It works! It works! ;-))))))  Way to go Dave, it puts out -17.3V into an
open load (-15.3 at 1.5-2.0ma load).  This generates the a great looking
contrast without even adjusting it.  I used a regular 232N and 1.0uF caps
and tiny little RadioShack 1N914/4148 glass diodes.

Thanks again, it works great.  ;-D  Plus, the added benefit of the 232
running off of a regulated supply keeps the contrast looking good until the
9V battery is just about dead.

michael brown

Dave D the genius wrote:

> If you'd rather roll your own charge pumper using more readily available
> parts, the attached diagram shows an approach using a MAX232A RS-232
> interface chip that'll give you about -18V unloaded.  You can feed a
> trimpot from that, to adjust the LCD contrast voltage.  If you use a
> MAX232 instead of a MAX232A, make all the capacitors ten times bigger to
> compensate for the slower switching frequency.  This design operates off
> the +5V supply, which I assume you're regulating; and that eliminates
> having to deal with a battery voltage that could be anywhere from 9V
> (new) down to about 6V (discharged).
>
> The MAX232A is a very useful chip.  The classical way of looking at it
> is as advertised: it's an RS-232 interface chip with a built-in charge
> pump for generating the interface voltages.  An alternate way of looking
> at it is a general-purpose charge pump chip that can generate +/- 8-10V
> from a +5V supply, and which also happens to have some built-in RS-232
> I/O, just in case you need it.
>
> They're real handy for generating +/- voltage for "funky" analog stuff.

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