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'negative voltage.'
1998\12\25@082453 by Darkness

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Hi people,
   Does anyone now how i can get a negative voltage to drive the
contrast of the 16x1 LCD display, FROM a single supply of +13.5

Thanx
 Tim

1998\12\25@103401 by WIL REEDER

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Wil Reeder
spam_OUTteachtechTakeThisOuTspambc.sympatico.ca
Vancouver,Canada

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1998\12\25@180859 by Geoff Thornton

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Another way if you dont want to use all the gates and transistors is to use
a 555 timer astable running at about 9kHz to drive the diode pump (ie the
two diodes and capacitors shown on Will's circuit.

{Quote hidden}

1998\12\26@002900 by Norm Rosenzweig

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Hi Tim
you wrote.
----------
> Hi people,
>     Does anyone now how i can get a negative voltage to drive the
> contrast of the 16x1 LCD display, FROM a single supply of +13.5
>
> Thanx
>   Tim
>

Have you tried the Si7660 or Si7661.
These chips are 8 pin devices, the 7660 will give you a up to a -10 from a +10 supply while the 7661 will give you up to -20 from a +20 volt rail.
All thats required is a 10uf Electro between pins 2 and 4 with the + toward pin 2.
Pin 3 in ground
Input is pin 8
pins 1, 6 & 7 not connected.
output is pin 5.
On pin 5 have a 10uF electro to ground + toward ground
NOTE + toward ground, output on pin 5 is negative.

All thats required is a Si766x and 2 x 10uF electros.

cheers Norm
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    Visit also my home page at.
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1998\12\26@003524 by Don McKenzie

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Norm Rosenzweig wrote:
>
> Hi Tim
> you wrote.
> ----------
> > Hi people,
> >     Does anyone now how i can get a negative voltage to drive the
> > contrast of the 16x1 LCD display, FROM a single supply of +13.5
> >
> > Thanx
> >   Tim
> >
>
> Have you tried the Si7660 or Si7661.
> These chips are 8 pin devices, the 7660 will give you a up to a -10
> from a +10 supply while the 7661 will give you up to -20 from a +20
> volt rail.

Of course, if you have RS-232 support on board, you will have that
negative source. It can be tapped off the appropriate pin of a Max-232
or similar.

Don McKenzie  @spam@donKILLspamspamdontronics.com   http://www.dontronics.com

Don's Download Dungeon http://www.dontronics.com/download.html
For more details, send a blank message to KILLspaminfoKILLspamspamdontronics.com
or RemoveMEsimstickTakeThisOuTspamdontronics.com or spamBeGonebasicsspamBeGonespamdontronics.com

1998\12\26@035257 by Pablo Bleyer Kocik

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Norm Rosenzweig wrote:

> Hi Tim
> you wrote.
> ----------
> > Hi people,
> >     Does anyone now how i can get a negative voltage to drive the
> > contrast of the 16x1 LCD display, FROM a single supply of +13.5
> >
> > Thanx
> >   Tim
> >
>
> Have you tried the Si7660 or Si7661.
> These chips are 8 pin devices, the 7660 will give you a up to a -10 from a +10
supply while the 7661 will give you up to -20 from a +20 volt rail.
> All thats required is a 10uf Electro between pins 2 and 4 with the + toward pi
n 2.
> Pin 3 in ground
> Input is pin 8
> pins 1, 6 & 7 not connected.
> output is pin 5.
> On pin 5 have a 10uF electro to ground + toward ground
> NOTE + toward ground, output on pin 5 is negative.
>
> All thats required is a Si766x and 2 x 10uF electros.
>

Check out http://www.national.com/pf/LM/LMC7660.html also.

The PDF file application note http://www.national.com/ds/LM/LMC7660.pdf is speci
ally useful.

Cheers & merry xmas!

--
Pablo Bleyer Kocik |"The most exciting phrase to hear in science, the
pbleyer            | one that heralds new discoveries, is not `Eureka!'
   @dgf.uchile.cl | (I found it!) but `That's funny...'" -Isaac Asimov

1998\12\26@043442 by Saise

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Geoff Thornton wrote:



> Another way if you dont want to use all the gates and transistors is

> to use a 555 timer astable running at about 9kHz to drive the diode

> pump (ie the two diodes and capacitors shown on Will's circuit.



 The NE555 is commonly touted as the answer to every problem.  It is

quite cheap.  It is however, rarely the optimum answer.  For *this*

particular problem:



1} The task is to drive a 10k bias potentiometer.  This (1.3mA) is the

total load.  The source is specified as 13.6V, presumably automotive

(thus, needs to be protected by a 22 ohm resistor and 15V 5W Zener or

equivalent).



2) An ICL7660/ Si7660 is *not* rated for this voltage.  You need the

7661.  It is the minimum component count (three capacitors; input,

output and shuttle) and most compact solution but it is not a regulator.
No other suggestion is either.



3) If you have a MAX232 (which you often do!) fed by 5V, you *have* a

-10V rail, (don't get confused; it is *not* -12V or -15V, but no matter

for this application), moderately regulated (depending on the RS-232

line load).  A MAX231 has in effect the 7661 included, mirroring the

supply voltage.



4) The transistors on Will's circuit are unnecessary in this

application; you will most likely obtain a greater voltage without them

(at 1.3mA that is!).  I would suggest instead, a 40106 with one gate as

oscillator and the other five paralleled as the current driver.  A B-

series gate may be preferable.  This is probably the *cheapest* option.



5) An NE-555 is a poor performer in this circuit, it's pull to the

negative rail is about 0.4V, while it's pull to the positive rail is

short by some 1.5V or so.  A 7555 is much better, but has little more

drive (not sure on this) than the 40106, costs a lot more and has the

same component count.  To assert the 7555 circuit is simpler is facile;
it has a smaller footprint.  It is more accurate but in this circuit
this is again irrelevant.



 You figure!

--

 Cheers,

       Paul B.

1998\12\28@090940 by paulb

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Pablo Bleyer Kocik wrote:

> Check out http://www.national.com/pf/LM/LMC7660.html also.

 Nice links!

 Interesting detail; no "7661" part found on NatSemi's site - they
don't make them.  Can't use 7660 over 10V input, though it's fine for
5V (and that's what you would mostly use for the LCD bias).
--
 Cheers,
       Paul B.

1998\12\29@124901 by John Payson

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|    Does anyone now how i can get a negative voltage to drive the
|contrast of the 16x1 LCD display, FROM a single supply of +13.5

I would suggest that if you have a semi-spare(*) PIC port pin
available, you may want to use it to drive a diode/cap charge
pump(**).  This would require less circuitry than other methods
and be somewhat more versatile(***).

(*) Depending upon how the circuit is designed, you may be able to
   share functions with another port pin.

(**) You may want to have the output filter cap be large relative
    to the series cap, in which case the drive voltage will be
    primarily dependent upon the ratio of (drive frequency times
    the series cap) to (resistive load on output cap).  The larger
    the output cap, the less ripple there will be on the output
    voltage.

    Another approach which works well on displays where you're
    generating the scanning clock is to forego the output diode
    and cap entirely and skip the LCD voltage pot.  The drive
    voltage will then oscillate between about 0 and -5 (more
    likely, between about 0.7 and -4ish); varying the duty cycle
    of this drive will control the display contrast.  I would
    not recommend this approach for the prefab text modules,
    though, unless you tap a wire on there to sense the scanning
    pattern (which would then open up some very nice possibilities
    beyond the scope of this post).

(***) With both of the approaches given above, the PIC can control
     the display's RMS drive voltage.  Consequently, there's no
     need for a pot to set display contrast--it can be done in
     software.  Cool, eh?


'negative voltage.'
1999\02\04@130357 by Marc
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> Another way if you dont want to use all the gates and transistors is to use
> a 555 timer astable running at about 9kHz to drive the diode pump (ie the
> two diodes and capacitors shown on Will's circuit.

Hey, you have a PIC already in your circuit. Use a PIC port pin instead of a 555
!

1999\02\04@211247 by paulb

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Marc wrote:

> Hey, you have a PIC already in your circuit.  Use a PIC port pin
> instead of a 555!

 Beware the 555 I say!  The "bipolar" version doesn't pull at all well
to the positive supply, while the CMOS version has poor current sourcing
capability - while it can sink 100mA, it will only source 10mA.
http://www-s.ti.com/sc/psheets/slfs043c/slfs043c.pdf  This suggests it
is not as symmetrical as might be thought.

 I therefore agree - you are likely to get better conversion
performance from a PIC pin.

 If you must add something, there's a lot to be said for using the
purpose-designed 7660 (or 7662 for over 6V).
--
 Cheers,
       Paul B.

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