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

Many thanks to SWLing Post contributor, Adi, who writes: Hi Thomas, I was just browsing eBay when I decided to see what was “new” in the receivers section. These popped up: https://ebay.us/iRSC87 [eBay partner link] RH1-1? [See photo above] Google doesn’t know much about it. https://ebay.us/RNQEy3 [eBay partner link] The seller from Poland writes, “The […]