Making Variable Capacitors By Stretching Aluminium Cans

Sometimes when you need a component, the best way to get it is by building it yourself. [North Carolina Prepper] did just that, creating his own trombone-style variable capacitor by stretching some aluminium beverage cans. 

The requirement was for a 26 pF to 472 pF capactitor, for a radio transmitting from 7 MHz to 30MHz. The concept was to use two beverage cans, one sliding inside the other, as a capacitor, with an insulating material in between.

To achieve this, a cheap exhaust-pipe expanding tool was used to stretch a regular can to the point where it would readily slide over an unmodified can, plus some additional gap to allow for a plastic insulating sheet in between. Annealing the can is important to stop it tearing up, but fundamentally, it’s a straightforward process.

The resulting trombone capacitor can readily be slid in and out to change its capacitance. The build as seen here achieved 33 pF to 690 pF without too much hassle, not far off the specs [North Carolina Prepper] was shooting for.

Radio hams are very creative at building their own equipment, especially when it comes to variable capacitors. Video after the break.

Continue reading “Making Variable Capacitors By Stretching Aluminium Cans”

A Variable Capacitor For Not A Lot

There’s one component which used to be ubiquitous in every experimenter’s junk box, but nowadays unless you happen to be a radio amateur the chances are you may not have seen one in a long time, if ever. We’re talking of course about the air-dielectric variable capacitor, the tuning element for millions of radio receivers back in the day but now long ago replaced by much flimsier polymer-dielectric parts. There’s still a need for variable capacitors though, in particular a high-voltage variant for use in magnetic loop antennas. It’s something that [Ben] had a need for, which he solved with a clever combination of PCB material and 3D printing.

While the variable capacitors of yore invariably used intersecting vanes on a rotor, this one has two large parallel plates that intersect as one is moved over the other with a lead screw. It’s cheap and effective, and best of all, the files to make it can be downloaded from Thingiverse. He claims a 34pF-164pF capacitance range, which, looking at the size of the plates we find to be believable (and which is a useful range for most HF applications). We like this solution, and believe it makes more sense than being scalped for an older example at a radio rally.

This isn’t the first variable capacitor we’ve shown you, though some previous examples have been more conventional.

Teardown: Analog Radionic Analyzer

Have you ever looked up a recipe online, and before you got to the ingredients, you had to scroll through somebody’s meandering life story? You just want to know how many cans of tomato paste to buy, but instead you’re reading about cozy winter nights at grandma’s house? Well, that’s where you are right now, friend. Except instead of wanting to know what goes in a lasagna, you just want to see the inside of some weirdo alternative medicine gadget. I get it, and wouldn’t blame you for skipping ahead, but I would be remiss to start this month’s teardown without a bit of explanation as to how it came into my possession.

So if you’ll indulge me for a moment, I’ll tell you a story about an exceptionally generous patron, and the incredible wealth of sham medical hokum that they have bestowed upon the Hackaday community…

Continue reading “Teardown: Analog Radionic Analyzer”

Surplus Syringes Make Satisfactory Tuner For Amateur Radio Experimentation

Amateur Radio as a hobby has a long history of encouraging experimentation using whatever one might have on hand. When [Tom Essenpreis] wanted to use his 14 MHz antenna outside of its designed frequency range, he knew he’d need an impedance matching circuit. The most common type is an L-Match circuit which uses a variable capacitor and a variable inductor to adjust the usable frequency range (resonance) of an antenna. While inefficient in some specific configurations, they excel at bridging the gap between the 50 ohm impedance of the radio and the unknown impedance of an antenna.

No doubt raiding his junk box for parts, [Tom] hacked together a variable capacitor and inductor using ferrite rods from AM radios, hot glue, magnet wire, copper tape, and some surplus 60ml syringes. You can see that he ground out the center of the plunger to make room for ferrite rods. Winding the outside of the syringe with magnet wire, the alignment of the ferrite can be adjusted via the plunger, changing the characteristics of the element to tune the circuit. [Tom] reports that he was able to make an on-air contact using his newly made tuner, and we’re sure he enjoyed putting his improvised equipment to use.

If Amateur Radio isn’t your thing, then maybe we can entice you with this syringe based rocket, syringe actuated 3D printed drill press, or vacuum syringe powered dragster. Have your own hack to share? By all means, submit it to the Tip Line!

DIY Tuning Capacitors From Washers And 3D-Printed Parts

The inside of classic radios holds wonders that the sterile chips and SMD components of today’s circuits can’t hold a candle to. Chunky resistors and capacitors, vacuum tubes with cathodes aglow, and seemingly free-form loops of wire forming inductors will all likely make an appearance. But the most fascinating bit of any old radio was connected to the tuning knob: the big variable capacitor with its interdigitating metal plates. Watching one at work, with its plates evenly and finely spaced, is still a joy to behold.

In an attempt to recapture a little of that magic, [Jeremy S. Cook] came up with this home-brew variable tuning capacitor. The frame is built mainly from 3D-printed parts, which supports a shaft made from a common bolt. Plates are fashioned from stainless steel fender washers cut in half; the fixed plates are press-fit into the frame while the rotary plates ride on the shaft. The spacing between the rotary plates is maintained by printed spacers, which also serve to lock the rotor into one solid unit. [Jeremy]’s prototype, for which he provides STL files, can be tuned between about 7 and 15 pF. Check out the build in the video below.

We love the look of this, and we can imagine custom tuning caps would come in handy for certain retro radio builds. The tuning range is a little narrow, but that could be fixed with more plates or closer spacing. That might be a tall order with thick steel washers, but we’ve seen really thin aluminum machined and closely spaced before, so this might be one approach to higher capacitance. Continue reading “DIY Tuning Capacitors From Washers And 3D-Printed Parts”

Make Your Own Simple VHF Tuning Capacitor

If you enjoy building radio projects you may have noticed something slightly worrying over the last few years in your component supply. Variable capacitors are no longer as plentiful as they used to be. There was a time when all radio receivers contained at least one, now with the advent of the varicap diode and the frequency synthesiser the traditional tuning capacitor is a rare breed. They are still made, but they’re not cheap and they won’t appear so readily in your junk box any more.

Fortunately a variable capacitor is a surprisingly simple device, and one you can make yourself if you are of a mind to do so. [Patrick] did just that with his home-made capacitor, in this case of a few tens of pF and suitable as a low-power trimmer capacitor or in a single-chip FM radio.

Rather than make a set of interlocking vanes as you’d find in a commercial design, he has gone for a screw in a tube. The capacitance is set by the length by which the screw is inserted into the tube. And his tube is not a tube in the traditional sense, instead he has used a coil of enamelled copper wire wound on the screw thread, whose insulation forms the dielectric. It looks wrong to use a coil in this way as you’d expect a similar coil to form the inductive part of a tuned circuit, but this coil is shorted out to prevent its inductance becoming a factor at the frequency in question.

It’s evidently not the answer to all variable capacitor problems, but it’s a neat piece of lateral thinking and it will make a simple working capacitor from readily available parts.

We’ve featured a couple of more traditional style home-made variable capacitors in the past on these pages, one made from thin aluminium sheet cut with scissors, and another one designed for use in higher power transmitters.

Thanks [PeterF] for the tip.

A Variable Capacitor Made From Junk


[Jezan] decided to introduce his son to electronics by building a small crystal radio. These crystal sets have been around for a long time, and make for a great beginner electronics project, but some of the required parts are a little hard to come by. The most difficult to source part for these radios is a variable capacitor, and not finding one in his parts bin, [Jezan] decided to make his own.

This variable capacitor comes directly from a piece of 1.5 mm thick aluminum sheet. Instead of fancy CNC machines, power tools, or even a pair of tin snips, [Jezan] cut the rotors and stators for his variable capacitors with a pair of scissors. The center hole was punched out with a piece of sharpened pipe, and all the pieces were filed down and sanded for a perfect finish.

Considering the variable caps you can get your hands on are either rare or very old, this looks like a great afternoon project for the budding electronics wizard or radio enthusiast. [Jezan]’s craftsmanship is incredible as well and the finished part looks like it came off an assembly line.