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.

28 thoughts on “A Variable Capacitor Made From Junk

  1. My first kit I got for Christmas way back when was a Tree of Knowledge kit aimed at beginners where you had to build the breadboard, build the pots and even this var cap. The instructions were even less helpful and after getting yelled at by the old man for not liking it (I wanted the spring connector rat shack like my buddy had) we managed to build the pot, get half the breadboard connectors lost/ in wrong and the dog chewed up the spindle. This led to more yelling and years of shame and if it hadn’t been for my buddies pedaling broken stereos to my house from the dumpster, I might not have taken up electronics. One of the main reason I still buy the kits in thrift shops if I see them. Their dedication to quality hasn’t stopped either, with many unopened kits missing parts and the rough Bulgarian guy that sends you pretty much anything. The instructions are just as bad, with errors in 3 of the schematics I attempted to build. Anyhoo, I buy those kits so no one else has to get yelled at by a non-plussed, Coors fueled father and lose interest in electronics. my CSB Called and even yelled at them for such a bad “educational” product but they continue under Elenco’s bigger bucks.

    Oh and KUDOS to Jezan on the var cap build- it looks very nice. Wish I had had ya around 30 years ago :)

    1. Wow, that’s the saddest thing I read the entire day. It paints a vividly realistic but bleak movie inside my mind:
      Franklin sits alone in the basement, grounded by his father for the twentieth time this month. He cannot know what day it is, with the dim light and all, but he has been crossing the days in the calendar in expectation of Christmas. He made his father promise him an electronics set for this day and for the first time in his adult life his father actually cared enough to make this sacrifice. Unfortunately, just as he was going shopping a new bar appeared in the way. Temptation was high and it left him feel ashamed afterwards, but maybe he could still buy something with half of the money…
      The kid receives the gift rather unceremoniously and begins tearing the packaging apart because he knew exactly what it was. Only he didn’t, his emotionally-challenged father is able to pick up a glimpse of unhappiness from his eyes and starts going into a tirade about gratitude…
      Kids from around the neighborhood come visit the kid through the small window just above ground level, bringing him gifts, hoping for him to be able to come out and play someday…
      30 years later Franklin gets a tear in the eye every time he sees an electronic kit, it fuels a mixture of rage and helplessness, of sad joy and a picture of a flickering yellowish bulb.

      Sorry, I had to do it

      1. Pretty close ;) You can add the macabre brown countryside and films about animal husbandry in the second draft. Oh and a garden left behind by field tenants that was way too big for 3 families- that was the worst. :)
        It did spawn the idea to weigh my gifts against the sears and jcpenney catalog weights to determine what they were a few years later (i gave a list and they chose 1). Santa was always a crapshoot. I got the same yellow car lego technics set 6 (no hyperbole) years in a row. In adulthood I asked mom a nice wtf and she just had no clue-had never noticed lol. She was apparently a stickler about setting things up all neat for xmas but said she gave up “the year you wanted that rocket train” leaving omega supreme safely in his box- whew.
        I now spend my christmases with a snifter of port by a redbox kiosk and constantly have to answer support texts from my relatives which consist of me reminding them “not to use dollar tree batteries in high-drain devices”, and waiting for loose tail as in anyone frequenting a redbox on xmas. Norman Rockwell kinda shit.
        ;) it is fun

  2. Variable caps are “Rare” until you try to sell them on e-bay or at hamfests… I got a big pile of various ones, because I thought they were “Rare” and “Hard to source”. Still can’t bring myself to recycle them as scrap metal, and still can’t get anyone to pay me scrap metal prices ($1/pound)….

  3. great work.
    If you want to increase the absolute capacitance values by about a factor of 100, it’s pretty easy to turn his kind of variable capacitor into a variable electrolytic Place in a container and cover completely with (flat) diet pepsi. Then pass a small current through the electrolytic cap to condition the plates -The phosphoric acid adds a layer of anodizing to the aluminium. next empty the pepsi out, wash, and finally half fill with tap water- The anodized layer is dielectric and an insulator so stops leakage. As the anodized layer is so thin/the water conducts pretty well (compared to the anodizing) the *effective* plate separation becomes really thin resulting in relatively high capacitance.

    one thing to watch out for with big variable capacitors changes… watch out for a voltage step up than can happen when one of these caps is charged to a voltage at high capacitance, then capacitance is reduced to a lower value.Not seen much discussion out there on this effect -despite a less extreme variant on it being the working principle of condenser microphones. -(NB It’s not the same as swapping a big capacitor for a small one!)

    Assuming capacitor is disconnected/doesn’t loose charge- the energy in the capacitor will remains constant . E=0.5*C_1*V_1^2 =0.5*C_2*V_2^2
    V_2=sqrt (V_1^2*C_1/C_2)
    V_2=V_1 *sqrt (*C_1/C_2)
    V_2/V_1=sqrt (*C_1/C_2)

    so if charged to say 5V, and capacitance is reduced to say 1/4 of it’s original value…
    the voltage across the now smaller capacitor will be increased to 10V. If the capacitor is supplied by the same voltage as an ICs such as a 555, The input pin can in theory end up double the IC’s supply voltage.Even with protection diodes, few ICs are likely to tolerate that kind of abuse.

    Now if capacitance is reduced to 1/100 of it’s original value. the voltage is stepped up 10*. so that 5V would become 50V- probably more than enough to blow a detector diode, let alone an IC, or especially sensitive CMOS devices.

    And just in case anyone gets the wrong idea and thinks this might be a claim for a over unity device…. as voltage is stepped up the available output current reduces in proportion, so power remains constant, just like a transformer, or DC-DC converter.

  4. Pretty slick, but air dielectric variable caps are not at all “rare”. I think radioshack or “the shack” the “shizz-nack” or whatever they are calling themselves these days still sells them even. If not, am-fm radios from goodwill or your thrift store of chose should have them, trimable even with all the little trimmer screws on them. ham radio stores have them, ebay has them… I think this guy just loves self torture…

      1. There are different types of hard drive platters. Older ones are made of aluminum, which should be perfect.

        Newer ones tend to be made out of glass or ceramic, but in my experience HDDs smaller (ie older) than the 1 TB era are usually metal.

  5. Interesting, yes using old HDD platters would probably work well as they are nice and flat PLUS also have a lubricating coating to stop stiction so the capacitance per unit area can be higher.
    I have a few laying around here somewhere along with a box of assorted varicons.

  6. Also amusing to kludge together, a signal/amplitude indicator using a 90V battery made from old coins, etc and one of those Russian IN-9 tubes.
    Current is actually pretty low so those batteries will last at least a year or so.

    If the signal strength is high enough NE2s can be used in relaxation mode…

  7. A couple of finned heatsinks might work if you could find 2 that mesh nicely. Use some insulated spacers to stop them from touching and slide them across each other.

  8. I.. I mean… Just… Wow. Heh, when I first saw it I was immediately reminded of a ridiculously-intricate lightsaber prop build I saw once; this looks a lot like the radiator the guy made for the interior details…only, if memory serves, he actually had access to a lathe and mill! I can’t freakin’ believe this was done with scissors and…I’m sorry…did you say a piece of sharpened pipe?! My mind has exploded into confetti.

  9. i am jezan and thank you for all your nice comments. i made this air variable cap for my son’s crystal radio that i also made. in this project it doesn’t only show you how to build a capacitor. but it teaches you how to do more with simple tools, a compass, a curved chisel, a portable lathe, make your own washers. and more. i am a fin. carpenter by profession and i love to build things. to enhance my skills further. it is great to be handy with tools, for me each tool has many use, just use your imagination.

  10. Might be great having the knowledge to research but it is a fssr better knowledge to be able to make your own or at least modify what you have on hand especially in todays world were many sadly lack the knowledge to make anything anymore and prices are soaring.

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