[sdiy] cv scaler

[Byron G. Jacquot]

>For my next mini-project, I want to build a circuit that will let me
>either amplify or attenuate any control voltage (up to audio
>frequencies), and be able to add a plus or minus offset.  I'm
>envisioning something with two pots - a scale and an offset - both with
>center detents.  With both in the center position, the signal would pass
>unchanged.  Does anyone have a circuit like this, or could someone give
>a beginner enough hints to "build from scratch"?  :)

Sounds like a job for a dual opamp: first stage as an inverting summer,
where you can add the input and offset together.  The offset V would come
form a pot wired with the ends to +V an -V, and you'd tap the mix from the
wiper.

The second stage would be an inverting buffer, possibly with some gain, to
get that amplification you want.  You'll need to decide where you want the
scaling to come.  If you want to only scale the input voltage, but not the
offset, you'd do it on the input to the summer.  If you want to scale them
both together, put an attenuator on the output of the second opamp.

You might also look around for some simple books of opamp circuits & theory.
This sort of project falls right into that kind of stuff...opamps are good
for addition & subtraction of voltages (even many operands at once), and
division/multiplication by fixed values.  There are several I could
recommend...Jung's Opamp Cookbook is a good start.  The little Forrest Mims
project books from Radio Shack have some good ideas in 'em, and are
affordable.  The "Encyclopedia of Electronics Circuits" by Graf (in many
redundant volumes) always has some interesting stuff, but it's light on
testing and theory...check what your local library has, too.  Eventually,
you'll probably want a copy of Horowitz & Hill's "Art of Electronics."

Byron Jacquot