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Old 08-30-2018, 01:40 AM   #1
sv2000
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Posts: 6
Karma: 17578
Join Date: Aug 2018
Device: Kobo Glo HD
[Hardware Mod] Warm light / redshift on Kobo Glo HD

Hi, this is my first post on this forum, hope you find it useful

I have a Kobo Glo HD, and find it to be a great e-reader, except for some shortcomings. Notably the lack of page turn buttons (something for another day) and the garish white frontlight. While newer e-readers come with adjustable frontlight tone, I didn't solely want to upgrade due to the light. Also many newer readers come with capacitive touch, which I don't like, and lack the removable storage from Glo HD. So I decided to make my Glo HD have a warm frontlight. While on LCD this can be achieved via SW tweaks (reducing the blue channel brightness), since the e-ink display is monochrome, lacking a color filter, we need to modify the HW to change the frontlight color temp.

Note 1: The below procedure involves advanced disassembly of your e-reader and could destroy the frontlight or in the worst case the whole e-reader. I am not responsible for any damages.

Note 2: While the newer models have an adjustable color temperature, since I replaced the white LED with orange, this permanently changes the frontlight color to orange. If you are creative, you could try to replace the white LEDs with RGB, and wire up some control circuit to control the color temperature. However I like to minimize the blue light from screens, so don't mind the orange light all the time. I have also super warm LEDs, almost red, you may want something on the yellow side

Step 1:
Pry open the back of the reader. Remove the four screws holding the mainboard to the display, and the four screws holding the display assembly to the chassis. Disconnect the battery connector and the frontlight and display ZIF

Step 2:
Lift away the mainboard from the display. Gently but firmly push the display from the front, to separate the display assembly from the chassis (be careful not to break the glass in this step.

Step 3:
Peel away the frontlight flex from the display. Desolder the white LED from the flex (SMD rework tweezers are very useful here), and replace with the desired LEDs. I used the LED part number SMS1105OC from digikey and I know it roughly fits, you can use this or any other similar part.

Step 4:
Follow steps 3 to 1 in reverse order to reassemble your reader.

Final thoughts -

The orange LED also has a lower voltage drop than the white. However since the series LED driver (presumably) operates in constant current mode, we can drop in the orange LED without burning them.

As you can also see in the final image, the front is less brighter than the original white light. This is due to the orange LED having a lower brightness rating than the white (almost by a factor of 10, couple of 100 mcd for the orange LED vs 2000+ mcd for typical WLED). However this is okay since I dont use the light at max brightness but at a small fraction.
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