Re: Capicitor tuning VCO/DRO

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Bill Vance <ccq@x> wrote:

> Hi folks;
>
> I've got a question about tuning capacitance. I've had some

thoughts about

> doing a DRO or otherwise, positional feed back to servo, using a

frequency

> counter circuit, fed by a VCO. I've never been able to find a

source for the

> prospective chip for doing this, but if I do, I'd like to play with

it.

> Originally the chip came out as the MC4024, (Motorola chip, not

CMOS), and was

> later designated as the 74424/74LS424. Its freqyuncy range is zip

to 25-30 MHz,

> and while its a VCO, it can also be controlled across its full

range with just

> a capictor.
>
> My thought is to make a vari-cap by way of taking some stiffish,

springy, steel

> wire like a guitar string or piano wire, insulating it, and running

that down a

> piece of narrow guage tubing. Capacitance can then be tuned by how

much wire is

> slid in or out of the tube.
>
> Now here is the meat of the question. Whatever the range of

capacitance this

> would amount to, is there a way to tune the ends of the above

discribed

> vari-cap such that it would then be able to feed the proper full

range of

> capicitance to the chip, without running above or below the

capacitance required

> by the chip, in a reasonably linear manner? Maybe the capacitive

equivalent of

> an R2R network with one side going to ground? Another, (ordinary),

vari-cap, or

> other circuit element(s)? I'm pretty sure this would require

shielding, which

> might affect things, too. Thanks in advance.
>
> Bill
>

Bill there are going to be several issues. 1.) the amount of
capacitance change is going to be relatively small (although you can
work within a narrow range of the VCO). One interesting circuit is a
one that would "read" the value of a capacitor. I have seen various
circuits that use cheap parts and output a DC voltage porportional to
the capacitor value. Google for "capacitance meter circuits"
or "measuring capacitance". Find ones that will read small values of
capacitors down into the mmfd ranges.

2.)A added issue will be the possible non-linearity of the wire in
the tube configuration. Honestly I don't know if the surface area of
the two plates of a capacitor and the distance apart are a linear
function. Surrounding masses of capacitance (people, machinery, etc)
could have effects. I once built a circuit that would sense the
capacitance of a human body at about 6'. It was notoriously unstable
because things change with different ground moisture and air humidity
conditions. RF worked a lot better.

I am sure there are measuring instruments out there that use
capacitance to measure distance.

You might be able to make a really neat random frequency transmitter
or an interesting theremin (audio frequency changes with capacitance)
project! Your neighbors will love you!