Weekend Workshop On Electronics: Homeschooling Myself On How A Radio Works
The weekend is winding down. I’ll be heading out to the roller rink shortly. AA1F is back from his trip to New York. I am experiencing that letdown you feel after a terrifically good time has come to an end, and real life is about to kick in again.
I decided last week that I was going to give myself the time and space to learn how a radio receiver works. AA1F jumped at a chance to take a solo road trip back to his hometown. I jumped at the chance to have the house to myself. Mostly because when left to my own devices, I can make the house look like a poorly run electronics factory. It’s a lot better to do this without anyone around! I only stumbled out to pick up some components at “You-Do-It” Electronics, perhaps the last remaining store of its type in the Greater Boston Area. And some sweet-and-sour chicken. I might have well stayed in my bathrobe (I didn’t!)
My curriculum for the weekend started Friday night with a binge watch of Scopes for Dopes, an excellent primer on how to use an oscilloscope.
I also picked a circuit to try to build. I chose one I found in this Instructables on how to build a 1.5V, 3 transistor, amplitude modulated (AM) radio. I will show the circuit, modified with results of my circuit analysis below. Please visit the instructables site to learn how this really should be done!
I fell asleep watching a playlist from The Radio Shop.
While the Instructables had a components list, I did not have a lot of experience in choosing correct components. What power rating would I need? I spent the morning with coffee and circuit analysis, trying to figure out what I should buy at the electronics store. Below you can see the circuit from Instructables (hopefully the author does not mind, but if you do, drop me a line and I will remove it!) In red is the circuit analysis I came up with for the resistors only. I did not figure in the other components except the loudspeaker which I treated as a resistor. I do not think that this is correct, nor did it match the completed circuit’s measured values, but it was worth attempting as an exercise. And proportionally, the resistor voltages were similar to what I eventually saw in the completed wired circuit.
I understood my circuit analysis to mean that resistors rated at 1/4 watt would be okay. With a full parts list, I headed to “You-Do-It” Electronics. I managed to find something suitable for everything I needed, with the exception of a ferrite rod, a variable trimmer capacitor, and a 0.47 microfarad electrolytic capacitor (C3 as indicated in the schematic, however it was listed as a 0.1 microfarad electrolytic capacitor in the parts list. Ultimately, I decided to use a 0.1 microfarad ceramic capacitor for no other reason than I had one.) I found an AM radio kit that had a trim cap and a ferrite rod, so I bought that too…
Mandarin food in hand, I went back home and watched the entire excellent series on Introduction to Radios while I ate. I now had a good grasp on the basics of a radio receiver, and I was ready to build my slightly amplified version.
I hand drew the AM transistor radio receiver schematic (because my printer is not printing) and taped the components onto their corresponding locations. My handwriting alone should be a dead giveaway that I am not an engineer!
I excitedly put the components onto a breadboard, and well, it just didn’t work. Voltage did not seem to be making its way through the entire circuit. In fact, I tried to fix it. But this just caused me to mess up the board even more. I decided that I would hang it up for the night. I needed to convert the schematic into a wiring diagram and try again… (Below is my first attempt.)
This morning I woke up ready to get back at it. Coffee brewed and my mind cleared from a good night’s sleep, I began to think through the AM receiver project. While I have no doubt there are much more sophisticated ways to work out circuit wiring, I just worked my way through each component and what I wanted to connect it to with paper and pencil. That is one good thing about learning on your own — no one can tell you that you are doing it wrong. Of course, the bad thing about learning on your own is that no one can tell you you are doing it wrong!
Regardless, the final result was a lot more organized and efficient looking than my original attempt… You can see a battery (now with a switch) at the top, the loudspeaker on your left, and the antenna composed of a trim cap and an inductor on your right.
A closer look at the wiring…
A view of the loudspeaker. It is sitting on a roll of clear tape.
And finally the antenna and tuning system. The rolls of tape are simply holding the antenna off of the table. That black knob is the trimmer variable capacitor.
For the antenna, I borrowed the ferrite rod from the commercial radio kit and covered it with Scotch 27 glass tape for protection. I then wound 14awg enameled wired (Thermaleze) around it as many times as I randomly cut the wire for.
I tested it with an LCR meter and proved it was indeed an inductor. As I ran to photograph the results, the meter’s non-standard battery died. But I swear, it was indeed registering as an inductor (no surprise there, I just cannot recall the value.) Of note, the wiring windings in the radio kit were made of incredibly fine thread-like wire. I just used what I had. This probably accounts for the reason this radio is completely deaf.
With the circuit now built, I could test my radio receiver. The most striking finding was that when powered, I could here a staticky discharge of sound in short bursts coming from the loudspeaker. There were no audible radio stations, and very limited sound at all. I tested the connections of all of the components while maintaining the positive lead at the positive battery terminal. The chart shows the voltage at the positive side of the resistors, capacitors, antenna, and loudspeaker, as well as the negative sides. The collector, emitter, and base values are shown for the transistors. While I know that my calculated voltages were most likely wrong, you can see that there is proportionality between what I derived theoretically from my flawed circuit analysis and what I actually measured. The “n/a” for the Q2 collector is due to the fact that it was not easily accessible to probing without shorting it.
So, while I don’t expect this blog to be decent DIY instructions on building an AM transistor radio receiver, I think it can be useful as a guide to how to approach the study of radio electronics. I learned a ton. In fact, I have been ignoring AA1F since he came home because I want to get this all written down. For me, this blog serves as a way of synthesizing the experiences I am having in the ham radio hobby. This weekend, I learned exactly how a receiver, in its most basic form (antenna, tuner, detector/demodulator, filter, speaker) works. I even was introduced to the electronic concept of amplification in a way that I had not previously understood.
So, even if my little AM radio receiver just crackles a tad bit, this self-study weekend was a resounding success!
Always,
KM1NDY