistor that is in series with it? If I touch either with my fingertip, while the circuit is live, radio stations play through the loudspeaker. These components form a loop from pin 8 to pin 1 of the LM386; these pins are the “gain” pins of this low voltage audio amplifier chip.

This is a lot like what happens back in my last receiver build attempt, except for this one, I needed to touch the potentiometer in order to pick up stations. I’ll repost the video from that build below so you know what I mean. Essentially I could remove the entire rest of the circuit and as long as I powered up the LM386 and touched the top of the potentiometer, I could hear a station through the loudspeaker.

I am not through debugging this current circuit or I would go into more detail about it. In fact, in preparing this blog, I can see I left one end of a capacitor floating. The cap in the arrow below should be sitting between pin 3 and pin 4 of the IC. Pin 3 is correct, but then you can tell instead of hitting the ground rail of the chip at pin 4, the other end of the capacitor is just freely hanging out in its own row in the breadboard. To be fixed! And if I make any headway, I’ll write up a more complete description of the circuit.

One of my most successful and useful builds is in action down below. This is the KM1NDY Voltage Converter that I designed out from scratch that uses 78xx series of linear voltage regulators in TO-220 packaging. The voltages are interchangeable, and for this receiver attempt I used a 5-volt 7805 chip. The power source is a 12 volt LiFePo battery. This system includes a replaceable fuse as well, as an attempt to minimize any potentially dangerous currents from reaching me when I accidentally short something out. This little device is actually quite handy! If I make another one, I’ll need to put a switch on it though.

Ok, now close your eyes if you are going to be squeamish, but it is probably too late. I just wanted to show the bloodshed that this ham radio hobby causes me. This cute little pattern of blood bubbles is what occurs when you send the pins of an IC socket deep into your finger. Don’t fret! I am okay!

So, yet another failure. But there is still some debugging left to do, so I won’t write off the entire project just yet. And there are some important mental successes. The first is that I can now start to see the various stages of a receiver circuit. They are making much more sense to me now. And I can see how you can work on each stage as a separate entity. I am already concerned that my antenna and tuning capacitor are not working properly. Or that there is not enough amplification at the RF amplifier stage. I have figured out inadvertently that the components of the audio amplifier stage work. I needed to substitute diodes, so am I not demodulating the AM properly? And I am understanding bit by bit how and what to probe, and with what instrument, to see what is working, and what is not.

Though not so secretly, I can’t wait for the day when I post a receiver build and it is actually a success. But I have always known that failure is not trying and failing. Failure is not trying at all.

That’s that.

KM1NDY