making crystal radio variable capacitor?

Question

im making a crystal radio for my science project. there are literally a million ways to make a variable capacitor. is there a simple homemade way to do this? i don't want to drive for 1h to a remote store just to buy a part.
i have seen one that looks like two semi circle and one which is basically two piece of wood board connected together.it would be nice if someone can tell me the EXACT process of how to build them

Answer 1

Hi "serpent":



I saw your other Y!A Question about coils for crystal radios, and had wondered if you were using a circuit design with fixed or variable capacitor, so this Q&A helped in that respect.



I was hoping (in the other discussion) that you were using one of the "old school" stock 365mmF (365 picoFarad) variable capacitors, to make answering your coil design Question easier. But making a simple two-plate variable isn't that hard (I made one over 40 years ago from a project book).



Fellow Y!A Contributor "Lare" made a good suggestion regarding single-side copper plated Printed Circuit Board (PCB), which I think you can still find at a Radio Shack. (It would require two squares of plated PCB mounted face to face with either the protective paper still attached, or a thin insulating layer in between.) But in case that's not convenient or affordable for you, I'll outline what I used in my wooden version similar to what you described in your Question:



●Cut 2 equal squares of Masonite (or other thin wood paneling about 1/8" thick) into 4"x4" size.

●Stack the two squares and drill a small hole (for a #6 or #8 bolt) through both sheets near one corner (this will form the "hinge" point for tuning).

●Cut two squares of aluminum foil slightly smaller than your 4"x4" panels (leave about a 1/4" border).

●Prepare two lengths of thin insulated wire (approx 8" each) by stripping 2" insulation off one end of each.

●Using regular white glue (Elmer's style) or a spray glue (Scotch #77 or equiv) coat the smooth side of each Masonite square completely from edge to edge.

●For each square, lay the 2" bare copper end of one of the wires near a corner adjacent the hinge hole (not at the hinge corner).

●Covering the wire end and the glue coated square, carefully lay down a foil square, smoothing carefully to prevent air bubbles. The bare wire should now be covered by foil, and the wood should show about a 1/4" border on the four sides. (Scrape away any foil near the hinge hole.)

●The next step has two options: Cut one or two 4"x4" squares of wax paper. Glue or tape a square of wax paper over the foil surface (adds both protection and the "dielectric" insulation). As long as one foil surface is covered, you're fine. Covering both protects the foil from scratches.

●Thread a short bolt & nut through the hinge hole, with both foil surfaces face-to-face and you're done!

●Connect the two wires to the rest of your crystal radio circuit, and have fun tuning around.



The amount of capacitance (in picoFarads) varies with how much the two squares overlap. You can calculate the maximum capacitance using this online calculator: http://www.daycounter.com/Calculators/Plate-Capacitor-Calculator.phtml

(There's a section for "inches" as well as "mm".) Use a "dielectric coefficient" estimate of 3 and a spacing "distance" of .004" for one wax paper layer or .008" for two.



The materials aren't critical: you can use regular wood and a nail hinge. You can use two squares of thick cardboard or plastic sheet or pieces from a CD cover.



Here's a link for the same idea using two CD's and the semi-circle concept you had: http://www.instructables.com/id/10-Minute-Variable-Capacitor/



hope this helps,

--Dennis C.

 

Answer 2

Homemade Variable Capacitor

Answer 3

even for crystal radio, a commercial capacitor is used as it make tuning easier. you can wind your own coil, that is not so critical. the problem with home built caps is it is not easy to get a high capacitance value needed to tune in the AM broadcast band. You can use the 2-sided printed circuit board to make a fixed cap but then you will have to make the coil tunable, a much more difficult chore, that is how the "Boy Scout" kits worked when i was a kid, and they weren't very satisfactory. And you will still have to make the hour drive to get the PCB material. if you live a hour from a major city, it is possible that there won't be a local AM station that can be received by a crystal detector either. crystal sets are not for weak signals.

Answer 4

hey buddy, try that translator again