Crystal Radio--DIY Air Tune Capacitor? Pt1 (4K)
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- Опубліковано 1 жов 2024
- With air tune capacitors getting more expensive/rare, I'm looking for a cheap & easy way to make my own. This is a first attempt that I think has potential.
Warning: the materials used to coat the HDD disk are extremely toxic and should not be ingested or inhaled.
When I was a kid I got a crystal radio kit and didnt like making tunable coils becasue I didnt want to ruin the wire by stripping it, so I made a variable cap for tuning from 2 circles made from cardboard cutouts. Stick a pin through the centre of both and glue aluminum foil to a half-circle on each side, and then carefully put a smooth single layer of plastic tape over the foil of one side. Then mate together the discs taped-foil side to foil side - the top disc can be rotated to adjust the surface area of aluminum separated by tape. I remade one recently from 12cm diamater discs I can pan from ~1pf-202pf. The ones I made as a kid were usually ~5cm diamater, which in hindsight is probably why I could only catch a few stations!
Works best when it has something pressing down the cardstock from above, like washers on the pin glued on, otherwise your tuning will jump as soon as you go to adjust it.
Scotch tape doesnt seem to have a publically avaliable dialectric constant but it works pretty well, probably becasue of the narrow gap it makes.
All the more reason to ( retain ! ) these parts from old ( busted ? ) radios ... these variable capacitors will NEVER be re- manufactured , except for ( VERY , VERY expensive ) lab items ....... DAVE™🛑
True, and at $25 to $35 each, they are **worth** keeping!
A much Easier to build & more "Antique" realistic looking is the vintage "Book" variable Cap used in the Earliest tube radios....1 pair of 2" x 3" rectangular metal Flat plates, Insulated & Hinged 1 long side (Like 2 book pages) compression adjusted by a fine machine screw makes a relatively easy to make 15--400pf stable Variable capacitor Identical to some earliest vacuum tube radio types...I've used them & Works fine
A sheet of window glass makes and excellent dielectric. You use copper or aluminum for the plates. For the stator I would glue copper foil to one dide of the glass. Put the 'rotor' on the other side, the metal on this side should be thick enough to be rigid. Glue a small hinge to the glass close to one edge of the aready attached plate. Attach the rotor plate to the hinge. Now build a small tower on the opposite side such that you now have a screw and nut assembly to push the plate towards the glass. Sprong load the plate so it is pushed away from the glass. The screw overcomes the spring pressure and brings the plates together. Attach a wire to each plate. Ta-da! You have a variable capacitor! To keep static noise down it would be best to insulate the screw and the spring from the plate.
This will enable you to make the capacitor physically smaller as glass has a high dielectric coefficient.
Hope that helps.
Use thin copper clad pcb for sections
Mica sheets are just a few dollars and copper plates just a few dollars. Copper, highly conductive and solderable -- as you do indeed know. Mica -- very thin and low dialectric constant.
Interesting. Watching to see more.
From the bottom up... A wood base. Drill a hole. Glue a threaded nut over the hole, to the bottom of the board. Flip the board and glue one disk to the base, centered over the hole. Glue a popsicle stick across the hole of the second disk. Thread a bolt through the nut and through the board. Glue the bolt to the center of the popsicle stick. Turn the bolt to precisely separate the two disk. The popsicle stick must be to the outside of the disk pair, before gluing the bolt to it. Otherwise, the popsicle stick will keep the disks apart. Just a first thought... Hope it made sense. I wonder if an old 45 RPM center disk adapter might be employed, somehow...
The contraption could also be turned on the vertical. Instead of turning the board flat, turn it vertical and make it narrow, like a pillar. Or, make two vertical stands, mount the bolt on the vertical, between them. Make both disks with the popsicle nut arrangement. Spin the disks apart...
I remember reading in a early Mother Earth news magazine of a Crystal radio build where the used a square of AL foil and a gum wrapper to make a variable cap. They also used a penny for the detector .if I remember correctly
I don't know about the penny detector, but that style of capacitor can work.
One main flaw I see with your idea is that if both plates touch each other and rub while tuning their surface will get damaged (at least over time) and their capacitance will change.
True. I mentioned in the video that one can put in an insulator (say paper) that could be replaced. Also, the operator will be adjusting the capacitance to tune, so if the min / max values are not perfect, it probably won't matter. You can adjust around that defect.
@@tsbrownie Overall I think your idea has quite potential 🙂👍
Interesting video!
Thanks for the visit.
Try Press-n-Seal plastic wrap as an insulator. It is sticky on one side and easy to work with.
Is the other side slippery? I'm not familiar with it.
Yes, and super glue won't stick to it. I used it to rebuild battery hinges on In the Ear Hearing aids.@@tsbrownie
@@mrdovie47 Interesting. Thank you.