LCD backlight light table

Function

A light table is a surface with light evenly shining through. It is used for looking at film, diapositive slides, xray films, tracing stuff on paper, and many other purposes.

A thin, lightweight and easily portable light table was desired to have in a workshop.

Method

A liquid crystal display is a light table, with a LCD panel laid over it.

Several discarded LCDs were obtained from a laptop repair shop. The displays were all cracked and the backlight CCFL tubes were either missing or broken. The backlight diffusers, made from optical-quality plastic and several layers of micropatterned foils, were however intact.

The cracked LCD panel was removed and set aside for salvaging the polarizing filters. The backlight tube was removed.

White SMD LEDs in the PLCC2 case (120°, 600-1200 mcd, ~3.3V, 20mA) were obtained. 20 pieces was bought, 15 were needed. (This turned out to be wise, as one was killed in early testing when an order of magnitude smaller resistor was used. The diode lit up very bright, then in matter of seconds its brightness gradually diminished to almost zero, when the test was ended.)

As the desired operation was from a 12V power supply, the topology was decided to be five strings of 3 LEDs each, with a 100-ohm resistor. 1206 SMD resistor was chosen, and attached directly to the terminal LED in the chain. Thin enameled wire was used to interconnect the LEDs. (In retrospect, a long thin strip of copper-clad would be better.) Five chains were prepared, with terminal wires of uneven lengths but adding to the width of the display panel bottom, with some added reserve. The ends of the chains were soldered together.

The LEDs were bonded to the side of the diffuser. Hot glue was attempted with low success; it did not satisfyingly bond to the LED cases. Using little pieces of adhesive foams on the LED backsides together with the original fluorescent tube holder to press them against the diffuser was also found to not be optimal. At the end, individual pieces of kapton tape were used to hold the LEDs in place.

A strip of black paper was placed over the LEDs and a centimeter-wide strip of the diffuser, to conceal the uneven light cones coming from the LEDs. A thin strip of black tape was used on the other side to mask the light leakage from the other end of the backlight.

A layer of 2mm thick glass, cut to exactly the size of the original LCD panel, was placed over the backlight, to serve as the work surface of the table. The aluminium frame of the display was then placed over the assembly. As it was found that the assembly is now a little bit thicker than the original, and the latches on the frame do not clip on anymore, adhesive tape was used to hold the frame to the bottom of the assembly. This turned out to be an expedient but surprisingly workable solution.

As the glass panel was found to be exactly matching the inner edge of the frame, a length of kapton tape was used to hold them together. This is an artefact of bad measurement, using the display panel as a master instead of the inner side of the frame. (Lesson learned.) The tape however does the job of holding the parts together well.

A 12V 2.1mm barrel connector of the cheapest plastic kind was used as the power connector, and attached to the side of the assembly with hot-melt adhesive.

The table produces fairly good amount of light and consumes only 100-120 mA at 12 volts.

Lessons learned

• The glass has to be cut to fit the frame, not just to match the LCD. 2-3 additional millimeters of height may be needed.
• Do not grossly overload a LED. It will lose brightness in seconds. Be wary of resistor calculations from head when tired.
• The diffuser is covered with several layers of foils with pattern of fine ridges, going in different directions. They catch dirt and fingerprints very easily. Specks of dirt are very visible. Bond the sides of the foils and diffuser together with tape as early as possible. Do not look between them.
• The parts that heat up the most are the 100Ω resistors. They do not overheat, however. The proximity of the metal frame is apparently sufficient to keep the LEDs and resistors at temperature that's just lukewarm to touch on the panel outside.

TODO

• RGB wall light panel, using the same approach and several times more RGB LEDs.
• Seal the glass-frame interface, so spilled liquid won't penetrate the enclosure.

Images

LCD panel, back side	LCD panel, front side	Backlight tube, detail	Backlight tube, placement
Diffuser backside	LCD without frame	LCD frame position	LCD frame position
LCD panel flipped off	Diffuser layers	Diffuser layers	Diffuser layers
LEDs, side of the diffuser	LED strings	LEDs in place	LEDs in place
LEDs in place	LEDs in place	LEDs in place	Glass panel
Backlight panel	Backlight, LEDs operating	Backlight, LEDs operating	Backlight, LEDs operating
Backlight, LEDs operating, hot-glue detail	LEDs, cover taped in	LEDs, cover taped in	LEDs, cover taped in
Black mask strip over the LEDs	Backlight with two rightmost LEDs nonworking	Backlight with two rightmost LEDs nonworking	LEDs taped in place
LEDs taped in place	LEDs taped in place	Power connector	Power connector
Power connector	Power connector	Power connector	Power connector
Finished light table	Finished light table	Finished light table, in operation	Finished light table, in operation
Finished light table, tilted to show the LED lights	Finished light table, tilted to show the LED lights	Finished light table, tilted to show the LED lights