"Mad Scientist Jr" wrote in message
ups.com...
I would like to hook up a variable capacitor to some audio circuits I
built for learning about synths and amps
(schematics are he
http://www.geocities.com/usenet_daug...generators.htm )
http://www.runoffgroove.com/littlegem.html
http://www.runoffgroove.com/ruby.html
http://makezine.com/09/crackerboxamp/
)

because in some of these the capacitors change the pitch, tone or
other qualities.

These circuits run off a 9V battery and use capacitors with ratings
such as
220 uF
100uF,
0.1 uF,
0.01 uF
0.047 uF
so I would want variable caps in this range.
Do these exist? Online all I am seeing is ones rated in pF.

Also, assuming I find these, and get them working in the circuit, and
find a desired setting for a capacitor, how do you measure the
capacitance? I have a multimeter but have really only used it to
measure ohms.

Thanks...

As others have mentioned, these values are not widely available in variables
partly because they are unweildy, and partly because they are rarely needed.

Take a look at your circuits. Many of the capacitors are "uninteresting".
For example, if you look at your "little gem" amplifier, the 100u is simply
filtering the supply (looks like a little overkill), the .01 appears to be
mostly DC blocking, although it is small enough it might have an efffect on
bass response. Ditto for the 220. The only one that looks like it might be
interesting to experiment with is the 0.047. The other circuits are
similar. Capacitors have all sorts of functions in various circuits, and
for many of those functions, playing with the values isn't very satisfying.

Before you go building these things and playing with them, I would suggest
modeling them in some SPICE flavor or another. This would allow you to
experiment with the frequency response without actually building the
circuit. Once you have seen some interesting SPICE responses, *then* go buy
some capacitors.

As far as measuring capacitance, you can buy meters, more commonly LC
meters, and many multimeters include them. You can also make an LC
oscillator, measure its frequency, then put the unknown C in parallel with
the C in the oscillator, and measure the frequency again. You can then
calculate the unknown C. Another way is to put the unknown C in series with
a known resistor, put some RF across it, and measure the voltage drops with
an RF probe. Again, a little math reveals the unknown C.

...