Building Radio Transmitter/Receiver circuits: Tips and tricks

Normally this is a project blog, but I’m going to “break the mold” today and share some of the things I wish I had known about radio circuits. I attempted to build radio transmitter circuits for years and always ran into unexplainable problems. Most of the time, I would a schematic off of the internet and attempt to recreate it, rather than writing my own, because I had no idea whatsoever about what actually made them work. I would build it to the minute specifications (more on that later) and would still end up with a completely non-functioning circuit. The few times I did end up with a working circuit would be after I unplugged a few components that I had previously assumed were vital to operation, thus causing more confusion. One particular circuit I attempted again and again was this one. Once I was finally able to obtain all the odd parts required for this project (and for such a small circuit, there are quite a few of them) I tried to actually build the circuit but to no avail, I simply couldn’t get it working. I tried looking around on the internet for information, but found surprisingly little that was actually helpful, which is why I am writing this now. Here is what I have found about radio transmitting/receiving circuits:

1. The Capacitor
90% of schematics that you will find for radio circuits involve a 4-40pf variable (“trimmer”) capacitor. This simple part is almost IMPOSSIBLE to find. Unless you have a RadioShack near you that sells 20 year-old RadioShack components OR you have access to an old transistor radio, you will likely never have access to this part. The good news? You don’t need it. The purpose of the variable capacitor is to tune the frequency, allowing you to select the exact frequency on which the device transmits. However, you can just use a 22pf capacitor and tune the frequency using the coil, as I will explain below. (If someone does, in fact, know a good source for these capacitors, shoot me an email at info@microcontrolled.com, and I’ll credit you in this blog post.)

2. The Coil
Every single radio circuit you could ever build will require one of these, a hand wound coil. Instructions in schematics can range anywhere from “5 – 8 turns of 22-24 gauge magnet wire over a straw” to “5 13/23 turns of 24 gauge enamel coated wire over a 0.847 inch air core”. Neither of these are correct. The performance of your coil depends heavily on many parameters that simply cannot be monitored. When winding a coil, standard 22 gauge enamel coated copper wire (“magnet wire”) is preferred, though with tuning, you could probably get away with using insulated 24 gauge wire. The main point here is TUNING. Yes, one thing that schematics often leave out is that you will need to TUNE your coil if you expect it to work. The preferred method of tuning is to use a ferrite bead, but a screw will also work. Let’s first discuss how to actually wind the coil. If you are transmitting or receiving on the standard AM/FM band, wind anywhere from 7-12 times around a screw. That’s it. Regardless of how many times it says to wind it, you can get away with just about anything here. Leave the screw in the circuit while you solder it together, and apply power to your circuit. Insure that everything is working right, then proceed to tune your coil. Turn on a radio and find an empty frequency to work with, then SLOWLY unscrew the bolt from your coil. It will make your life easier if you use a plastic screwdriver, as a metal screwdriver (or your hand) is conductive and will mess with the frequency tuning, causing you more trouble than it’s worth. Even a scrap of plastic that you can twist the screw with will work better. Anyway, twist the screw out until you hear something on the radio. If you DON’T hear something on the radio, try adding more turns on your coil. (and make sure that you are actually transmitting something) Once you get it tuned, glue the whole thing down and don’t move it until the glue drys. One little movement could mess up the coil.

3. The Antenna
Though this can technically be a simple strip of wire, a good antenna will save you a lot of trouble in the end. Like the coil, movement of the antenna can highly affect the functionality of the device. Even the antenna touching metal could mess up the transmission, even if the wire is insulated, so I would suggest you spend $5-$20 on a good antenna if you want no trouble with it. If you can’t, bend a wire antenna around the frame of your circuit board and glue it down. You will need to tune the coil accordingly, but it works. I have made several transmitters that work like this.

4. The Construction
This can really cause frustration if you don’t know about it: DO NOT BUILD RADIO CIRCUITS ON A BREADBOARD! It will not work once you transfer it to a protoboard and solder it down, or even if you build another circuit on the breadboard. Each metal strip in the breadboard causes electrical “noise” that interferes with the radio transmission, so you should just avoid it entirely.

I think that this is conclusive, if you need help assembling your own radio circuits, contact me at info@microcontrolled.com and I will help you out. If you have your own contribution to make, I will post it here and credit you with the idea.

Thank you for reading!

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