Picture of 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

Picture of 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.
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Whoa! This is incredible, true DIY!
Light_Lab4 months ago

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.

Jezan (author) 6 years ago
Here is the finished radio. All the wirings are done under the plastic base. I will publish an ible of this crystal set soon.
Drakekay Jezan4 years ago
How would you measure this type of Capacitor? I'm guessing it would have a resistance range? Do you think the number of plates would add variability to those values? Perhaps adding in a few extra plates at the top end only. You could do this by using 1/4 and 1/8 of a circle for a few of the rotor plates instead of the 1/2 circle rotor plates. I'm not sure this will translate into capacitance the way I'm image it. Varying the degree of the rotor plate to achieve a larger range?

You can get a reasonable estimate of the maximum capacity (totally meshed) by simple geometry. The capacity in Farads of two parallel conductive plates separated in air is approximately 8.85×10−12 times the Area of the plates in square Meters divided by the Distance apart in Meters. This applies to the area of plates meshed but some capacity still exists between the unmeshed plates so you can't get down to zero capacity.

You need a meter that measures Farads. An Air Variable like this for Broadcast band should go from 0-365 pF.
Air Variables can be had from MidnightScience.com for about $12 +/- US.
Adding or subtracting plates will increase or decrease the caps value.

If you can make caps like you suggest then you should have a Ham Radio License and build your own Transmitter and Receiver to operate.
raypsi Dr.Bill3 years ago
A little FYI goes a long way:
the equation for a air plate capacitor is:

C= {k*0.2248*A}/d

this is in inches:
C is the capacitance of 2 plates
k is dielectric constant air and vacuum are 1
A is the area of one plate in square inches
d is the distance between the plates in inches

The maximum capacitance of a multi plate variable capacitor would be:

Ct= {n-1}*C

Ct is the total maximum capacitance
n is the number of plates
and C is the capacitance in pico farads of just 2 plates

If you do like the Japanese do and put a dielectric between the plates it can make the capacitor smaller no real advantage in a crystal set. but if you were shipping tens of thousands of them it would save you sum money.

Say you take a 3 inch diameter circle the area fro the caqpacitor would be r^2*PI= A
or about 7 square inches then you only will be using half of the circle so making A=3.5 inches for the capacitor equation.

say you make the distance .04 inches between the plates
{1*3.5*.2248}/.04=79.4 pf for 2 plates

so 365/79.4=4.59 so 5 plates total will get you close

6 plates would be

it is perfectly ok to have even about of stator and rotor plates
or you can tweak the diameter with a spread sheet to get an odd number of plates

de N8ZU

Dr.Bill raypsi3 years ago
I just stick ah meter on it.

de WH7WP
zwheel Dr.Bill2 years ago
Or.. for the purpose of building a crystal set... why bother with the meter. Just hook it up and see what frequencies you get. That could be part of the fun!
Dr.Bill zwheel2 years ago
When I build an X-Tal set I tend to use what I have in the junk box.
I have BCB air variables from the tube era that I use that are not near 365pF but they usually have trimmer caps on their sides. If that do the trick I have been known to adjust the capacitance with fixed caps and micro switches. My last X-Tal set I had set up under the transmitting antenna, at a Boy Scout Jamboree, of a 100 watt HF transmitter. My long wire was lower and parallel to the antenna on the transmitter. With the switched caps on my air variable I was able to have complete separation of frequency's. HF radio did not bleed over to the BC X-Tal set. I had 2 meters on the thing to detect null spots for each tank circuit on the X-Tal set. I had a Push Pull coil and a spider web coil all working together with a matched pair of 1N34 detector diodes. No "Taps" on any of the coils. The results I had were astonishing to me. I was able to tune from below 53kHz AM all the way into the SW Bands to around 40 meters somewhere.
The separation was great. The 2 Meters I used as tuning aids. Best X-Tal Set I ever had. Next time I use 600/44 Litz wire for the coils.

WH7WP de
zwheel Dr.Bill2 years ago
That capacitor doesn't look like it would go as high as 365pf. But.. he didn't say Broadcast Band, he said Shortwave. That wouldn't require as large a capacitor so it makes sense.

If you are going to build something like this shortwave might be more fun anyway. AM broadcast radios are still so common. I'm sure we all have several even if we never use them. It's fun to build anything, just to be able to say you built it yourself but better yet when it is something you don't already have!
Tachyon2 years ago
This is a really great Instructable. I especially like how you did everything using simple tools and scrap.
Every crystal radio buff out there is going to love you for this as those darn variable caps are hard to source.
Nice work!
Jezan (author)  Tachyon1 year ago


Dr.Bill Tachyon2 years ago
http://www.midnightscience.com  Source Air Variables

Hey it is really amazing to make a capacitor for just scrap aluminium. Its very good use of wasted aluminum. Appreciate you a lot. Thank you for sharing this idea.


Jezan (author)  Hinesdarrel1 year ago

thanks hines

dditlya1 year ago
Or you can get a variety of variable capacitors from us - spraguegoodman.com
micchow2 years ago
Just got this Instructable in my e-mail digest. I see that the plates are made from aluminum. Would it be possible to make the plates from the aluminum from an old drink can?

Thanks for the wonderful build.
Late to the party, but this is really cool!
tlynch14 years ago
wow neat
my grandfather built a bunch of those old radios theirs even a couple sitting in my dads basement
Dr.Bill4 years ago
In the begining of Amateur Radio operators built All the parts needed to assemble Transmitters and Receivers.
wirecutter4 years ago
Wow! First class! A True instructable and a real make your own radio. Will it be a make your own crystal next.?
This has taken me back to the 50s when I use to go around the Second hand shops, and scrounge 'Old Faulty Radios' for a few pence of my pocket money, and either make them work or use the parts. I did find a box of Carborundum crystals but the best sensitivity was far less than an OA90 diode.
I had thought that no one was doing crystal sets any more, as in Europe T.P.T.B. (the powers that be) have sold off for "Data" use most of the AM and FM . frequencies and pushing for every one to listen to D.A.B. ( and I am not saying the other use of those initials!) You try building a receiver for digital out of just six simple items, and explain how it works to a youngster, if three of them are little black multi-legged chips!
Oh yes, for those that wanted to know how to find out the value of of 'C' the mid range of digital meters have an "F" or "C" range. Once again what a great instructable.
joliwana5 years ago
way to go Jezan! keep it up!
genesis_tan5 years ago
nice!! great job tito!!
Re-design5 years ago
You take DIY seriously don't you!
Jezan (author)  Re-design5 years ago
It's habit forming. lols  
Jezan (author) 5 years ago
Thanks a lot for all the nice comments.
parker2165 years ago
Jezan, I really like what you did here. Your instructions were exact and precise. I love this because I try to build EVERY component of a crystal set myself. I would like to suggest the book "The Voice Of The Crystal". There are a lot of diy crystal radio projects inside. You might be able to find it on amazon, but midnightscience.com has it in stock and they are a great crystal radio club with many books and parts. Thanks for this instructable!
scottm_1135 years ago
Being an ex-radio operator (that made Most of his toys) I have to say that this has GOT to be one of the BEST Instructables that I've seen to date! Well Done!
seandogue6 years ago
Ok, I'm officially impressed. That's ingenuity and gumption
Jezan (author)  seandogue6 years ago
Thank you Sean.
Nucleus6 years ago
where should I hook up the terminals using this capacitor?
Jezan (author)  Nucleus6 years ago
Hook up terminals on the rotor using the thin copper sheet ( photo step 17.) for the stator part you can hook up wires on any of the four nuts that holds the housing. Or you can put a terminal lug on each nut if you are to mount it on a PCB. These are like washers that have a portion to crimp a wire in place
gmoon6 years ago
You did an absolutely fantastic job--really! (but radio-type variable capacitors are fairly cheap and easy to find...)
Jezan (author)  gmoon6 years ago
Yeah right, cheap and easy to find. Im in the middle of the desert nowhere to find and nowhere to buy. :-) thanks..
gmoon Jezan6 years ago
Oh, sorry, you've got a delivery / shipping problem? If I'd had a way to get it to you, I'd have sent you one...
Jezan (author)  gmoon6 years ago
And so do you, lols
gmoon Jezan6 years ago
Not so much--on this end. ;-) Good luck. Looking forward to seeing your the finished radio.
Stuffses6 years ago
how much can the capacitor hold
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