Monday, February 1, 2010

DIY LCD backlight

Simple tutorial how to retrofit old type LCD by adding custom backlight of any color.

The same technique can also be used to repair or modify LCD of any size.
Main idea of this method is to place custom built light emitting panel behind modified LCD glass assembly. 

Source of light are bright 3mm LEDs embedded into edges of panel. It's a simple and very reliable approach. 
LED backlight requires only one low voltage power supply, it doesn't flicker or make noise and brightness can be controlled with PWM.

DIY LCD backlight.

This method doesn't involve use of complicated equipment, just regular set of hand tools, small file, sanding paper of fine grit, soldering iron.

For small LCD, 4 LEDs provide enough light to evenly illuminate whole LCD area. 
Each LED should be connected in series with current limited resistor to maintain no more than 20 mA current trough it. Resistor's value depends on type of LED and supply voltage. For blue LED with 2.5V forward voltage and 5V supply I used 150R resistor.

Light emitting panel can be made from any kind of transparent plastic. For this project I used piece of clear acrylic found in my "scrap" box.

Materials for project: clear acrylic, LEDs, resistors, LCD to be modified.
Measure gap between PCB and LCD frame to choose material with corresponding thickness. 

LCD clearance determines thickness of light emitting panel.

To disassemble LCD unfold metal tubs on a back side of PCB to release LCD frame.
Make sure you note location of top side of glass panel. It will not work if display panel is installed upside down. 

LCD dissected.
Glass LCD panel is sandwiched between two polarizing filters.
Since this LCD is not equipped with backlight there's reflective film on the back side of LCD.
In order to let light shine through glass panel reflective film should be removed.

Removing reflective film.

On this stage it's easy to remove polarizing filter together with reflective film and, of course, LCD will not work without second filter.
If this mistake has happened, carefully separate filter from backing film and keep it to install later. 

Semi transparent film is polarizing filter. Silver reflective film should be removed. 
Light emitting panel can be made from clear acrylic or similar material.
Front surface should be sanded with fine grit sand paper to diffuse light.
To mount LEDs squire slots should be cut in both sides of panel.

Light emitting panel. Sanded piece of acrylic with cut mounting notches for LEDs.
LEDs also require shaping in order to be fitted into panel.  Thickness of LED should be the same as thickness of light emitting panel. LEDs can be secured in its place with hot glue or any other adhesive.  

Mounting LEDs.
To improve backlight brightness, piece of white matte paper can be added to back side of light emitting panel.
Another option is to cover back side of panel with white paint.

LED illumination test.

Modification job is over and now is time to put PCB, white backing paper, light emitting panel, polarizing filter, glass panel and metal frame together in one piece.

One important thing to remember. Elastomer connector (zebra strip) consists of alternating conductors and insulators. Conductors should exactly match golden pads on PCB.
If connector was shifted during assembling (or golden pads got dirty), LCD may show some sort of garbage instead of normal characters.
In that case disassemble LCD, adjust connector or clean PCB and assemble it again.

Parts of modified LCD are ready for assembling.  
Final step is wiring.
All LED circuits should be joined in one parallel network and connected to LCD power source.
Standard 16-pins LED connector has dedicated pin 15 (positive) and pin 16 (ground) for backlight power so  backlight can be powered by independent DC source or can be controlled by PWM signal.
If old LCD is equipped with 14-pins connector then backlight should be connected to pin 2 (+5V) and pin 1 (ground).

Current limiting resistors wired to LCD connector.
There are many other cool ideas that can be implemented with this method like mentioned before PWM control, using as light source RGB LEDs or maybe low power lasers.

DIY LCD backlight. It works.

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