How to replace the light in your laptop screen with surface-mount LEDs.
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Difficulty level: Moderate to Painful. Fixing the screen is easy, putting Apples back together is hell.
In this instructable I will show you some of the steps involved in reviving my venerable laptop.
But first let me describe the problem. It was the inverter. My laptop's LCD screen was slowly dying. The back light would turn off by itself. It would come back on after I closed the lid a few times. Then this trick stopped working. The back light would flash on briefly and then poop out when I woke the computer from sleep. The screen would still function but without the back light, so the image was nearly invisible. Some Googling informed me of a common problem with a reed switch cable. But I cracked open my laptop and all the cables were fine. The bulb still lit up briefly so it was probably fine. This left only the inverter and more Googling described similar problems in other computers.
I could try to repair or replace the inverter but I wanted something more robust. Light emitting diodes (LEDs). After all I'd rather re-design around the problem rather than fix the same thing again later.
This guy describes a similar retrofit but with 12 volts incoming. This allows him to wire some LEDs in series. In my case I had 5 volts so all the LEDs here are in parallel.
So here's how I did it...
Step 1: Parts/Tools
10x or more surface mount LEDs (3.6volt, 35mA max, 30mA ideal. These put out 2600mcd at 30mA) My laptop is 12 inches so a bigger screen would need more LEDs. Get about 1 per 2cm, 2/3inch of screen width.
~1 meter enameled wire. I chose the second thinnest wire in the shop. It was compact but strong enough for my manhandling.
1x teeny 500 ohm potentiometer (variable resistor) 10x 4 ohm resistors would be safer and more efficient but I was lazy.
1x thin 100K ohm potentiometer (this was way too much resistance but, meh. It works)
Clear scotch tape.
Paper (for mapping out where about 50 screws go back to.
Detailed disassembly instructions: iFixit.com (thanks for the tip Snowpenguin
Credit card (spudger)
Multimeter (nothing fancy, mine was like $10)
Small Philips screwdriver set. You know, cross-shaped like this +. (I used a kitchen knife instead but that sucked)
Allen key/hex wrench set with small keys.
Soldering gun, solder.
Step 2: Open Your Laptop
There are too many screws to remember so I drew several confusing diagrams for myself.
I think you can do this repair without taking apart the main body of the laptop. You probably only need to open the screen.
Use a credit card as a spudger. As you remove screws from your laptop, you'll find the plastic shell won't open. You have to stick a credit card in the seams and pry it apart.
Step 3: Carefully Rip Out the Inverter and LCD
Take the screen apart and pull out the LCD.
Don't break the bulb!
There is one bulb along the bottom of the screen. It is about 2mm ~1/16th of an inch wide. Be gentle when you pull the bulb out. It contains mercury: bad poison.
Step 4: Notice the Construction
You will find a sheet of glass inside the screen. This glass is tapered; it is thicker (about 4 millimeters) along the bottom of the screen and quite thin at the top. We will be pressing the LEDs up against the thick bottom edge of this glass.
Step 5: Wiring Diagram
This is how it goes together.
The power source is a set of 4 wires that terminate in a plug with four contacts.
Step 6: Wire Up Your LEDs
Available electricity. My crappy multimeter told me that one of the wires feeding the inverter had 5 volts incoming and another wire was ground. Do some trial and error to figure out which wires these are on your screen. If you have a 12 volt feed try some LEDs in series like this.
Seeing as I had 5 volts available I couldn't put any LEDs in series. So I wired them in parallel with one resister as you can see in the first picture.
Initially my LEDs blinked intermittently when I poked at them. The problem was I did a bad solder job. A bit of resoldering fixed this.
Step 7: The Resistor(s)
Decision time: This website suggests one resistor per LED. Design wise, the online calculators all agreed this would be the most efficient. Also wiring LEDs in parallel is supposed to be a bit risky. I think the danger is something to do with a cascading overload effect if one LED poops out. If you know about this risk please let me know.
This site told me that one 4.7 ohm resistor in series with 10 LEDs in parallel would work.
The bottom line was that I didn't want to shop for 10 very specific resistors when I could take one small variable resistor and dial it in to whatever I needed.
So I used one potentiometer as a single resistor.
I also put in another potentiometer to act as a dimmer.
Step 8: Make a Hole for the Dimmer
Make a hole for the dimmer.
Step 9: Put Everything Together
Tape your LED string to the wide bottom of the LCD's glass backing.
You may have a few screws left over. Their proper place will forever remain a mystery...