Resistors are to potentiometers as...

[Single Coil]

Capacitors are to what? Is there such a thing as an adjustable capacitor?

[Seljer]

variable capacitors do exist, however only for very small values (only in the low picofarad range, which is enough for tuning a radio to the right frequency).

http://images.google.com/images?hl=en&q=variable%20capacitor&btnG=Google+Search&sa=N&tab=wi

a capacitor is basically just 2 conductors, seperated by an insulator, and you just move around one of the plates to change the capacitance (only the overlapping area is working as the capacitor), or you could increase the distance between the 2 conductors

[tcobretti]

As I recall, the old DOD Wah used some new fangled variable capacitor instead of a pot.  However, this didn't save it from sucking.

Just found this:

http://www.diyguitarist.com/DIYStompboxes/FX-17.htm

So it is possible.

[captntasty]

Yep...  however, the only ones I've ever seen are small values within the realm of stompboxes - say, 5pf-900pf (not exact numbers but you get the idea).  The most common usage I've seen is the tuning mechanisms in older radios although it could be used to adjust any kind of "frequency".  They would be about the size of a 24mm pot and larger.  Check out mouser.com for trimmer capacitors for devices that physically could be used in a stompbox but the values (<1pf-200pf) are so small that I don't see them being suitable for any application in a stompbox.

edit: wow we all jumped on this one, huh? 
edit#2: forgot about this one and I don't remember where the image originated, but... http://www.diystompboxes.com/smfforum/index.php?topic=49306.0 

[Mark Hammer]

Capacitance is produced by having two plates and a gap to be crossed by the current.  Most of the time this consists of two sheets of conductive foil with a very thin sheet of insulating material between them.  Larger capacitance values are produced by having a much larger surface area, although we get to see it as a rolled up little cylinder, like a capacitance cinnamon roll.

What that means is that varying the potential surface area where the charge can "jump" involves creating reconfigurable surface areas; either surfaces that can become bigger or paralleling a whole stack of them that become only a little bit bigger.  Physically, they are like a deck of cards being shuffled in slow motion as you tune the capacitor, with each of two sets of plates moving closer together or farther apart like alternate cards.  Take apart an old receiver sitting in someone's driveway on garbage day and you'll see what I mean.  Variable capacitors can often be too large to fit inside a 1590BB.  It shouldn't surprise you then, that small-sized PC-mount variable caps tend to be very low capacitance values, such as those indicated by previous posters.

If it was the case that the only way to tune in radio and TV stations was the "old-fashioned" way via variable capacitors in an LCR circuit, then I imagine you'd see the further development of of high density multi-plate variable capacitors that could be the size of, say, a 10uf electrolytic cap, the same way we've seen the development of incredibly small multi-platter hard-drives for devices like iPods and laptops.  Unfortunately, it would seem that variable capacitors as tuning devices have been eclipsed by other ways in the digital domain, making them "legacy technology"......i.e., largely obsolete, hence ignored by developers.

[Single Coil]

wow, there was already a thread on this...I didn't even think to look first...thanks for pointing that out.

That vari-cap potentimeter looks really cool. I may try it. I was wondering about a variable capacitor for the sweep cap in my GCB-95 wah. which originally is 0.01 and the recommended change is to a 0.022. Not a huge change, but enough to improve. But I was also wondering about using this for the treble bleed mod on the guitars volume pot.

That vari-cap may be the solution. very cool.

[Rectangular]

I was really hoping that the criteria for the next FX project (Oct) would be usage of a variable capacitor in an fx circuit. this seems to very fairly uncommon, but as mark hammer pointed out to me in the past, even a few pf can make a lot of difference in BBD/clock circuits. I'm sure there are much more interesting things that could be done though

[idlechatterbox]

QUOTE:
"Capacitance is produced by having two plates and a gap to be crossed by the current.  Most of the time this consists of two sheets of conductive foil with a very thin sheet of insulating material between them. "


You can make a McGuyver type radio using sheets of aluminum foil alternating with sheets of wax paper (about the size of a floppy disk), and an old diode (and headphones). You "tune" the station by lifting up the sheets, to put more or less distance between them. It's nerdy, but it does sort of drive home the "two plates" concept. 

[A.S.P.]

http://www.diystompboxes.com/smfforum/index.php?topic=29898.0
http://www.diystompboxes.com/smfforum/index.php?topic=31312.0
http://www.diystompboxes.com/smfforum/index.php?topic=24307.0

[Sir H C]

Diodes are also used as variable capacitance devices.  By varying the reverse voltage on them you can change the capacitance.  Varactors are what they are called when designed to maximise this property.  Still they don't have much capacitance or range.  Other ways to make a variable cap include the miller effect combined with varying the circuit gain, and some other more convoluted techniques.

[Single Coil]

Diodes are also used as variable capacitance devices.  By varying the reverse voltage on them you can change the capacitance.  Varactors are what they are called when designed to maximise this property.  Still they don't have much capacitance or range.  Other ways to make a variable cap include the miller effect combined with varying the circuit gain, and some other more convoluted techniques.

Let me ask this...Can diodes be substituted for capacitors? Which cap values can be replaced with a diode?

[Seljer]

You can make a McGuyver type radio using sheets of aluminum foil alternating with sheets of wax paper (about the size of a floppy disk), and an old diode (and headphones). You "tune" the station by lifting up the sheets, to put more or less distance between them. It's nerdy, but it does sort of drive home the "two plates" concept. 

you might as well go all the way on the McGuyver radio: http://sci-toys.com/scitoys/scitoys/radio/homemade_radio.html

[Sir H C]

Diodes are also used as variable capacitance devices.  By varying the reverse voltage on them you can change the capacitance.  Varactors are what they are called when designed to maximise this property.  Still they don't have much capacitance or range.  Other ways to make a variable cap include the miller effect combined with varying the circuit gain, and some other more convoluted techniques.

Let me ask this...Can diodes be substituted for capacitors? Which cap values can be replaced with a diode?

In specific situations yes.  The problems are that you have a very non-linear capacitance so you have to look at the AC and DC across the device and see that it is not introducing too much distortion.  Also the cap value is very small as these devices are mainly used for RF VCOs and the like.  That said, PAiA used diodes to control the corner of a filter:

http://www.freeinfosociety.com/electronics/schemview.php?id=390

not using the variable cap of the diodes but the variable impedance with DC voltage.

[Seljer]

^^
ah, so thats how the Tim Escobedo Phcugnosis (or however its spelled) works?

[idlechatterbox]

QUOTE:
"you might as well go all the way on the McGuyver radio: http://sci-toys.com/scitoys/scitoys/radio/homemade_radio.html"


You got me there!

I can see it on the old show too: "... then I remembered that I had an old #2 pencil in my shoe, and that, along with the razor blade I had found in the bathroom, the graphite in the pencil could be used to....."

That was outstanding technical writing; I liked it so much I read how to make a transmitter out of alligator clips (though to be honest, those are way too conventional for McGuyver) and a 9volt! 

great link

[Seljer]

Well the thing predates McGuyver a little bit its mentioned they were used by soldiers in WWII, they were known as foxhole radios.

and yeah, that website is awesome