LCD TV modification to create a diffused light source

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My Samsung LCD TV broke down after 3.5 years of service. After failure analysis, it appears that the LCD panel is dead. You would expect it but the price to repair it exceeds the price of a new TV …

Then what to do with this TV? LCD TV is electronic boards and an LCD panel in front of a backlight. The backlight is still working as well as electronics. Why not use it as a light source ? It is still difficult to find a device that can generate an almost constant brightness over an area of ​​about 0.5 m2. This could be used as a light table, a diffuse light source for photography or macro photography. In my side I think rather to use it as a source of extra lighting in trompe l’oeil, a a fake window.

Let’s see how to transform an LCD TV Samsung LE40B620 which is 101cm diagonal to a lighting source.

 

Unmounting the TV

LE40B620_1 The stand of the TV have been removed beforehand. The opening of the unit is done by removing all 15 screw present on rear of the TV. The screws are located on the periphery of the housing (red dots on the picture).
The plastic part can be easily removed after full unscrewing.
LE40B620_2 We discover at the back of the TV the three main boards present in most LCD TV:

  • the POWER board, which in this case includes also the inverter parts that are generating the high voltage (> 3000V) used for fluorescent tubes
  • The MAIN board integrating connectivity and the main chips that are managing the device. It is connected to the TCON board by an LVDS cable.
  • The TCON board is receiving images through the LVDS interface and is generating signals and timing required for LCD panel
LE40B620_3 Après avoir déconnecté les haut-parleurs ( en bas à gauche et droite) ainsi que la carte de réception infrarouge ( en bas à gauche ), le chassis acier se désolidarise facilement de la face avant du téléviseur (pas de vis).

En débranchant les câbles, en particulier le câble noir d’alimentation, on peut voir à travers les fentes du chassis que le rétroéclairage s’allume immédiatement . L’écran reste quand à lui noir car la dalle LCD reste opaque. Une résistance de Pull-down doit être présente sur le carte Main, l’absence de connexion entraîne l’allumage du rétroéclairage.

Il n’y a donc pas de modification de l’électronique à prévoir pour forcer l’allumage de l’éclairage, il suffit de déconnecter les câbles !

LE40B620_4 After disconnection of speakers (bottom left and right) and infrared receiver board (bottom left), the steel frame can be easily disengaged from the front of the TV (no screws).

When disconnecting the cables, especially the black power cable, you can see through the slots of the chassis that the backlight turns on immediately. The screen remains black however because of the LCD panel that remains opaque. A pull-down resistor must be present on the MAIN board, no connection causes the ignition of the backlight.

There are therefore no modification of the electronic expected to activate backlight, simply disconnect the cables !

LE40B620_5 When removed diffuser filter, you can see 3 superimposed polarizing filters performing a first level of light scattering. CCFL tubes appears also through the filters.
LE40B620_6 All filters being removed (3 polarizing filters, one filter diffuser), and you have direct view onto the CCFL tubes.

Be careful not to bend the frame, it isn’t very rigid. An important twist might break the tubes.

Mounting light source

LE40B620_7 Warning: the device have been replugged before putting back protections. This is very dangerous ! Some conductive parts on circuit boards are connected tomains (230V ~), but also other high voltage generated for CCFL tubes (> 3000V).

Let’s have a look to the effects of the different filters on the light scattering.

First on the photo at the letf is the light coming directly from CCFL tubes without the presence of any filter.

LE40B620_8 After establishment of the three polarizing filters, a first level of diffusion is achieved.
LE40B620_9 The last diffuser filter shows an almost constant brightness over the entire surface. It remains slightly possible to distinguish the alignment of the tubes.
Reassembly is done in reverse order, possibly omitting some parts that are not useful for you :

  • putting back the front steel frame front and screwing it. The thickness of the missing panel is easily caught by the flexibility of the framewor.
  • the front frame is no more interesting for me, it takes space and is useless
  • fixing the steel frame in the bottom plastic shell using screws + nuts
  • fixing steel stiffener for the footage of the TV
  • foot attachment

About electronic boards, I myself have kept only the Power boards, other boards are useless.

LE40B620_10 Final result : a TV format with a white front panel
LE40B620_11 This light source is distributed on a surface of 0.44 m2. The brightness is surprisingly strong (see comparison below) due to the suppression of the LCD panel. The light temperature is quite cold.

Results

I tried to compare the results between this new light source and commercial bulbs. The photos were taken with the same sensitivity settings, aperture and speed. The color temperature was set on the camera for a fluorescent type. The photos have not been modified, so they can be used to compare the results under the same test conditions.

Overview One detail
Lighting type
eclairage_1 eclairage_zoom1 Compact fluorescent bulb
Power = 13W
Color temperature = 2700K ( chaud )
eclairage_2 eclairage_zoom2 Compact fluorescent bulb
Power = 18W
eclairage_3 eclairage_zoom3 Halogen bulb
Power = 105 W
eclairage_4 eclairage_zoom4 TV panel lighting
Power = No measurement

I don’t have unfortunately any bulb at home that rivals to light generated by this new light source. It probably equals to a compact fluorescent bulb of about 25-30W .

GamutCIE_sRGB_NTSCThe color temperature is much cooler than bulbs. This is really visible in the photos above, but it is not a problem for photographic use if you take care is to enable the automatic adjustment of color temperature.

The spectral range obtained with our panel TV should be also better than average bulbs. Chromaticity is an aspect that is standardized for TV broadcasting , which is not the case for the general public lighting. TV manufacturers must ensure that the spectral content of the images is large enough to get closer to this standard. This is only possible with a backlight source of good quality, the LCD panel itself is only filtering this light flux coming out of the screen.

We can also see on those pictures the interest of having a diffuse light over a large area . Let’s compare the following two photo extracts :

eclairage_zoom3_nb

Bulb lighting

eclairage_zoom4_nb

Diffuse lighting

The shadow casted by the teapot on the left to the one on the right is much less pronounced, or non-existent, on the photo with diffuse lighting. Furthermore, the reflection of the light source on the object is not limited to a single point but to a larger area that is closer to a light source like a window.

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Vincent Recipon

Propriétaire de ce blog. Owner of this blog.

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2 Responses

  1. electrodydy says:

    Bonjour,
    j’ai eu la même idée que vous il y a quelques jours de sa sauf que pour moi c’est différent, j’ai démonter un écran lcd externe qui se branche sur le 230V, la dalle lcd était cassée je l’ai alors complètement démonter ! j’ai pu trouver le backlight une alimentation et une carte analogique (sans parler de la petite carte de commande a boutons), j’ai fais quelques test et j’ai fais des conclusions:
    le backlight ne peut a priori pas fonctionner sans la carte analogique.
    si je le laisse allumé avec la carte analogique branché l’écran se met en veille au bout de 1min…
    j’ai donc au début penser a reconfigurer la carte analogique par port serie/vga avec un ordinateur, mais je n’ai pas le materiel ni les connaissances pour… et je n’étais même pas sur que sa pouvais être possible de virer la mise en veille grâce a une reconfiguration par port série,
    j’ai donc regardé de plus prés la carte d’alimentation et j’ai remarqué que les broches de connections du pont alimentation/carte analogique étais nommées:
    -1: 8
    -2: VADJ
    -3: ON/OFF
    -4: GND
    -5: GND
    -6: +5V
    -7: +5V
    -8: Sync
    j’en reconnais quelques unes:
    le ON/OFF et une broche qui attend un voltage bien spécifique pour que l’alimentation démarre
    le +5V et tout simplement une tension de sortie de 5V produite par l’alimentation
    le GND et la masse commune le – si on peut dire
    je soupçonne le sync en le vadj a être des broches qui s’occupe de l’intensité de la lumière produite par le backlight, mais se n’est qu’une suggestion…
    la broche nommée 8… eh bien la c’est le mystère le plus total car un numéro ne pourra pas m’aider a trouver l’utilisation de cette broche…
    j’ai donc essayé de branché le +5V sur le ON/OFF…
    et sa a marché ! seul petit bémol c’est que la luminosité n’a absolument rien a voir quand je laissais l’alim tourner avec la carte analogique, elle était très faible …
    et c’est la que je bloque … dois je aussi croire qu’il y a une broche PWM dans cet assortiment ? esque c’est la broche sync ou VADJ ? dois je injecter un voltage dans l’une des broches pour faire varier la luminosité ?
    en tout cas si vous pouvez répondre a mes questions se serait très gentil 🙂 en attendant bonne nuit !

    • Bonjour,
      Merci de partager vos expérimentations.
      Je n’ai pas eu besoin d’aller si loin dans l’analyse. Au vu du pinning que vous présentez, et de quelques infos sur internet ( recherchez “inverter” et “vadj”), la pin vadj sert à régler la luminosité.
      Je pense qu’il serait donc utile d’appliquer une tension sur cette pin pour augmenter la luminosité. Il y a de fortes chances que la luminosité soit maximale pour une tension de 5V (donc liaison directe au 5V), mais rien n’est sûr, et le composant pourrait mal supporter une tension trop importante. Dans le doute le mieux serait de faire un pont diviseur avec un potentiomètre de 10kohms entre la masse et le +5V et de connecter le point milieu au VADJ. Attention à démarrer le test avec le potar dans la position fournissant 0V au VADJ, et de ne plus augmenter la tension dès lors que la luminosité ne varie plus…

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