Cgs synth

I am building a joystick controller based on the schematic at CGS
site. The offset pot is specified as 25K but I would like to use 50K
because that's all I have onhand. Will this work, would I need to
change the 22K resistors coming from the +/- supplies to different
values ?
Also, I have spring centered joysticks and would like to get
negative voltages when I pull down and positive going up. Can I just
attach the pot end that goes to ground to the negative supply ? If so
will I need a resistor matched to my joystick in series.
This probably "potentiometers 101" for most so forgive me. Any and
all responses greatly appreciated.

Peng

--- peng3002 <peng3002@...> wrote:

> I am building a joystick controller based on the schematic at CGS
> site. The offset pot is specified as 25K but I would like to use
> 50K because that's all I have onhand. Will this work, would I need
> to change the 22K resistors coming from the +/- supplies to
> different values ?

You do not have to change the resistors, but you may want to. You can
use any size pot and resistors that you want (as long as they are not
too small). Once you pick your pot, you will find that you use the
resistors on each end to set the overall range of the offset pot.

Look at the string from +15 to -15 and realize this is a simple
voltage divider. If each element is the same size, you will get
equal voltage drop across each. So, the range of the pot will be 10
volts (+/-5). If you want the offset range bigger, reduce the size
of the resistors compared to the pot and vice versa. So, if you
change to a 50K pot and like the 10 volt offset range, change to 47 K
resistors.

Or, you can just leave the end resistors out, and increase the size
of the 47K resistor that connects the pot wiper it to the op amp to
150K.

BTW, all this assumes you have the range trimmer turned to full 47K.
The more you reduce the size of the trimmer, the smaller your actual
output range will be.

> Also, I have spring centered joysticks and would like to get
> negative voltages when I pull down and positive going up. Can I
> just attach the pot end that goes to ground to the negative
> supply ? If so will I need a resistor matched to my joystick in
> series.

Yes. That will also give your joystick a 10 volt range (+/-5)

Larry H

__________________________________
Do you Yahoo!?
Yahoo! Mail SpamGuard - Read only the mail you want.
http://antispam.yahoo.com/tools

>I am building a joystick controller based on the schematic at CGS
>site. The offset pot is specified as 25K but I would like to use 50K
>because that's all I have onhand. Will this work, would I need to
>change the 22K resistors coming from the +/- supplies to different
>values ?

Increase the 22k resistors ro 47k.

> Also, I have spring centered joysticks and would like to get
>negative voltages when I pull down and positive going up. Can I just
>attach the pot end that goes to ground to the negative supply ? If so
>will I need a resistor matched to my joystick in series.

Yes and yes. It is the ratios that are important, not the exact values.

Ken
_______________________________________________________________________
Ken Stone sasami@...
Modular Synth PCBs for sale <http://www.blaze.net.au/~sasami/synth/>
Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/>