Air Variable Capacitor From Scrap Aluminum Sheets





Introduction: Air Variable Capacitor From Scrap Aluminum Sheets

I was building a crystal set for my son, but it came to a halt. When i found out that i have no variable capacitor in my pile of junk.
Scavenging one from an old radio was not an option. Since most of new radios uses analog tuning. And the ones with air variable capacitors are very rare, and are collectors item.
I have read an article once about building an air variable capacitor. So i decided to build my own from scraps of aluminum sheets and from things that are easily found around the house.
If you have a drill, a scissors, a file and some sandpaper. You can easily build this one. It doesn't require much skills.:-)
I made mine from aluminum sheets 1.5 and 2mm thick. The housing is from an aluminum heatsink, i also manage to get 3 pieces of bolts with a few nuts from my junk box. The plastic bushing which also acts as an insulator is from a plastic pen and some plastic jar caps.
I also made my own washers/spacers from the 2mm thick aluminum sheet.
The contacts of the rotor which also acts as a tensioner. Were from a broken washingmachine timer.
You can experiment with the number of plates and with the size op gaps by adding more plates and washer, in the rotor and stator.

Step 1: Marking the Center for Pilot Holes

On this step we will do the measurements.
Lay the aluminum sheet flat on your work bench. From on end draw a square measuring 5cm x 5cm. Using a sharp object like a nail or file.
Get the center and punch a hole in it using a small nail. Punch a hole for every 5 centimeters along the straight line as shown on the photo.

Step 2: Draw Some Circles

Here we will draw circles on the sheet. We can draw circles perfectly by using a compass.
If you dont have a compass.You can improvise, by driving two small nails on a small piece of wood.
Insert one nail on the hole that you made on the sheet and turn it. It is important that the circles doesn't overlap. So that we will have enough space when we cut them out later.
You can make as many circles as you like, if you have enough aluminum sheet. It is better to have a spare you if you made a wrong cut. Than to repeat the same process if we you ran out of pieces at the middle of your project.

Step 3: Marking Lines for the Stator Plates

Using a tri-square, draw a line from the edge of the sheet up to the center of the arc. (The one that is pointed by a pen on the photo)
It is important to do this, because we need each part to be uniform in size. And this also serves as a guide when we cut them out.

Step 4: Cutting

With a big scissors and a strong grip. You can easily cut out all of the pieces. And then flatten each piece with a rubber mallet.
You can also use a hammer. But be very gentle.

Step 5: Marking the Holes for the Stator

Get a piece and draw a line from the center. Going to the left corner.
Get the centerline (from corner to arc) and punch a hole in it.
Mark this piece as pattern.
Put it on top of another piece. And puch a hole on the second piece under it. Using the hole on the first piece as a template. Flip the pattern and punch the second hole.
We do this because we will drill all the rotor and stator plates one at a time. I dont recommend stacking all the pieces and drill them all at one time. The drill bit always tend to bend. Or you may if you have a drill press

Step 6: Marking the Inner Circle

Again improvise a compass to draw the inner circle.
Using a scissors cut it from both end, up to the arc of the inner circle. Be very carefull not to cut it all the way to the center. :-)
when all of the pieces are cut. Flatten them again with a rubber mallet.

Step 7: Cutting the Arc

Here we will cut the arc that still connects the rotor and the stator plates. You can always cut it with a scissors.
But it's easier and faster if you use a curved chisel. (the one's that wood carvers use)
I have one but i don't want to ruin it.:-)
So i made one from a small pipe. Sharpen it at one end with a file. And removed half of it. So that it will be just like a curved chisel.
Dont get confused when cutting. The rotor must have the part with a hole in it. (This is where the spare pieces come in handy:-)

Step 8: Drilling

Now that we have our rotor and stator. It is time to make the holes bigger. With the use of a drill. Find bolts (3pcs.) with nuts ( i used 12 pcs. Of nuts because i removed the head of each bolt so i can open my varicap on both ends.) i used a 4mm. drill bit. Find bolts that will fit the holes perfectly, specially for the rotor

Step 9: Turning With a Drill

At this stage we will make our rotor plates perfect arcs. And to remove burrs and sharp edges. With the help of a drill and a coarse file and some sand paper.
Stack all of the rotor plates facing each other forming a circle. (we do this to avoid vibrations when we turn them with a drill) Insert the bolt and tighten the nut.
Insert the end of the bolt on the chuck of the drill. Just like a normal drill bit. Turn on the drill and use the file to smoothen the sides of the rotor plates. Be very carefull set the drill at low speed. Do not press the drill swith for a long time. (the nut may loosen) if this happens reverse the direction of the drill.
Finish with a fine sand paper

Step 10: Shaping the Stator Plates

We will shape the stator plates at this part of my instructable.
Like what we did with our rotor plates. We will also stack all of the stator plates. Insert the bolts on each hole, and tighten the nuts. If you have a bench vise so much the better.
If you can see the photo clearly. I colored the parts to be remove with a black pen. Or just simply follow the arc and avoid the head of the bolt and the nut. Be patient you can do this with a coarse file. Remember aluminum is not that hard to work with. :-)

Step 11: From This to That

At this stage, your pieces must look like the 3rd piece on this photo. Well done.
At this point you can also remove the paint. If they have paint on. And check again for curves or if ever they are crooked.

Step 12: Making Your Own Washers

Make your own washers/spacers. From a heavier guage or from the same sheet. But you will need 2 pcs. Of washers/spacers for each gap. I recommend using the same guage as the stator and rotor plates.Lighter guage is much easier to cut.
Cut a strip of aluminum about 1cm. wide. Drill holes in it using the same drill bit you used for the stator and rotor plates. Cut the strip into squares. Making sure that the hole you drilled is on the center. Insert the bolt with its head on top. Then cut the corners using the head of the bolt as a guide, as seen on the photo.

Step 13: Finishing Your Washers

As what you did with the rotor plates. Hammer them gently to make them flat. Stack them up, insert the bolt, tighten the nut. And again with the use of a drill turn them and smothen with a file and finish with a fine sand paper.

Step 14: Rotor and Stator Assembly

Remove the head of each bolt. Put one nut on one end of eah bolt. Turn the nut until 5cm. of the bolt protrudes from the nut. Get one of the bolts, insert on one stator plate followed by two washers. And again a stator plate and two washers. Repeat this step until you finish. Leave enough space for 2 bolts and two washers and dont forget the shaft on where to put a knob

Step 15: The Housing

I made the housing from an aluminum heatsink salvaged from a television chassis. It is more or less 2mm. thick.
I placed the rotor and stator on top of it. leaving a distance of 1mm. Between the rotor and stator. Marked the 3 holes to be drilled using the rotor and stator holes as template. Then drilled all the holes using the same drill bit used for the rotor and stator plate. Then i just cut it in a triangular shape and rounded all of the corners.

Step 16: The Insulator

Look for something that will insulate the rotor's axel from the end plate/chassis. Like for example a rubber hose from a fuel line of an automobile. Since i don't own a car. I just used a plastic pen for a bushing. Note: look for the right pen before you make the top hole bigger. Don not drill until you have your bushing:-)

Step 17: Plastic Jar Caps

Cut two triangular shaped plastic from jar caps. Or any plastic of your choice. This plastic will insulate the rotor contact from the aluminum housing. I just glued my contact temporarily. I will later change it with a copper connector. The ones that look like washers like the ones used for ground connection.
Now you just put the two triangular plastic on both sides. Followed by the two triangular shaped aluminum and you are done.
Remember if you have two stator plates, you must have three rotor plates. 4/5, 5/6 and so on.
I hope that this instructable will be of much help. Thanks :-)

Step 18: Finish

Here is the finished variable capacitor. Installed on a crystal shortwave receiver.



    • Pocket-Sized Contest

      Pocket-Sized Contest
    • Pro Tips Challenge

      Pro Tips Challenge
    • Science of Cooking

      Science of Cooking

    We have a be nice policy.
    Please be positive and constructive.




    Whoa! This is incredible, true DIY!

    Would this work with copper? I'm very ignorant about crystal radios but would like to give it a try.

    It will work with any metal. Aluminum is just easy to work with, is soft, light, cheap and doesn't rust.

    Nice job and you could make it bigger and with a longer shaft you could make several in series of different capacities that all tune together

    Where are you making your wire connections to your variable capacitor?

    Wow not only did you give a very good write up, but the detail to your craftsmanship / work is great for the tools you used. I can use this to make variable air caps for my magnetic loop antenna project(s) when I get around to doing them. This will even help someone If you have one variable air cap and need more of the same or with less or more rotors and stators, all you need to do is disassemble it and copy (trace ) the parts and follow this to help make it correctly. Thank you very much for making this with such detail.

    thank you.

    thank you.

    What are the dimensions for the parts for the capacitor?

    Beautiful piece of work. I got a start in electronics when I was a kid by my Dad who bought me a crystal radio. I learned to make my own and went all the way to building amps and digital circuits.
    How well does your crystal radio design work? I noticed just a telescoping aerial, I had to use about 50 feet of wire strung across our back yard. I also gave up on variable capacitors and used smaller coils with a sliding rod of ferrite for tuning. I was listening to the AM band not shortwave so that would make a difference.