Step 1: LCD Surgery
Open the casing, be careful for remaining charge in capacitors: 94 V!
Work your way through taking the thing apart, take the display unit out. The unit is framed in thin metal, usually held together with tape and some glue. From front to back, the unit has the following layers:
- A horizontal polarizing filter in the form of a sticky plastic sheet glued on the LCD glass.
-The actual LCD unit: looking like greenish glass with ribbon film cables coming out of it (in reality it is 2 sheets of glass with grids of wire, transistors, transparent electrodes, 4 color filters per pixel embedded, and of course the LCD sandwiched in between.
- Another sticky plastic sheet, polarized in vertical direction, glued on the backside of the LCD glass.
- A matte transparent plastic sheet (in my monitor this was also polarized).
- A Fresnel prism sheet, to direct the back light to the left and right, for easy viewing from the sides.
- An acrylic wedge with small notches: It directs the light from a cold cathode tube above the wedge evenly throughout the screen (from now on called ' the wedge').
- A white transparent plastic sheet and a metal back for reflection.
Step 2: Exploring the Panel
The polarizing sheets and LCD panel are not needed for the project. The cold cathode tube is too fragile and too pink (to compensate the greenish hue of the LCD panel). Besides, it fried a 12 V ballast in 3 seconds!!! Instead, LEDs will be used, in the same position where the old tube was(= shining into the wedge).
Stacking the remaining layers in the same order as in the original shows the possibilities of the panel : shine a flashlight beam into the wedge to see the nice even distribution!
Step 3: Light Source
Amber LEDs would illuminate the screen in the proper color. But I still had lots of white LEDs laying around, which can be used with a yellow sheet filter, while amber LEDs would have to be ordered.
I did try 2 ultra brights, but more are needed to get an even illumination. Too much load on the batteries, and too costly.
2 strips of perf board were used to seat the LEDs in parallel. 7 LEDs, fed by 3 (rechargeable) AAA batteries (3.6V in total) with a switch. The battery holder is made from a E 1.49 flash light, which has a nice switch.
The LEDs are kept at about 2 cm distance from the wedge, attached to the frame, to let their light beam spread out and avoid an uneven appearance of beams in the display.
Of course theory says to put them in series with resistors, but the flashlights from which the LEDs are salvaged don't have these, but seem to work just fine! The panel light will not see frequent use, so hopefully I can get away with it... If not, I will regret it, because replacing a blown LED will require a total dis-assembly of the glued and caulked unit!!!
The battery holders and switch will fit in a project box, with some velcro to attach to the trailer, bike or the unit itself.
Step 4: The Frame
One problem: the display is too square...(4:3). It would be great if I had a wide screen display, or 2 identical displays, but alas...
So instead, the panel has to be cut in two, and the two halves rotated. The lights now come from the sides, and the resulting new panel has a much better shape (3:1). The prism sheet has to be cut differently!! The light still has to be refracted horizontally. No good to deflect to the sky & ground...
Just like the example in step 1 (fallen off a truck), this panel has a red frame. It is made of aluminum L profile. Red LEDs are inserted all around, through holes, fixed with hot glue.
On the sides, the frame contains an air space for the LED strips, with some reflecting, but non-conducting mylar foil (from a salami sausage wrapping) as a reflector and strips of yellow filter and prism sheet stuck onto the wedge (very important, diffuses the narrow beam of the LEDs!).
After wiring and testing, everything is sealed with silicone caulk and insulated with tape/ hot glue.
Step 5: Unit Assembly
To protect the fragile plastic sheets, an acrylic panel cover should be added on the outside. The air space between the prism sheet and wedge will have to be moisture proof.
The wedge is made of extremely brittle plastic. Even with a lot of care, going slowly with the hacksaw, trying to support both sides, 2 small (ca. 1 cm) cracks appeared while cutting, at the thin side of the wedge. I can mask them with the frame, but hopefully they won't propagate, because a crack will block the light beam, making the wedge useless.
The prism sheet is cut and stuck with clear glue to the wedges (there's 2 now). The prism side should be facing outward, otherwise the glue will be very visible and ugly. It is hard to see which side is which, but a small drop of water reveals the active side. (Better not use glue, but double sided tape)
(This part I screwed up: cut the wrong way, and even with the glue on the proper size, it shows up badly)
In (day-) light, or when the power is off, the unit would look bad, a white/ grayish color. Yellow plastic, from a office binder can replace the original white sheet. This way, it's presentable both day and night...
By pure coincidence, I had a metal back plate of the exact same size (!!!) from a plasma TV surgery project laying around. This will go on the back, making the unit look custom/ factory made.
The panel unit has to be removable, to prevent theft, damage and weathering. The bike, or trailer needs a receiving unit. The back panel of the unit is equipped with 2 hooks. The trailer has bolts to receive. In my opinion the unit is too wide for a bike, but a receiver could be fastened to the seat post.
Step 6: Mistakes, Possible Improvements
Things to be avoided:
Making a completely new design always involves mistakes, choosing wrong materials or methods. If I run into another LCD monitor, I would do the following things differently:
- Don't glue the sheets, but fasten them, on the edges,out of sight, with double sided tape.
The prism sheet only works with plastic/ air transition. If the active side gets wet, or glued, it will loose it's properties. Even the flat side, unevenly glued, shows up as stains.
- (And not have forgotten to put the matte sheet between the prism sheet and the cover panel (now all glued up, forget about it))!
- When available, use 2 panels, even if only to obtain larger cuts of the prism sheet.
- Don't use salami wrapper. better use aluminum tape... Put more space between the LEDs and the wedges, and point them more straight to the wedges.
About my unit: the mistakes are not severe. The appearance of the unit seems somewhat worn. Which is actually my style anyway...
suggestions for further improvement/ advanced models:
Make a unit from picture/ video frames. This way it could show messages, signs or cool colors! Of course this is more costly, and will consume more power. With falling prizes, a computerized display on the back of a car could be made without costing an arm or leg (replacing the number plate with an active display could save a lot of money on traffic tickets ...! - You might ask your local cop if this is permitted in your area!).