Variable capacitor

[ecollen]

That is not complicated at all,here is the Cap/Vr graph from the BB112 datasheet .
you can see the points you need (typical value).
Since Vr/Cap is a logarithmic relation in Vr, it is better to use a logarithmic pot. to obtain the Vr needed.
You can usea 1K ohm pot and 1M,resulting in a higher current consumption from the 9V battery.

View attachment 23581

Hi Dorke
Many thanks for this. I won't pretend to understand what it's all about, but I'll use the example to try to find one of the locally available ones that will (perhaps!) to do the job.

 

[ecollen]

Hey,
No doubt variable capacitors are very hard to find. There are some dealers providing variable capacitors from USA and here is one variable Gen Rad1422: http://www.used-line.com/capacitors/capacitor-air/gen-rad-1422/item-10591756

May be it will help a bit.

Hi James
Many thanks for your reply. That looks like quite a sophisticated variable capacitor, and I would get a quote for it and the postage, but unfortunately our postal system has become so bad out here lately that it's not worth risking the cost just yet. I'm checking out the other options first, but I'll definitely remember this.

 

[ecollen]

OK, I see your better at decryption of Colin's posts than I am but I still don't get his reference to a Variometer. Did you click the link I provided that describes a Variometer? Maybe he had a brain fart and meant Varicap.

BTW: His website beats the hell out of his posts!

Chris

Hi Chris
Yes, I reckon that's what he meant anyway. His website is chock a block with wonderful information, that's for sure. Not always too easy to locate, but it's there.
Sorry, I should've mentioned it, it's not that kind of variometer. The one he means is the arrangement with two moving coils, It sounds fascinating, and I'm definitely going to try it the minute I get a chance. The problem is I'm always short of time - 20 projects on the go, and not enough time to do half a one, you know how it is. But this is one I'm planning to try soon. I think first in line is probably the variable capacitor made with sheets of aluminium foil and paper though.
Have a look on Colin's page http://www.talkingelectronics.com/projects/CrystalSetRadio/CrystalSet.html, scroll a good way down and you'll find the description and pictures of "An inductive tuning coil called a variometer"

 

[ecollen]

Not really. General Radio was never known for consumer electronics. They were known world wide for top of the line (expensive) electronics instrumentation. Your link points to a capacitor lab standard.

Chris

Hi Chris
Is that what it is? It looked like quite a fancy variable capacitor to me. Shows you how much I know! Anyway with my Problems with the Post Office (sounds like the title of a poem, doesn't it?), it wouldn't really be an option for me. The Crystal Set Society in America also sells them, beautiful looking jobs, but the postage they charge means I'll probably never risk ordering one. (I'd be better off buying a plane ticket to the US and picking it up personally!)

 

[ecollen]

Not really. General Radio was never known for consumer electronics. They were known world wide for top of the line (expensive) electronics instrumentation. Your link points to a capacitor lab standard.

Chris

Hi Chris
By the way, way off topic here, if that's you on that bike, shouldn't you be right over on the right of the road? I rode for many years and loved it (nowadays you'd take your life in your hands going on to the roads here) and one of the unwritten rules for survival on the road was to keep well away from anywhere where the traffic goes.

 

[CDRIVE]

Hi Chris
By the way, way off topic here, if that's you on that bike, shouldn't you be right over on the right of the road? I rode for many years and loved it (nowadays you'd take your life in your hands going on to the roads here) and one of the unwritten rules for survival on the road was to keep well away from anywhere where the traffic goes.

Florida law dictates that if there's no bike lane then you should be in the lane for your direction of travel. Positioning yourself along the right curb with a (Right Lane Must Turn Right) puts you in a more dangerous position than being out in traffic. The vehicles turning right will run you down or simply not let you move. In the case of this photo it's both a straight and right turn lane. I position myself on the left of this lane so right turning vehicles are not impeded. My wife was riding drag when taking that photo but if I wasn't hauling that pipe she'd have been beside me. This is because that's a dual direction lane and some idiots going straight will try to pass us on the right side!

Here's a tip for other members that call Florida home. Florida law states that if a lane is 11 feet wide or less the cyclist has the right to take charge of the lane. IE ride down the center of the lane. This is because Florida law also states that passing vehicles must keep at least 3 feet between their side view mirror and the cyclist.

In the end it doesn't matter how a cyclist handles road sharing. Might makes right and motorist don't give a sh!t! In the last three years I've cycled in excess of 25,000 miles. I consider most of those miles as mortal combat. When I'm King I will decree that bicycle gun ships shall be considered the order of the day.

FYI: My wife has been side swiped and left on the pavement by a hit and run driver and I was left skidding on the pavement when trying to avoid a car blowing a stop sign.

Chris

 

[CDRIVE]

Hi Chris
Is that what it is? It looked like quite a fancy variable capacitor to me. Shows you how much I know!

It could very well be a variable capacitor or two fixed caps with a null control to adjust for stray capacitance. Either way it's definitely a lab standard. It's certainly not something I'd build a regen receiver around.

Chris

 

[ecollen]

Florida law dictates that if there's no bike lane then you should be in the lane for your direction of travel. Positioning yourself along the right curb with a (Right Lane Must Turn Right) puts you in a more dangerous position than being out in traffic. The vehicles turning right will run you down or simply not let you move. In the case of this photo it's both a straight and right turn lane. I position myself on the left of this lane so right turning vehicles are not impeded. My wife was riding drag when taking that photo but if I wasn't hauling that pipe she'd have been beside me. This is because that's a dual direction lane and some idiots going straight will try to pass us on the right side!

Here's a tip for other members that call Florida home. Florida law states that if a lane is 11 feet wide or less the cyclist has the right to take charge of the lane. IE ride down the center of the lane. This is because Florida law also states that passing vehicles must keep at least 3 feet between their side view mirror and the cyclist.

In the end it doesn't matter how a cyclist handles road sharing. Might makes right and motorist don't give a sh!t! In the last three years I've cycled in excess of 25,000 miles. I consider most of those miles as mortal combat. When I'm King I will decree that bicycle gun ships shall be considered the order of the day.

FYI: My wife has been side swiped and left on the pavement by a hit and run driver and I was left skidding on the pavement when trying to avoid a car blowing a stop sign.

Chris

Hi Chris
Ow, man! That looks like cycling here in Johannesburg! All in all, I guess we're lucky you're still alive to tell the tale.
Yes, I really like cycling and have covered miles and miles in years gone by, but as the scars and broken bones started mounting up, every one a bit worse than the previous one, I think I seem to have abandoned the road. Such a pity, because it's so nice.
I follow your description of road usage, but it seems as if there, like here, it's only cyclists who know the rules. Motorists' only rule seems to be to flatten as many cyclists as possible.
Thanks for the explanation. Interesting.
But before the others wonder if this is now a cycling forum and not an electronics forum any more, thanks for the feedback on that piece of equipment that James told us about. Yes, even if I didn't know any better, I probably wouldn't try using it in the tiny receivers that I'm still making - components just screwed into a block of wood! Can you believe anything so primitive? But at least it does have the advantage that if the thing doesn't work I can't blame my (questionable) soldering.

 

[CDRIVE]

thanks for the feedback on that piece of equipment that James told us about. Yes, even if I didn't know any better, I probably wouldn't try using it in the tiny receivers that I'm still making - components just screwed into a block of wood! Can you believe anything so primitive? But at least it does have the advantage that if the thing doesn't work I can't blame my (questionable) soldering.

Hey Collen, whatever floats your boat! There's nothing wrong with prototyping on a wood board. When I was a young sprout that was invariably my protoboard of choice. If you research historic photos and drawings of electronics inventions you will find exquisitely hand made Mahogany platforms adorned with brass and Bakelite terminals. That's what you'll see in museums too! I regret that I lost it but there's a nice fellow that's devoted to crystal radio sets. He has reproduced and created a host of eye popping wood based sets.

Chris

 

[ecollen]

Hey Collen, whatever floats your boat! There's nothing wrong with prototyping on a wood board. When I was a young sprout that was invariably my protoboard of choice. If you research historic photos and drawings of electronics inventions you will find exquisitely hand made Mahogany platforms adorned with brass and Bakelite terminals. That's what you'll see in museums too! I regret that I lost it but there's a nice fellow that's devoted to crystal radio sets. He has reproduced and created a host of eye popping wood based sets.

Chris

Hi Chris
Well, thank you for your kind words. If someone had said that to me "when I were a nipper" I may well have chosen electronics as a career, as I should've done.
Yes, I've seen that site with those beautiful crystal sets, or one like it anyway. They are superb. I'll report back with the URL of that site, if I can find it again. But, um, mine don't look anything like that! Mine are pretty rough and ready.When I was young I stuck crystal sets together and was amazed at the magic of them working. Nowadays I still stick them together and still marvel at them working. The only difference is that now I'm learning how and why they work. Fascinating. One of my projects is still one of those foxhole radios made out of the simplest possible parts - razor blade, rusty nail, perhaps, I think.

 

[CDRIVE]

Oh yeah, that was the first detector I made as a kid. It looked like this. I drew it from memory for another thread some time ago.

Chris

 

[Ratch]

Help for a newbie, please. I’m planning to build a regen receiver (schematic attached), but I can’t seem to get hold of the required variable capacitor. Is there any way to adapt the circuit so that I can get away without the variable capacitor, or any other way around the problem?

Also it seems strange that variable capacitors are so difficult to find. Surely they're still used in many small transistor radios nowadays.

All assistance will be much appreciated.

Errol

Perhaps this might be of assistance to you. http://www.falstad.com/circuit/e-capmult.html

Ratch

 

[CDRIVE]

Collen, you're probably scratching your head after clicking that link to the animated capacitor multiplier. Don't loose too much sleep over it. They don't find much (if any) application in RF circuitry. They're kinda cool though!

Chris

 

[ecollen]

Oh yeah, that was the first detector I made as a kid. It looked like this. I drew it from memory for another thread some time ago.

Chris

Hi Chris
That looks real neat. I hope to try something like that sooner or later, as soon as time allows.

 

[ecollen]

Collen, you're probably scratching your head after clicking that link to the animated capacitor multiplier. Don't loose too much sleep over it. They don't find much (if any) application in RF circuitry. They're kinda cool though!

Chris

You're right! And it looks cool too. I was wondering whether it's Andy Warhol but, seriously, no, I can't really appreciate it, though I'm sure it can do all sorts of wonderful things. Electronics is such a great subject, isn't it.

 

[ecollen]

Perhaps this might be of assistance to you. http://www.falstad.com/circuit/e-capmult.html

Ratch

Hi Ratch
Thank you very much for your reply. Unfortunately it's a bit too advanced for me to be able to understand it or derive much benefit from it yet - hopefully in time to come. No problem though, I'll keep looking at it until I get some vague idea of what's going on. Thanks for your interest

 

[CDRIVE]

Collen if we had award categories for the most responsive, polite, appreciative new member I'd bet you would win all three categories hands down! I'd also bet that anyone responding to your topics would vote that way too!

Impressed!
Chris

 

[(*steve*)]

I've been toying with the idea of making a variable capacitor from single sided PCB (just for the hell of it). PCB has a far higher dissipative loss than air, so the Q of the tuned circuit is far lower.

I also have some brass foil which is more easily cut, and I'm trying to come up with a design for supports that can be laser cut.

If I get further than just thinking about it I'll get back to you. It week be tho later for your project but it may still be interesting.

 

[CDRIVE]

Steve, why not use a double sided PCB? You'd get more capacitance for the square inch. What, no double sided boards in the junk box?

Chris

 

[(*steve*)]

Steve, why not use a double sided PCB?

I'm thinking of using the board as the dielectric. Imagine a stack of boards with alternate pieces electrically connected. The alternate pieces can be meshed and unmeshed tho vary the capacitance.

Double sided board (or just foil) with an air gap is the alternative, but if you ever played with a large variable capacitor and damaged any of the vanes you'll appreciate that mechanically maintaining the gap between the plates requires pretty tight tolerances.