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'[PIC][EE] Voltage Reference for HD44780 LCD displa'
2005\12\14@221745 by Rolf

face picon face
Hi all.

I am trying to find a convenient way to (interactively) set the contrast
on a "standard" character LCD display, and I wanted to avoid using a
mechanical Pot., so I had the hair-brained idea that I could use the
comparator's voltage reference to specify the contrast voltage for the
LCD display. Basically, the contrast voltage is "guaranteed" to be
between the Vss and Vdd, and so is Vref, so can I use the resistor
ladder in the PIC to set the Vref (and allow it to be changed
dynamically when the LCD is in sub-zero temps for e.g. ....).

I have searched for specs on both "typical" LCD character displays, and
on the PIC itself, but the LCD specs do not give the current
requirements of Vo, and apart from the pin maximum sink/source current,
I have not found a current max for the Vref. I am using the PIC18F4320,
and there is the statement (page 228/229 of datasheet) that "The RA2 pin
can be used as a simple D/A output with limited drive capability", but I
can not find anywhere that indicates what the drive capability actually
is. Will I need to create the Op-Amp "buffer" as described in that same
section? Table 26.3 indicates the  Voltage Reference Specifications, but
nowhere does it indicate what the max supply current is....

Additionally, I can't seem to find the input current requirements for
the LCD display. Initial investigation seems to indicate that it is only
used for potential, and no current actually flows... based on the
references: http://www.pacificdisplay.com/lcd_cont_adj.htm and
http://www.microchip.com/stellent/groups/picmicro_sg/documents/devicedoc/en023491.pdf
Essentially, the LCD voltage (Vo) is applied accross the LCD 'pixel',
which acts as a capacitor, and thus no significant current flows.

I guess I could just measure the current in to the pin, but I would like
third-party confirmation that I could just plug the Vref of the PIC to
the Vo of the LCD module, and get an easy "Dynamic" non-mechanical
contrast adjuster.

While I am on the subject, I also thought I could connect the backlight
of the LCD module via a resistor and MOSFET to the PWM output of the CCP
module, and thus effectively have dynamically adjustable "brightness" as
well. More duty-cycle = more brightness.

Is there anything in here I have got horribly (or even a little) wrong?

Rolf

2005\12\14@225211 by Harold Hallikainen

face picon face
I just did a similar project last week. A PIC was driving a 120x64 graphic
LCD. It needed an adjustable -12V for the contrast/viewing angle adjust. I
took one of the PWM outputs and ran it through an inverting amplifier with
a gain of 12/5 and put a capacitor across the feedback resistor to get rid
of the PWM ripple. That handled the viewing angle.

I took another PWM output and drove an N channel FET. The backlight LED
went to +5V through a current limit resistor. The PWM varied the
brigthness.

Worked great!

Harold


{Quote hidden}

> -

2005\12\14@225742 by Jinx

face picon face
> I have searched for specs on both "typical" LCD character
> displays, and on the PIC itself, but the LCD specs do not
> give the current requirements of Vo

I don't believe Vo goes to the LCD ICs (eg 44780 or 44100).
It's part of the PSU, and is often shown supplied via the wiper
of a 10k pot

VRUR (unit resistance value R) is stated as 2k ohms for the
F88's Voltage Reference. Figure 14-1 of DS30487C shows

Vdd - 8R - (16 * R taps) - 8R - Vss

so it probably doesn't have enough ooomph for an LCD. But
there's one way to find out for sure

2005\12\15@041131 by Jan-Erik Soderholm

face picon face
Rolf wrote :

> Basically, the contrast voltage is "guaranteed" to be
> between the Vss and Vdd, and so is Vref, so can I use the resistor
> ladder in the PIC to set the Vref (and allow it to be changed
> dynamically when the LCD is in sub-zero temps for e.g. ....).

Doesn't "extended temp range" LCDs normaly have a
neg voltage contrast adj. ?


Jan-Erik.



2005\12\21@075537 by Rolf

face picon face
Hello again, all.

Thought I would provide an update, and maybe it'll help someone, even me!

After much thought, and gnashing of teeth, I put together the circuit I
had conceived. I found that (as expected), the CVref was not able to
"drive" the LCD contrast. I put together an op-amp voltage follower to
try to accomplish a buffer, but still it did not work. Finally, I took
some measurements, and realised that the voltage on the contrast "pin"
was at 0.8V when the contrast was most "pleasing", and this was below
the threshold for the op-amp (in the single-supply config I had), so it
was actually driving the voltage to the 5V rail when the input was lower
than about 1V.

Anyways, I played with diodes to drop the voltage, and other "extreme"
measures.

Bottom line is that I made a "discovery", and I wish I had known it
before. The Vo (contrast) pin on a "standard"  LCD is not supposed to be
"Driven", it is supposed to be grounded! All the documents that talk
about a "voltage divider" 10K Pot to control contrast are misleading. In
playing with my LCD module at least, I discovered that the contrast is
proportional to the resistance between Vo and ground, not the voltage on
that pin (or perhaps the LCD Module uses the resistance on Vo as part of
an internal volt divider).

Anyways, to make a long story short, you can not effectively use the
CVRef to adjust the contrast of an LCD module because the contrast is
not volt dependent, but resistance dependent. Additionally, the 10KPot
commonly used for "contrast" adjustment could be simplified to a
rheostat config, and that would save some energy from the power source.

In my case, adding about 1K fixed resistance in series with a 1K
Variable resistor, placed between Vo and Ground, gave me good coverage
of the required contrast range. Additionally, there was about 0.3mA
current from Vo to Ground, wheras with a 10Kpot there would be 0.8mA in
a 5V system.

Thus, unless someone points out something I have missed, I intend to
investigate a system using 4 pins (I have them spare... I think) on the
PIC to ground out different rungs on a resistor ladder in the following
arrangement, thus giving me 5 contrast settings.... I may have to play
with the actual resistor values a little...



 LCD Vo ----
           |
           \
           / 1K Ohm
           \
           |
Pin a ------|
           |
           \
           / 250 Ohm
           \
           |
Pin b ------|
           |
           \
           / 250 Ohm
           \
           |
Pin c ------|
           |
           \
           / 250 Ohm
           \
           |
Pin d ------|
           |
           \
           / 250 Ohm
           \
           |
     Gnd ---

Anyways, I learned something I though should be shared....

Rolf

Rolf wrote:
{Quote hidden}

2005\12\21@081402 by olin piclist

face picon face
Rolf wrote:
> Bottom line is that I made a "discovery", and I wish I had known it
> before. The Vo (contrast) pin on a "standard"  LCD is not supposed to be
> "Driven", it is supposed to be grounded!

But that would not allow for any adjustment, whether resistive or a voltage.

> All the documents that talk
> about a "voltage divider" 10K Pot to control contrast are misleading. In
> playing with my LCD module at least, I discovered that the contrast is
> proportional to the resistance between Vo and ground, not the voltage on
> that pin

And how did you tell the difference?  Didn't the voltage vary as the
resistance varied?  How can you decide which is driving and which one is
following along?

> Thus, unless someone points out something I have missed,

The data sheet apparently.  Any one LCD at a particular temperature and
other operating point characteristics probably exhibits a fixed relationship
between contrast pin voltage and resistance to ground on that pin.  However
due to the way LCDs work, it's the voltage on that pin that is the important
attribute.  By providing only a resistance, you are counting on the
effective impedence of the contrast pin to set the voltage.  This is is not
well specified and will probably vary over parts and conditions much more
than the voltage to produce the same contrast.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2005\12\21@085230 by Rolf

face picon face
See inline.

Olin Lathrop wrote:
> Rolf wrote:
>  
>> Bottom line is that I made a "discovery", and I wish I had known it
>> before. The Vo (contrast) pin on a "standard"  LCD is not supposed to be
>> "Driven", it is supposed to be grounded!
>>    
>
> But that would not allow for any adjustment, whether resistive or a voltage.
>  
You are correct in the pedantic sense, but you missed the part where i
describe a resistor ladder to implement adjustment. I thought it would
be relatively obvious given the remainder of my mail.
{Quote hidden}

Actually, I could tell the difference by lacing an LED in-line with Vo.
Current flows FROM Vo to ground, and not the other way around.
{Quote hidden}

Quoting your words: "However due to the way LCDs work, it's the voltage
on that pin that is the important attribute.", it is exactly this that I
have read elsewhere, and it is exactly this that is wrong (in my case)!.

It is NOT *only* the voltage that counts. If this were the case, then you could put a Diode between the wiper of your POT, and the Vo, like so:

Vdd ----
      |
      \ 10K
      /
      \<------------->|--- Vo
      /
      |
      |
Gnd ----

When in fact, the above will not work even if the Voltage at Vo is
correct, the current still needs to flow (from out of Vo).

In fact, I recall in my experimenting that the circuits:

Gnd   -------/\/\/\/\------Vo

and

Gnd --------/\/\/\/\-----|<--------Vo

produced the same contrast for any given resistance, and thus, the Vo
must have been different in each case. I will confirm this at home again.

> ******************************************************************
> Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
> consultant in 2004 program year.  http://www.embedinc.com/products
>  

Imperial evidence (my testing) has revealed that there is more to the
issue than just the voltage.... and in fact, the voltage (at least in my
module) is not the determining variable, it is the dependant variable.
In all cases, it is the resistance between Vo and Ground that influenced
the Contrast.

Thus, while what you say makes sense (and it did to me before I started
this exercise...), the reality I have on the desk at home does not
follow what I originally understood to be the case. Which is why I am
raising this issue.

Rolf

2005\12\21@093717 by olin piclist

face picon face
Rolf wrote:
>> And how did you tell the difference?  Didn't the voltage vary as the
>> resistance varied?  How can you decide which is driving and which one
>> is following along?
>>
> Actually, I could tell the difference by lacing an LED in-line with Vo.
> Current flows FROM Vo to ground, and not the other way around.

So?  That only shows the direction of current flow.  It says nothing about
whether voltage or resistance is a better means to control the contrast.
You are completely missing the point I made.

{Quote hidden}

No.  That only shows that current flows out of the Vo pin instead of into
it, which has nothing to do with your premise that contrast is inherently
adjusted by resistance.

> Imperial evidence (my testing) has revealed that there is more to the
> issue than just the voltage.... and in fact, the voltage (at least in my
> module) is not the determining variable, it is the dependant variable.
> In all cases, it is the resistance between Vo and Ground that influenced
> the Contrast.

But didn't voltage vary as you varied the resistance (I'm sure it did)?  So
how can you possibly say which was the direct cause of the contrast change
when both varied?  In other words this is a flawed experiment because there
was no control for the voltage variable.

Ideally you would want to hold the voltage contstant while changing the
resistance.  However, the LCD already dictates a relationship between
voltage and current, and this is therefore impossible.  About the only way
you can do this experiment correctly is to find different operating points
for the LCD where there are different relationships between the voltage and
current.  Then at each of those operating points apply a fixed voltage and
separately a fixed resistance and measure if the resulting contrasts had
less variation accross fixed voltages or fixed resistances.  What the data
sheet is trying to tell you (and which makes sense from how LCDs work) is
that you will get a smaller variation with fixed voltage accross the range
of operating points.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2005\12\21@102538 by Rolf

face picon face
Olin Lathrop wrote:
{Quote hidden}

Let's back it up a bit.

Here's the rub. I thought, and based on previous mails:
1. I could use the CVref to create the necessary (adjustable) voltage to
set the contrast.
2. If I could not do it directly, that I could use an Op-Amp as a Buffer
/ Voltage Follower to accomplish the same.

Here's the problem...
with 1. the resistor network in CVref is to Vdd, and thus, even though
the voltage may be right, the current can not flow, and the LCD is "blank".
with 2. putting the LCD Vo at the output of the OpAmp causes reverse
currents through the OpAmp (I think), and it is not stable at the low
voltages required for contrast. In my case, it would be too "dim", and
then, as it approached "close", the op-amp would jump to the Vdd rail,
and the display would "blank". Placing voltage-dropping Diodes after the
Op-amp to get the right voltage range did not solve the problem, and
screen is still blank.

Bottom line for me.... Controlling *just* the voltage at Vo does not
control contrast.

Futher experiments using a voltage-divider after the buffer (and thus
putting the buffer in to it's linear range) discovered that it did not
matter what voltage I put at Vo, it was the resistance between Vo and
ground that changed contrast.... in other words, I had the following:

CVRef -------+|\
             | \
             |  >----------/\/\/\/\/\-----Gnd
             | /  |            ^
           |-|/   |            |
           |      |            ---------Vo
           -------
                 

Experimenting indicated that it did not matter what the Voltage at CVref
was (from 0.0V through 3.8V), that it was only the position of the Pot
that mattered (Admittedly, below the Min input voltage of the buffer,
the op-amp would go to the +5V rail.....).

So, I concluded that the Vo is not Voltage dependant, but resistance
dependant. I then confirmed it by removing all components other than
what became the rheostat, and behaviour was consistant with just a
resistance (to ground) controlled device.

Admittedly, I am new to most of this, op-amps included, and everything I
know about them I learned in the past week. Additionally, my tools are
crude, and methods are in-experienced.... yet, to me this is pretty
conclusive....

Rolf

2005\12\21@110458 by Wouter van Ooijen

face picon face
> Here's the problem...
> (snip)
> Bottom line for me.... Controlling *just* the voltage at Vo does not
> control contrast.

If I read your post correctly you made a bad voltage buffer, it did not
work, and now your conclusion is that voltage driving does not work????

> yet, to me this is pretty conclusive....

It is, but conclusive of what excatly? That you need much more
experience before you jump to conclusions?

A free tip (as in free beer): Olin is not known for his subtle human
interface, but I don't recall him being wrong on electrical matters.


Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2005\12\21@112101 by Spehro Pefhany

picon face
At 05:04 PM 12/21/2005 +0100, you wrote:
> > Here's the problem...
> > (snip)
> > Bottom line for me.... Controlling *just* the voltage at Vo does not
> > control contrast.
>
>If I read your post correctly you made a bad voltage buffer, it did not
>work, and now your conclusion is that voltage driving does not work????
>
> > yet, to me this is pretty conclusive....
>
>It is, but conclusive of what excatly? That you need much more
>experience before you jump to conclusions?
>
>A free tip (as in free beer): Olin is not known for his subtle human
>interface, but I don't recall him being wrong on electrical matters.

Yes. Ideally you feed Vo a voltage V(T) which varies with temperature. The
function of temperature depends on the LCD type. You can add a BF
for the user to fiddle with the contrast depending on viewing angle, lighting
conditions and personal preference.

Sounds like he was using a bipolar op-amp for a buffer than would not swing
to the -ve rail, and further one that had the phase reversal "feature".


Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


2005\12\21@114543 by olin piclist

face picon face
Rolf wrote:
> Here's the rub. I thought, and based on previous mails:
> 1. I could use the CVref to create the necessary (adjustable) voltage to
> set the contrast.

What is CVref?  I don't remember you defining this.

> 2. If I could not do it directly, that I could use an Op-Amp as a Buffer
> / Voltage Follower to accomplish the same.

Probably true.  What is the useful range of Vo according to the data sheet?

> Here's the problem...
> with 1. the resistor network in CVref is to Vdd,

I have no idea what that is supposed to mean.

> and thus, even though
> the voltage may be right, the current can not flow, and the LCD is
> "blank".

OK, you saw a blank LCD.  I'm not convinced your assumed reasons for that
are correct.  I particularly don't get the part about the voltage being
right but the current not flowing.  This leads me to suspect a
misunderstanding of the circuit, and perhaps some of electronics in general.

> with 2. putting the LCD Vo at the output of the OpAmp causes
> reverse currents through the OpAmp (I think), and it is not stable at
> the low voltages required for contrast. In my case, it would be too
> "dim", and then, as it approached "close", the op-amp would jump to the
> Vdd rail, and the display would "blank".

I'm not totally sure what the circuit was, but it sounds like a bad opamp
circuit or the wrong opamp for the job.  It sounds like you need an opamp
with output that is "rail to rail" or at least goes down to the low rail.
Unfortunately there is a lot gray area and sometimes outright misinformation
in spec sheets pertaining to this.  You have to know what kind of fish to
wave and exactly how to wave it.  In general, us a CMOS opamp.  All else
being equal, these are more likely to be able to drive their outputs close
to the low rail without jumps or other unexpected behavior.

> Placing voltage-dropping Diodes after the
> Op-amp to get the right voltage range did not solve the problem, and
> screen is still blank.

That makes sense as the voltage drop on a diode will make the opamp output
rail-to-rail problem worse.

> Bottom line for me.... Controlling *just* the voltage at Vo does not
> control contrast.

Not really.  I think the problem was that you weren't controlling the
voltage as you thought.

> Futher experiments using a voltage-divider after the buffer (and thus
> putting the buffer in to it's linear range) discovered that it did not
> matter what voltage I put at Vo, it was the resistance between Vo and
> ground that changed contrast.

I find this very hard to believe.  Did you measure Vo as you changed the
resistance?  I'm sure it changed.  The difference is that now you had a
circuit that was able to change the voltage, even if it was only loosely
controlled by controlling a resistance.

> Experimenting indicated that it did not matter what the Voltage at CVref
> was (from 0.0V through 3.8V), that it was only the position of the Pot
> that mattered

Again, what is CVref and what does it have to do with this?  I thought the
job was to control Vo.  The load on Vo may be non-linear with voltage, maybe
your driving impedence was wrong, I don't know.  Measure Vo as you change
the pot and you will see it change.

> So, I concluded that the Vo is not Voltage dependant, but resistance
> dependant. I then confirmed it by removing all components other than
> what became the rheostat, and behaviour was consistant with just a
> resistance (to ground) controlled device.

Again, measure the Vo voltage as you are doing this.  You will see it
change.

> yet, to me this is pretty conclusive....

Sorry, but it isn't since you haven't (can't) separate the Vo voltage from
the resistive load applied to Vo.

Another thing to consider.  LCDs are inhernetly switched devices.  The
current coming out of Vo may be very pulsed.  The voltage probably only
matters during one phase of this pulse.  All this means there should
probably be a capacitor on Vo to keep it at a "constant" voltage.  I'm just
guessing so check the data sheet.  Unless it says not to, try putting 100nF
on Vo with respect to ground.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2005\12\21@123952 by Rolf

face picon face
Spehro Pefhany wrote:
{Quote hidden}

That's what I like about this list .....

I was using TL082 http://www.national.com/pf/TL/TL082.html

Wouter, I have "lurked" around long enough to get a sense of the regular
players here at piclist, and I take no offense with Olin. In fact, I am
always happy to be corrected, and so on. In my professional capacity (as
opposed to PICs which are a hobby) I find that it is sometimes
refreshing for a "un-filtered" honest opinion. I am fully aware that
Olin's and most other people's experience far exceeds mine (otherwise I
would not have asked questions).

As I expressed in a post a while ago, one of the values of experience in
Electronics appears to be in being able to choose the right part for the
job, and I guess the Op-Amp I chose was not the right one. I had based
some of my experiments on
http://www.maxim-ic.com/appnotes.cfm/appnote_number/3658/ln/en and that
is why I was so confused by the lack of correlation between my
expectations and "reality".

I will get a "rail to rail" opamp and see if I get better results.

While I have everyone's ear, what chip would be suggested in my
situation... ;-)
Narrowing the field down, MicroChip has OpAmps, and given the Maxim
app-note above with the Op-Amp controlling the Backlight as well, I
think a dual op-amp would be useful for me. Additionally, I am
bread-boarding things... (so DIP is good). Finally samples are great,
so, I would think that either of: MCP6002 or MCP6042 would work for me.
There are a bunch of others to choose from at
http://www.microchip.com/ParamChartSearch/chart.aspx?branchID=110153&mid=11&lang=en&pageId=79

I see that Olin has another response... I will respond to that too.

Thanks all

Rolf

2005\12\21@123952 by Rolf

face picon face
Spehro Pefhany wrote:
{Quote hidden}

That's what I like about this list .....

I was using TL082 http://www.national.com/pf/TL/TL082.html

Wouter, I have "lurked" around long enough to get a sense of the regular
players here at piclist, and I take no offense with Olin. In fact, I am
always happy to be corrected, and so on. In my professional capacity (as
opposed to PICs which are a hobby) I find that it is sometimes
refreshing for a "un-filtered" honest opinion. I am fully aware that
Olin's and most other people's experience far exceeds mine (otherwise I
would not have asked questions).

As I expressed in a post a while ago, one of the values of experience in
Electronics appears to be in being able to choose the right part for the
job, and I guess the Op-Amp I chose was not the right one. I had based
some of my experiments on
http://www.maxim-ic.com/appnotes.cfm/appnote_number/3658/ln/en and that
is why I was so confused by the lack of correlation between my
expectations and "reality".

I will get a "rail to rail" opamp and see if I get better results.

While I have everyone's ear, what chip would be suggested in my
situation... ;-)
Narrowing the field down, MicroChip has OpAmps, and given the Maxim
app-note above with the Op-Amp controlling the Backlight as well, I
think a dual op-amp would be useful for me. Additionally, I am
bread-boarding things... (so DIP is good). Finally samples are great,
so, I would think that either of: MCP6002 or MCP6042 would work for me.
There are a bunch of others to choose from at
http://www.microchip.com/ParamChartSearch/chart.aspx?branchID=110153&mid=11&lang=en&pageId=79

I see that Olin has another response... I will respond to that too.

Thanks all

Rolf

2005\12\21@132945 by Rolf

face picon face
Olin Lathrop wrote:
> Rolf wrote:
>  
>> Here's the rub. I thought, and based on previous mails:
>> 1. I could use the CVref to create the necessary (adjustable) voltage to
>> set the contrast.
>>    
>
> What is CVref?  I don't remember you defining this.
>  

It is the Comparator Voltage Reference output.
>  
>> 2. If I could not do it directly, that I could use an Op-Amp as a Buffer
>> / Voltage Follower to accomplish the same.
>>    
>
> Probably true.  What is the useful range of Vo according to the data sheet?
>  
Hmmm... got me there - no-name brand LCD module with real HD44780 chip -
found in surplus store. No useful identifying features I could find.
>  
>> Here's the problem...
>> with 1. the resistor network in CVref is to Vdd,
>>    
>
> I have no idea what that is supposed to mean.
>  
Looking at block-diagram of Comparator Voltage Reference module of
PIC18F4320 datasheet
(http://www.microchip.com/stellent/idcplgidcplg?IdcService=SS_GET_PAGE&nodeId=1335&dDocName=en010287)
I see the resistor network may in fact be referenced to Vss, but the Mux
is a "black box". Regardless, I can't seem to be able to sink any
current through CVref pin when enabled.
{Quote hidden}

I'm getting a better understanding of my deficiencies with op-amps, and
I am sure your last statements are accurate!
{Quote hidden}

I gather that (now).
{Quote hidden}

Well, I did a *lot* of things. And much of it is in the too-early hours
of the morning. I certainly did measure Voltage at Vo, and I changed Vo
by changing the voltage at the top of the voltage divider by changing
the CVref output from the PIC. I did measure very similar voltages at
the PIC Pin, and at the buffer output. The voltage divider 10Kpot did
change the contrast, but if I kept the pot unchanged, then changing
CVref did not *appear* to change contrast. But, perhaps my voltage
divider was such that a change in the voltage at the top of the Pot was
not suffient to change the Vo noticably (visible contrast wise).
{Quote hidden}

I did draw an ASCII circuit.... ;-)

CVRef -------+|\
             | \
             |  >----------/\/\/\/\/\-----Gnd
             | /  |            ^
           |-|/   |            |
           |      |            ---------Vo
           -------

Again, CVref is the Reference voltage generated by the Comparator
Voltage Reference Module. I fed it in to the OpAmp (TL082) and I was
able to measure (mostly) linear Vout from the buffer with a 1:1 gain.
>> So, I concluded that the Vo is not Voltage dependant, but resistance
>> dependant. I then confirmed it by removing all components other than
>> what became the rheostat, and behaviour was consistant with just a
>> resistance (to ground) controlled device.
>>    
>
> Again, measure the Vo voltage as you are doing this.  You will see it
> change.
>
>  
I will do more homework tonight. My recollection fuzzy with these
details, but is that it may have changed, although the contrast
"appearance" did not.... But I may be wrong with that as well. I now
think that everything was happening so close to the low-end of the
Op-amp's linear region that all the observations are meaningless.
{Quote hidden}

Thanks for the input... I will try things with better (more appropriate)
parts (will probably be after Christmas....). I will have to try harder
to find a datasheet for my particular LCD module, the one I have gives
absolute-maxes and that's about it. It mostly regurgitates the HD44780
"standard" interface documentation (bad translations and all).

I thknk I should halod off more experiments until I get a better buffer
set up.

Rolf

2005\12\21@135603 by Peter

picon face


> Bottom line is that I made a "discovery", and I wish I had known it before.
> The Vo (contrast) pin on a "standard"  LCD is not supposed to be "Driven", it
> is supposed to be grounded! All the documents that talk about a "voltage
> divider" 10K Pot to control contrast are misleading. In playing with my LCD
> module at least, I discovered that the contrast is proportional to the
> resistance between Vo and ground, not the voltage on that pin (or perhaps the
> LCD Module uses the resistance on Vo as part of an internal volt divider).

The innards of the display decide this. Usually the 10k contrast
potentiometer is a part of an internal voltage divider that sets the
drive voltages to the LCD. It is better to do it as the schematic says,
because on other displays (and VFDs for example), the Vlc is internally
buffered and using a rheostat to ground or -Vss will make it impossible
to set the contrast.

Peter

2005\12\21@160221 by Jinx
face picon face
> The Vo (contrast) pin on a "standard"  LCD is not supposed to be
> "Driven", it is supposed to be grounded! All the documents that talk
> about a "voltage divider" 10K Pot to control contrast are misleading

In this document (page 6)

http://www.densitron.com/editor/pdfs/appnotes.pdf

Densitron say "Contrast can also be controlled digitally with a
digital potentiometer or DAC"

But whether they actually tried a DAC ........?

In this document (Table 4.2, page 9)

http://www.thinkgoldfish.com/downloads_lcd_pdfs/MDL16166.PDF

There are two current figures. Idd (1.3mA) for the display and
logic and Io (0.2mA) for the LCD supply

What you need to do is find a manufacturer's detailed description
of the bias module, not the typical "pin connections" datasheet

2005\12\21@231253 by Rolf

face picon face
Olin Lathrop wrote:
> Rolf wrote:
[snip...]
> I'm not totally sure what the circuit was, but it sounds like a bad opamp
> circuit or the wrong opamp for the job.  It sounds like you need an opamp
> with output that is "rail to rail" or at least goes down to the low rail.
> Unfortunately there is a lot gray area and sometimes outright misinformation
> in spec sheets pertaining to this.  You have to know what kind of fish to
> wave and exactly how to wave it.  In general, us a CMOS opamp.  All else
> being equal, these are more likely to be able to drive their outputs close
> to the low rail without jumps or other unexpected behavior.
> [snip...]

Well, I replaced the TL082CP op=amp with a CMOS TLC272, and it works
just fine.....

Real conclusion.... good part selection is very important for good
results. Never knew op-amps were Pisces, but I sure got the right fish now.

Thanks for all the help everyone.

For the record, piclist has gone awful quiet.... did the holiday come
early and no-one told me?

Rolf

2005\12\22@160534 by Jinx

face picon face
part 1 1038 bytes content-type:multipart/related; (decoded 7bit)

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Content-Transfer-Encoding: 7bit

Dear Joe,

The Vo (wiper) pin is an input.  This DC Voltage that is applied to the
IC Chip, then converts to a voltage waveform to drive the LCD Glass.
This is technically the voltage  that controls the contrast on an LCD.

Best Regards,
Bill Spinney
__________________________

Bill Spinney
Regional Sales Engineer
Densitron Corporation
Tel: (562) 941-5000 xx244
Fax: (562) 941-5757


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part 2 35 bytes content-type:text/plain; charset="us-ascii"
(decoded 7bit)

2005\12\22@165351 by Wouter van Ooijen

face picon face
> Wouter, I have "lurked" around long enough to get a sense of
> the regular players here at piclist, and I take no offense with Olin.

That is good. The next step is to take his 'electrical' opinion (and not
only his) very very serious, and as far as I could see you did not do
that.

(snip)

Given that the normal value of a contrast potemeter is 10k, why would
you need an opamp at all? Do you have a voltage source with a very high
impedance?

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2005\12\22@181856 by olin piclist

face picon face
Rolf wrote:
> I was using TL082

That explains a lot.  It's reasonably fast opamp with very high imput
impedence, but needs a lot of headroom from the rails.  Not a good choice in
this situation.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2005\12\22@184508 by Jinx

face picon face
Dear Joe,

The Vo (wiper) pin is an input.  This DC Voltage that is applied to the
IC Chip, then converts to a voltage waveform to drive the LCD Glass.
This is technically the voltage  that controls the contrast on an LCD.

Best Regards,
Bill Spinney
__________________________

Bill Spinney
Regional Sales Engineer
Densitron Corporation
Tel: (562) 941-5000 xx244
Fax: (562) 941-5757

2005\12\23@031500 by Peter

picon face


On Wed, 21 Dec 2005, Rolf wrote:

> For the record, piclist has gone awful quiet.... did the holiday come early
> and no-one told me?

Rudolph bumped a server's power cable.

Peter

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