Forums

Force LCD monitor "white"/"black"

Started by Don Y July 10, 2014
Hi,

Anyone know how the drive to an LCD panel works?  Specifically,
how one can force the display to appear "all white" or "all
black" WITHOUT relying on the electronics already present
in the monitor?

E.g., imagine having *just* the LCD panel and being able to
operate it as a "light gate" (full on, full off).

Thx!
> Anyone know how the drive to an LCD panel works? Specifically, > how one can force the display to appear "all white" or "all > black" WITHOUT relying on the electronics already present > in the monitor?
All black is easy, just cut the power. All white is a little more difficult, and need a little bit of electronics. It depends on the panel, but usually, 8 to 10 data bits, plus hsync, vsync and dot clock. You can definitely do it with a small uC, even with some simple display test patterns.
On 7/10/2014 12:32 PM, edward.ming.lee@gmail.com wrote:
>> Anyone know how the drive to an LCD panel works? Specifically, how >> one can force the display to appear "all white" or "all black" >> WITHOUT relying on the electronics already present in the monitor? > > All black is easy, just cut the power.
<grin>
> All white is a little more difficult, and need a little bit of > electronics. It depends on the panel, but usually, 8 to 10 data > bits, plus hsync, vsync and dot clock.
No, when I refer to having just the "panel", I mean "just the glass"
> You can definitely do it with a small uC, even with some simple > display test patterns. >
On 10.7.14 22:52, Don Y wrote:
> On 7/10/2014 12:32 PM, edward.ming.lee@gmail.com wrote: >>> Anyone know how the drive to an LCD panel works? Specifically, how >>> one can force the display to appear "all white" or "all black" >>> WITHOUT relying on the electronics already present in the monitor? >> >> All black is easy, just cut the power. > > <grin> > >> All white is a little more difficult, and need a little bit of >> electronics. It depends on the panel, but usually, 8 to 10 data >> bits, plus hsync, vsync and dot clock. > > No, when I refer to having just the "panel", I mean "just the glass" > >> You can definitely do it with a small uC, even with some simple >> display test patterns.
You might be able to tie all rows together and all columns together, and drive the rows and columns with antiphase square waves. The hard part will be to find the proper voltage levels. -- Tauno Voipio
On Thursday, July 10, 2014 3:52:55 PM UTC-4, Don Y wrote:
> On 7/10/2014 12:32 PM, edward.ming.lee@gmail.com wrote: > > >> Anyone know how the drive to an LCD panel works? Specifically, how > > >> one can force the display to appear "all white" or "all black" > > >> WITHOUT relying on the electronics already present in the monitor? > > > > > > All black is easy, just cut the power. > > > > <grin> > > > > > All white is a little more difficult, and need a little bit of > > > electronics. It depends on the panel, but usually, 8 to 10 data > > > bits, plus hsync, vsync and dot clock. > > > > No, when I refer to having just the "panel", I mean "just the glass"
Hi Don, Knowing almost nothing, I'd guess there are some different technologies. I certainly remember a prof who did liquid Xtal stuff back at Uni. He used an electric field which caused the crystals to line up or not line up and this resulted in more or less scattering of the light. So the short answer is maybe an electric field. Perhaps you can "scuff up" some high voltage on a carpet and put your finger near an lcd. (Why the question.. and why not start at wiki?) George H.
> > > > You can definitely do it with a small uC, even with some simple > > > display test patterns. > > >
 
> No, when I refer to having just the "panel", I mean "just the glass"
Without controller, the glass is just 1000x800 (typical) dot matrix. Typical connection pads are in (sub)micron.
On Thursday, July 10, 2014 4:04:16 PM UTC-4, George Herold wrote:
> On Thursday, July 10, 2014 3:52:55 PM UTC-4, Don Y wrote: >=20 > > On 7/10/2014 12:32 PM, edward.ming.lee@gmail.com wrote: >=20 > >=20 >=20 > > >> Anyone know how the drive to an LCD panel works? Specifically, how >=20
>=20 > Hi Don, Knowing almost nothing, I'd guess there are some different techno=
logies.
>=20 > I certainly remember a prof who did liquid Xtal stuff back at Uni. >=20 > He used an electric field which caused the crystals to line up or not lin=
e up and this resulted in more or less scattering of the light. =20
>=20 >=20 >=20 > So the short answer is maybe an electric field. =20 >=20 > Perhaps you can "scuff up" some high voltage on a carpet=20 >=20 > and put your finger near an lcd. >=20 >=20 >=20 > (Why the question.. and why not start at wiki?)=20 >=20
from here, http://en.wikipedia.org/wiki/Liquid_crystal#External_influences_on_liquid_c= rystals "Scientists and engineers are able to use liquid crystals in a variety of a= pplications because external perturbation can cause significant changes in = the macroscopic properties of the liquid crystal system. Both electric and = magnetic fields can be used to induce these changes. The magnitude of the f= ields, as well as the speed at which the molecules align are important char= acteristics industry deals with. Special surface treatments can be used in = liquid crystal devices to force specific orientations of the director."
> George H. >=20 > >=20 >=20 > >=20 >=20 > > > You can definitely do it with a small uC, even with some simple >=20 > >=20 >=20 > > > display test patterns. >=20 > >=20 >=20 > > >
=20
> You might be able to tie all rows together and all columns together, > and drive the rows and columns with antiphase square waves. The > hard part will be to find the proper voltage levels.
Making connections are more of a problem. Unless you have a custom glass p= anel, all the H & V lines are routed to a small chip area, typically with <= .1mm pitch. You can't attach PCB directly to them. The controller interf= ace pads are usually bigger, for PCB attachment. Basically, unless you are making your own custom controller. It will be di= fficult to connect properly.
Not as easy as you would think, it is NOT a CRT. There is a board called th=
e Tcon for "Tin=3Dming Control" because they could never have enough wires =
to address alll the pixels. They are done sequentially. the Tcon isn't even=
 the last thing in the chain, there is also either logic roght in th epanel=
y os possibly COFs (Chip On Film) before it gets to the actual pixels. Only=
 way to do it. Plasma is similar but since there is no variable twist to mo=
dulate the light, the brighhtness of each pixel is controlled by PWM. the l=
onger it is fired, the brighter.=20

If you look at the lines going to the Tcon from the main board, there are p=
airs opeertated in LVDS, which is Low Voltage Differential Signal. You cna =
actually see the video on a scope on those, though unless you use different=
ial probes/V.amps you don't really get a good signal. All the ones I've see=
n are still synch negative. Frequently there are more than three pairs, it =
is not necessarily one pair per color, it superimposes OSD and all that, do=
es half blanking or whaver the engoneer decided that day with the extras. T=
here are technical resons for all that but it doesn't matter right now.=20

The Tcon, even though there is no deflection in the conventional sense, doe=
s need synch to operate. You could most likely find a way to fake it out. A=
ll you owuld need is one signal connected to all of them since you want whi=
te. If you want different colors you will have to separate them of course.=
=20

But first, you could try to get a manual for it and se if it has a test mod=
e. Some of them do have a function just like that for servicing. /usually i=
t is a jumper on the man board, though I am pretty sure it could be put on =
the Tcon instead, so some models might have it there.=20

There are online resources for getting these manuals. I am not all that fam=
iliar with them because when I was doin alot of that servuice I was a membe=
r of a website whe I could hget all that. But i thknk if you join Encompass=
' website (they sell parts)you can get downloads there. There is also somet=
hing like electrotanys or something like that I've used a couple of times.=
=20

would be a shame to kludge something together to do this and find out all y=
ou needed was a clip lead.
On Thursday, July 10, 2014 12:16:28 PM UTC-7, Don Y wrote:

> Anyone know how the drive to an LCD panel works? Specifically, > how one can force the display to appear "all white" or "all > black" WITHOUT relying on the electronics already present > in the monitor?
> E.g., imagine having *just* the LCD panel and being able to > operate it as a "light gate" (full on, full off).
Most of the light from the backlight DOESN'T show up when the screen is 'white', because your LCD display has a mask, color filters, and polarizer all blocking off some of the light. Photochromic glass, and the skin of cuttlefish, are better light switches. There are liquid crystal solutions, too, but more of the transparent/milky sort than transparent/black, and those use completely different materials (which are nonetheless still liquid crystals, just not twisted-nematic).