Introduction: 10 Minute Variable Capacitor

Supplies you will need:
1.) Sharpie
2.) Superglue
3.) CD case (slim)
4.) 2 CD's
5.) Wire (26 AWG)
6.) Heavy Paper
7.) Glue Stick
8.) Tinfoil sheet
9.) Pocket or Exacto knife
10.) Plastic bottle cap

Step 1: PREPAIR "PLATES"

a.) Turn both CD's upside down and cover them half-way with glue from the glue stick
b.) Place CD's glue side down on tinfoil so only the glue covered part of the CD is on the tinfoil
c.) Wait for Glue to dry and then trim excess foil of with the knife
d.) Cut desired length of wire off and remove insulator about 1/4 inch
e.) pass wire thru center opening from non-tinfoil side to tinfoil side and tape exposed portion to foil
e.) Set aside for now

Step 2: Prepair the Holder

a.) Place 1 CD in the jewel case and close lid
b.) Center the Bottle cap on the lid open side up
c.) Use the sharpie to trace the outline of the bottle cap
d.) Remove the lid from the jewel case
e.) VERY CAREFULLY use the back of the knife tip to scrap the plastic away under the sharpie line until the center piece falls out

Step 3: Finish Second Plate Assembly

a.) Use point of knife to open a small hole in the center of the top of the bottle lid
b.) add a ring of superglue to the top of the lid and attach to the second CD on the non-tinfoil side
c.) cut off piece of wire and strip the insulation off for 1/4 inch and pass thru hole in lide
d.) Tape exposed portion of wire to tinfoil

Step 4: Assembly

a.) install first plate tinfoil side up in the CD holder with wire hanging out one side
b.) Use superglue to permanently attach the CD to the holder
c.) trim Sheet of paper to completely cover the CD and be easily installed into holder
d.) reattach the jewel case lid
e.)  pass bottlecap of plate 2 thru the opening in the jewel case and close the lid.
f.) You should be able to easily spin the top CD now, but it should not spin on its own
g.) tape the lid closed (loose enough for cd to still spin)

Comments

author
Dave Kruschke (author)2015-04-04

It certainly took more than 10 minutes for me to fabricate this device. 110 minutes is more like I required. And when I tried to cut a hole in the plastic CD case with the "back of a knife tip," there were many cracks and near shattering. But I suspect that maybe all CD cases are not identical.

That said, your project introduced me to new, creative ways to use different glues to assemble craft like artifacts - well worth the 110 minutes...

author
meddler (author)2010-05-25

I have been trying to find plans to build a crystal radio. Would this work as a station tuner?

author
dloranger (author)meddler2010-05-26

It should work for your needs, just need to find out what value of capacitance it creates so you can build your inductors appropriately.

author
Dave Kruschke (author)dloranger2015-04-04

Take the middle of the AM radio dial to be 1000 kHz. Take the middle value of the 44 pf to 145 pf capacitor to be 94 pf. Doing the math gave me a value of 270 uH. Be prepared to add more windings if you want to receive the lower end of the AM dial.

author
Dave Kruschke (author)meddler2015-04-04

Yes but it wouldn't be very "selective," that is, you might mostly hear the strongest stations or even have stations overlapping. You would need more than the typical 200 uH coil to match this capacitor.

author
7654321 (author)2011-01-17

I tried making a capacitor with aluminum foil and press n seal. It didnt work at all lol. Before I try this, can you give any kind of rough estimate? Like 100-500uf?

author
Dave Kruschke (author)76543212015-04-04

44 to 145 pf or .000044 to .000145 uf.

author
Phil B (author)2010-05-13

This is an interesting use of old CDs.  Have you done any readings with a capacitance meter of any kind?  Would readings be in picofarads?  Would there be enough capacitance that the readings would be in microfarads?  It would be great if you could utilize the metallic layer already in the CDs.

author
dloranger (author)Phil B2010-05-14

I wish I had access to an LRC, would make my life much easier!  I would be interested in getting some approximations as I am trying to build a completely "home brew" radio receiver for a 20 MHz AM signal for school

author
psychotron (author)dloranger2011-08-12

capacitance depends on the area of the plates and inversely to the thickness of the dielectric and also the dielectric constant of the dielectric.
http://www.daycounter.com/Calculators/Plate-Capacitor-Calculator.phtml
http://members.gcronline.com/cbrauda/0007.htm

using wild guesses I get about 750 pF

author
Dave Kruschke (author)psychotron2015-04-04

The highest value I measured was 145 pf.

author
Phil B (author)dloranger2010-05-15

Capacitors pass AC current.  How about lighting a small bulb with low voltage AC current through capacitors of known values and comparing the brightness of the bulb when the same current passes through your capacitor?  It would be crude, but it could give you an idea of the values.

author
Dr.Bill (author)Phil B2011-08-14

Or you could measure the Farads with a meter that measures capacitance.

author
Dave Kruschke (author)Dr.Bill2015-04-04

Yes, pico farads where 1 farad equals a million million farads...

author
Dave Kruschke (author)Phil B2015-04-04

I know firsthand that 150 pf in series with a neon test light at 120 volts ac will result in the neon bulb lighting. The current draw would be 1 milliamp or less and this current wouldn't light many incandescent bulbs made today.

author
Dave Kruschke (author)dloranger2015-04-04

44 pf to 145 pf, values I measured when I built my own capacitor, would be close to workable with 20 mHz. If not, a fixed capacitor in series would lower the overall capacitance.

author
Dave Kruschke (author)Phil B2015-04-04

I just made one of these and my capacitance meter revealed values from 44 pico farads to 145 pico farads, depending on the knob adjustment.

author
psychotron (author)Phil B2011-08-12

the metal layer is beneath a very thin lacquer layer, so paint thinner might do it.

author
raypsi (author)2012-05-22

The Equation of a plate capacitor is:

C=[0.2248*k*A]/d

Where k is dielectric constant most plastics it's 2.4
A is the area of one plate in square inches
and d is the distance between the 2 metal plates in inches

Voltage breakdown is about 600volts per 0.001 of an inch up to 0.005 inch at which point the max voltage can't be exceeded by much unless you add more plates in series.

73
de N8ZU

author
Dave Kruschke (author)raypsi2015-04-04

I just made one of these and if I followed the instructions correctly, there is paper (not plastic) between the two semi-circle of aluminum foil...

author
Snappy83 (author)2011-06-09

Ha ha, ok thats scary! I thought about using cd's to make a variable cap, good minds think alike! nice job!

author
35Timmy (author)2010-08-24

if your looking for a crystal radio check out am radios at
http://electronics.howstuffworks.com/radio9.htm
& use resonance &
use the capacitor to tune into the fequncy & radio station you want

author
rimar2000 (author)2010-05-14

Clever!

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