Wednesday, November 22, 2006

Replacing a Cold Cathode Tube with LEDs.

I recently opened up my old Toshiba Satellite 2510 CDS and replaced the worn out tube and the burnt inverter with an array of Light emitting diodes. It took me approximately 2 hours to do this mod and it’s an easy one. The Satellite 2510 CDS is equipped with a 12 inch Sharp DSTN display which has a Cold cathode tube mounted on the top. The diffuser and the aluminium plate along with a white acrylic sheet at the back ensure that the light lights up the surface of the diffuser evenly. This diffuser is placed right behind the LCD and lights up the LCD screen. I was actually thinking of making a projector; but then I scrapped the idea of a movie projector because watching movies on the sluggish DSTN display that has a resolution of only 800X600 isn’t exactly fun.
For this experiment; you need a Multi-meter, 25 LEDs, a soldering gun and some soldering tin and 2 stripped wires. I used a thin cardboard sheet; clamped the legs of the LEDs on opposite sides of the cardboard and soldered 2 stripped wires on the opposite sides. The good thing about the white LED is that it operates between 2.8 volts to 4 volts. Anything above that and you may end up burning your LED.
By mounting those LEDs on the cardboard strip I made an array of LEDs that was equivalent to the width of the screen. I removed the cathode tube (see the picture) and the inverter. Mounting the LEDs in place of the tube was an easy job.
NEXT STEP: I needed 3 volts+ to power those LEDs and I probed around a few voltage regulator ICs on the mainboard and I managed to find 2 points on the mainboard that gave me 4.5 volts DC. The positive terminal was directly taken from a voltage regulator IC and the negative terminal from the heatsink that is mounted on the CPU. You can attach a variable resistor in parallel to fine tune the brightness of the LED; since the out put is 4.5 volts. I removed the old cables that fed the power hungry inverter and cathode tube and inserted those 2 wires through the channel where there is a folding mount. After connecting the respective terminals to the LED array in parallel to the Variable resistor; I turned the laptop on and it worked well. On an average; the batteries last for 50 minutes; but I was able to use the laptop for 1.5 hours. The screen appears bluish; but adjusting the gamma settings in the display properties usually solves this problem.


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