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

This was a little rabbit hole. On some amateur radio forum somewhere someone mentioned the MFJ-802B RF Field Strength Meter. This piqued my interest because I had not realized that a field strength meter was a consumer accessible device. Turns out there are all sorts of RF meters and detectors available to purchase. Who knew?

This collided me into the homebrew versions of RF detectors. One that caught my eye, and inspired this project was “The Squeakie” by VK3YE. It requires a handful of components, all of which I had. I built it mostly to spec on a breadboard, adding only an extra trimpot to control the volume. Then I soldered it onto perfboard and it, after a bit of troubleshooting, worked exactly as advertised. In short, it squeaks at a proportionally higher pitch when it encounters radio waves. This means it is useful as an audible RF field strength meter. You could use it to detect RF while fox-hunting, or to test the radiation pattern of an antenna, or to annoy your significant others… In short, it is a voltage-controlled oscillator that increases in frequency in the presence of an RF signal.

So I built The Squeakie, and sure enough one tap on my HT, and the little guy was shrieking. Below gives you a rough sketch of how I positioned and connected everything. This perfboard is of the type that all of the holes in each individual row are interconnected (and represented by the printed lines on the page).

So how exactly does The Squeakie work? My modification of VK3YE’s Field Strength meter is shown in part below (circuit constructed with Multisim).

Let’s start with D1, which is the switching diode 1N4148. In this application, the diode is acting as a rectifier, transforming an RF signal coming from the antenna into direct current.

Next let’s discuss the 555 Timer IC. This circuit is set up in “astable” mode, which means that the 555 timer is going to oscillate between an output of VCC and zero volts indefinitely with a regular and predictable voltage-dependent frequency. The tying of the threshold (pin 6) and trigger (pin 2) together puts the 555 in a permanently unstable state flipping between “on” (with a voltage equivalent to that seen on pin 8, i.e. VCC) and “off” (zero voltage) producing an endless square wave.

Resistors R1 and R2, along with capacitor C3 work together to produce the frequency of the 555 Timer output. In the situation where VCC is the only voltage source, the threshold/trigger flip-flop produces a stable oscillation. But in our circuit, the antenna acts as an additional voltage source in the presence of RF, adding to the VCC, and resulting in more swift oscillations (i.e., an increase in frequency of the output square wave signal).

The VCC crosses the R5 resistor, 20kΩ potentiometer, and the R4 resistor , which act as an adjustable voltage divider, creating a forward bias on the 1N4148 diode. The 20k POT can be adjusted to increase or decrease the voltage either toward the diode or toward ground, and in turn, change the frequency of the timer output. An additional voltage supplied by the antenna and rectified by the diode will increase that frequency.

The output voltage, rhythmically oscillating between VCC and zero volts, will cross through an electrolytic capacitor and to an 8Ω 0.5W loudspeaker whose volume is controlled by a 200Ω potentiometer. Running in parallel to the loudspeaker is also an LED that’s brightness will be increased or decreased with the frequency (and duty cycle) of the timer. And also an output lead to provide a readable input to a microcontroller is also available. In this case I used an Arduino Nano as well as a 1602 LCD screen.

The above EasyEDA schematic shows my entire finalized project. Or I supposed at least version 1.0. This includes placement of a DPDT switch (SW1) to switch from the Nano to the loudspeaker. A second switch (SW2) to switch the output to the Nano on or off. Three 2-port terminal blocks for the voltage source, antenna, and loudspeaker. The Arduino Nano and it’s connections to the 1602 LCD screen. And finally, the code I programmed for the Nano that would convert the output reading of the RF meter into a corresponding number. I’ll talk more about the code later on.

Below shows the entire breadboarded project, fully operational. In my version, not only does it squeak in higher frequencies (and have an adjustable volume) in the presence of RF above ambient background noise, it also brightens an LED proportional to RF signal strength, and displays a relative numerical meter reading. The antenna is just a length of wire tied into the positive port of the terminal block.

And just a close-up of the placement of the components. You will notice that the readout number is much less bright than the words “RF Strength”. I think this is due to the very quick oscillations of the device, of which I tried to account for somewhat in my rudimentary code. Hey, what can I say? It needs work.

I did go ahead and convert my EasyEDA schematic into a PCB through JLCPCB. Remember, I do not accept sponsorship from anyone. I actually cannot believe how easy it is to acquire a circuit board from this company.

The unpopulated rendering of the PCB is shown below. And the partially populated board is above.

And an exact Bill of Materials below.

And finally the code!

#include <LiquidCrystal.h>

int rs=7;
int en=8;
int d4=9;
int d5=10;
int d6=11;
int d7=12;
LiquidCrystal lcd(rs,en,d4,d5,d6,d7);

int maxVal = 0;
uint32_t lastSample = 0;

void setup() {
  Serial.begin(9600);
  lcd.begin(16,2);

}

void loop() {

    if (millis() - lastSample > 1000) {
        lastSample = millis();
        Serial.println(maxVal);
        maxVal = 0;

    }
    int reading = analogRead(1);
    if (reading > maxVal) {
        maxVal = reading;
    }
        lcd.setCursor(0,0);
        lcd.print("RF Strength: ");
        lcd.print(maxVal);
        lcd.clear();
      

}


What can I say? The firmware sure isn't pretty, and it doesn't even give you great results. But it does kinda work. I will probably spend some time working out different ways to produce more meaningful results from the readable Nano lead of this device.

This was a great and instructive project to work through for me. It also lends itself to a really easy soldering project for hams, or students, or individuals, particularly if you only build the part with the loudspeaker, or even the original “The Squeakie”. I think it is putting me one step closer to building my own receiver, which is something I have made a few attempts at so far.

May we meet again!
KM1NDY

For last year’s Field Day, I took a stab at networking a couple of computers together with an ethernet cable so that our N1MM+ logging software could be synced up. It was both surprisingly easy to do, but equally daunting due to the curious lack of reasonably digestible tutorials tackling the topic on the interwebs. So now that Field Day is again upon us, I felt that same sort of dread that comes from staring up at a steep learning curve. Because quite frankly, I could not remember at all how to create a N1MM+ computer network. I checked back at my blog page on the topic, and was dismayed at how little of the process I documented. So, I am here to rectify that.

Here is my use case. I want to have three computers with Windows 10 operating systems host logging software (N1MM Logger Plus) for a multiple station ARRL Field Day event. All of the computers need to be synchronized with each other in order to avoid such dreaded contesting faux pas as “dupes”, i.e., getting the same person twice. I also do not want to have to rely on an internet in order to maintain communication between these computers.

As far as hardware goes, I already was in possession of three (quite aged) computers. I splurged on three new 25′ ethernet (CAT 6) cables (one for each computer), and a Linksys 8-Port Gigabit Ethernet Switch. I set up the computers simply by plugging one end of an ethernet cable into its ethernet port, and the other end of the cable into the switch. Remember the gigabit switch does need power to operate!

Before I began networking the computers, I had updated all of the necessary software, including Windows and N1MM+. All of the computers need to have the exact same version of N1MM+, as well as exactly the same inputted contest information, before N1MM+ is able to synchronize between multiple stations.

Once the hardware was gathered and the software was up-to-date, I followed the step-by-step procedure documented below.

Step 1: Go to internet icon, click, and “Open Network & Internet Settings”.

Step 2: Select “Ethernet” on left, and then “Network and Sharing Center” on right.

Step 3: The “Unidentified Network” is set to “private” which is what I want it to be. For contrast, my wifi network is set to “Public” (see arrow on the left). Click on the “Ethernet” hyperlink.

Step 4: Click on “Properties” of the first box that pops up. Then click on “Internet Protocol Version 4 (TCP/IPv4)”.

Step 5: Click “Use the following IP address” and add in “192.168.1.200” for “IP address”. The “Subnet mask” should just show up as 255.255.255.0. While I am no expert by any means in networking computers, I do think you can choose the last three digits of your IP address from 1 to 255 254 [Correction sent to me by my favorite critic, AC1JR!] I picked “200” rather arbitrarily. Once you are done, click “ok”, “ok”, and “close” on the multiple windows.

Step 6: If you need to make your network private because it is showing as public (see Step 3 above), you need type “secpol.msc” into the search bar and press enter.

Step 7: In the pop-up window, click on the “Network List Manager Policies” under the “Security Settings” tab. Then click “Unidentified Networks”. In the next pop-up, choose “Private”. Hit “Apply” and then “Ok”. Your “Unidentified Network” settings should now say “Private”.

Step 8: Open the file manager and click on “Network”. Your computer’s name should be listed there. My computer is named “PC-1”.

Step 9: Now it is time to network your second computer. Go back through Steps 1-8, but this time on the new computer. Below shows all of the steps ordered numerically. Don’t forget to change the ethernet connection to “Private” as shown above. The only difference is that you want to assign this computer a different IP address than the first one. I chose 192.168.1.201.

Step 10: Check the “Network” tab in the file manager to make sure the second computer (in my case, “PC-2”) shows up.

Step 11: Repeat these steps as many times as you need to in order to connect all of your computers to the network. Just change the last digits of the newly assigned static IP address, as they all have to be something different. I have three computers that are now linked together.

Step 12: Once your computers are all networked, open N1MM. Under the “Window” menu, find and click “Network Status”.

Step 13: Make sure that the most recent version of N1MM is installed or else you will get an error message when attempting to connect to the other networked computers (in red below). You also need to make sure that everything else about N1MM is identical, in particular that the contest information for the log is the same.

Step 14: When all of the computers are identically set-up, with the same software versions and contest information, open up the “Network Status” window. A bubble will show that gives you an option to turn on “Networked Computer Mode”. Click it!

Step 15: If you see all of your computers listed with no red error messages, your networking efforts are a success! Make sure you have designated one of the computers as the “Master” by checking the appropriate box.

There you have it! N1MM Logger Plus synchronized across multiple stations for Field Day! I hope to catch you on the air!

Forever,

KM1NDY

And boy-oh-boy did we make a lot of noise! Running three generators, a cement mixer, and a tractor equipped with a scary-looking auger, we managed to dig…a hole!

Congrats AA1F, on not only digging three feet into the ground, but on fixing your auger after it snapped a shear bolt when it wedged itself under a cow-sized rock.

It may not be radio, but darn it…just go ahead and look at that cement mixer…What more do you need? Besides a chainsaw and chipper of course.

Really yours, I mean it.

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