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Old 01-02-2013, 10:09 AM   #61
geekmaster
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Quote:
Originally Posted by ProDigit View Post
Their forward voltage is 3V, their backwards voltage could sometimes range in the tens of Volts. ...
Enough unsubstantiated claims. Here is some ACCURATE information about LEDs:
http://www.maximintegrated.com/app-n...ex.mvp/id/3070
Quote:
...
Standard red, green, and yellow LEDs have forward voltages in the range 1.4V to 2.6V, depending on the desired brightness and the choice of forward current. For forward currents below 10mA, the forward voltage varies only a few hundreds of millivolts.
...
A trick is employed to make white LEDs. Blue-emitting InGaN base material is covered with a converter material that emits yellow light when stimulated by the blue light. The result is a mixture of blue and yellow light that is perceived by the eye as white.
...
Many portable and battery-operated devices use white LEDs for background illumination. In particular, the colored displays of PDAs need a white backlight to obtain color reproduction that is close to the original.
...
In most cases a single white LED is not sufficient, so several must be operated together. Special steps must be taken to make sure their intensity and color is matched, even as battery charge and other conditions vary.
...
At first, when the battery is fully charged, all diodes are illuminated but with different shades of light intensity and color. As battery voltage drops to its nominal level, the light intensities decrease and the differences in white become stronger. The designer must therefore consider the value of battery voltage and diode forward voltage for which the series resistor is calculated.
...
The goal of an LED power supply is to provide a sufficiently high output voltage, and to force the same current through all LEDs connected in parallel. Note (Figure 5) that if all the white LEDs of a parallel configuration have identical currents, all will have the same chromacity coordinates. Maxim offers a charge pump with current control for that purpose (MAX1912).
FYI, a charge pump is a voltage boost convertor that steps the output voltage UP, so it is higher than the supply voltage. It is common practice to do this when driving LEDs from a lithium battery. The kindles also use even higher voltages internally, such as the high-voltage waveform used to electrostatically control the eink display. The battery is not the highest voltage (even for the LEDs).

Note that the referenced LED documentation mentions that LED brightness may be controlled by simple hardware or software-controlled PWM as I mentioned, or it may be controlled by varying the supply CURRENT (not voltage) at the expense of inconsistent emitted light color and more complex control circuitry. PWM results in a more consistent white color at all brightness levels, and does not require extra current-control circuitry for software control.

EDIT: If you refer to ANY spec sheets for LEDs, you will see that NONE of the commonly available colors use 3V forward voltage as you claim. In fact, blue LEDs typically use about 4V, and other colors typically use about 2V. Some super-bright LEDs may have lower reverse breakdown voltages, such as a super-bright blue LED that uses 3.3V. You can measure that voltage drop across the LED while lighting it using a common button cell battery such as a CR2032. The actual battery supply voltage depends on the load, and the internal resistance of the battery will drop the measured voltage to what the LED actually uses. Supplying more current than the LED can dissipate (such as using a benchtop power supply) will burn out the LED if it gets hot enough to initiate thermal runaway. At a minimum, you need a series current limiting resistor to prevent overdriving the LED to destruction. But with small batteries, the internal resistance (and lack of high current capacity) is enough to protect the LED.

You can drive a blue or white LED from a lower voltage battery (such as 3V, or even 1.5V) using a simple boost converter such as a "Joule Thief" (a fun little project in itself):
http://en.wikipedia.org/wiki/Joule_thief

EDIT2: The "tens of volts" reverse breakdown voltage of which you speak applies only to small signal diodes and power rectifier diodes, not LEDs (which typically use 5V reverse breakdown). Zener diodes may have even smaller reverse breakdown (such as 3.3V) depending on how they are doped. Reverse bias beyond breakdown is only destructive with insufficient current limiting (and in fact is the NORMAL mode of operation for Zener diodes).

Last edited by geekmaster; 01-02-2013 at 10:32 AM.
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Old 01-02-2013, 10:23 AM   #62
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Quote:
Originally Posted by geekmaster View Post
The kindles also use even higher voltages internally, such as the high-voltage waveform used to electrostatically control the eink display. The battery is not the highest voltage (even for the LEDs).
Oh thank you for that. I found the waveforms yesterday on the Kindle and (wrongly) assumed they have something to do with the LEDs. Maybe some magic that is calibrated towards each individual PW so that all 4 LEDs output creates the desired effect. Learn something every day.
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Old 01-02-2013, 02:29 PM   #63
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DIY turn of the light

For now I will simply post it as DIY instructions. Not sure when and if I get around to get it packaged (never packaged anything, so my learning curve starts at ZERO).

You need USB Network access to your Paperwhite. I do not wish that anyone just follow these instructions unless they are comfortable doing it. If you scratch your head, because something doesn't make sense, then DON'T DO ANY OF THIS. Please don't complain later that I did not warn you.

Instructions are in the Spoiler:
Spoiler:

The Hack, as you can see, is rather simple and short. That doesn't mean it is not doing what it should. It also means that it is less likely to misfunction. What does it do? When you wake up from sleep (either by pushing the power button or by opening your cover) the light of the Paperwhite gets turned of. This happens if and only if these conditions are met:
- the file TURN_LIGHT_OFF exists in folder /mnt/us (empty or not doesn't matter)
- the light is set to level 0 (no bars showing) according the setting screen of the Paperwhite

Also make sure that lipcd is running and using the correct settings. Easiest way to make sure is to stop it (stop libcd) and restarting it (start lipcd) after you made the change to the config file. Once it is running, it doesn't need to be restarted again. No need to go back into the USB Network for it to keep working after eventual restarts.

Now to install it you need to:
STEP 1)
Create the file /usr/bin/lightoff.sh with the following content. When I made did it I copied another shell script to the new file and replaced the content. (That was easier for me than to figure out what permissions the Kindle likes to have)
Code:
#!/bin/sh
#
FLINTENSITY=`lipc-get-prop com.lab126.powerd flIntensity`
if [ -e "/mnt/us/TURN_LIGHT_OFF" ]
then
        if [ "$FLINTENSITY" -eq 0 ]
        then
                sleep 1;
                /bin/echo -n 0 > /sys/devices/system/fl_tps6116x/fl_tps6116x0/fl_intensity;
        fi
fi
Here is also the chance to modify it to a desired (other than 0) intensity setting, if you so wish. Please note that the intensity I check against is the setting of the bar, not the actual setting the light is set on (so it should be 0-24). Who knows what logic Amazon uses to translate light bars into real intensity values - plus it might change. I gave my reasons before on why I won't change it to anything else than 0 or to make it customizable. Not going to repeat myself. That I coincidentally made it so easy to customize has nothing to do with my foresight of it beeing a possibility. In fact, if it would have created cleaner code, I would have avoided this. You might also notice the lack of comments in the script. That is on purpose as well, it practically reads itself. If it doesn't for you, then don't mess with it. A little side note for the the necessity of the delay in there: If you take the call to sleep out, the whole functionality will break. I believe it has something to do with calling and finish executing the script BEFORE the Kindle had time to turn the light on. If the light is not on, then turning it off is redundant. Even worse, the light would stay on, contrary to what you want to do. (Doh )

Step 2)
The file /etc/lipc-daemon-events.conf needs the last line added (in green). This is the complete file, the non-green part is the default file. It even includes (and I did not edit that) the typo evetns (in red) .
Code:
## This file defines what evetns are handled by lipc-daemon
## The format of the file is
##<event name>   <event publisher>   <command to execute when event is received>
##
## NOTE: By default the event name and any parameters are passed to the script as arguments.  If you want to ignore the arguments,
## you must add a semi-colon (;) to the end of the script command.
##
FrameworkStarted  com.lab126.framework  rm -f /var/local/system/.framework_retries /var/local/system/.framework_reboots
tzRequested       com.lab126.framework  settz
interfaceChange   com.lab126.cmd        /usr/sbin/updatetime;
triggerTimeChange com.lab126.cmd        /usr/sbin/updatetime
networkTime       com.lab126.wifid      /usr/sbin/setdate

## Uncomment the following line if you want to see when lipc-daemon received
## outOfScreenSaver event.  Useful for debugging out of suspend performance
#outOfScreenSaver    com.lab126.powerd  . /etc/rc.d/functions; msg "lipc-daemon got outOfScreenSaver at `date +%s`s" I; true;
outOfScreenSaver com.lab126.powerd  /usr/bin/lightoff.sh
Please note: If you ever want to edit this file again and try out to uncomment the other outOfScreenSaver, then you MUST comment out the one for the light. If you have 2 lines with the same event, it does not call any of them. Also, you NEED to restart the lipc daemon after you edited the file. (see above)

last not least:
Step 3)

The file TURN_LIGHT_OFF needs to be in usb partition (/mnt/us) (so that it can be disabled simply by deleting or renaming the file - just like USE_ALT_FONTS). That is an easy way to turn the light hack completely on or off with only access to the usb-visible disk.

Simplest way to make it is like this:
Code:
/bin/touch /mnt/us/TURN_LIGHT_OFF


I hope this is useful. If I get enough time this weeking, I will try to start figuring out on how to package it. It all but looks like a short weekend aka nonexistant. Shutdown over Christmas / New Year usually comes to haunt us with a vengence in the form of overtime. Tonight it is time for me to go back to work .

p.s.: If someone wants to have a go at packaging it, please feel free to do so and use all knowledge and code. If you change functionality (e.g. different light level, customizable light level or anything) please don't claim that it was my work. The code I made is robust, but I won't voucher for it if it is changed. And yes if you want to give me credit for inspiration, feel free to do so. If it wasn't for others, even this would not have been possible. Special thanks to: knc1 and twobob (they know why)

p.p.s: Just seen ixtab has the margins fixed in JBPatch (see here). Yay good timing on his part. Time to tinker with that a bit.

Update: decided to lay the packaging on ice for now. This is really easy enough to do if you have usb network installed. Plus since I discovered a way to turn the light off w/o jailbreak, there is no incentive for me right now to learn a new trick. See post #78 for the joilbreak free method.

Last edited by DuckieTigger; 01-07-2013 at 02:27 AM. Reason: updated
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Old 01-02-2013, 03:06 PM   #64
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Oh I forgot to mention: The really cool part is, the way I did it, the light is still fully useable no matter what. If it turned off, and you really really want it turned on at level 0, then you need to go to your light, change the setting to 1 (or really anything), the light will turn on. Then you can turn it back down to 0 and it will stay on (at 0) until you put PW back to sleep with light at level 0 and waking back up. win-win for everybody. The ones that don't want the light at all, leave it at 0. The ones that never have the light that low, but want to occasional turn it off, there you go. If you frequently change from one level to next, including off, all you need is turn to 0 and put to sleep and wake right back up.

Like I said, win win for everyone. Light is always still fully functional.
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Old 01-02-2013, 03:09 PM   #65
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Quote:
Originally Posted by geekmaster View Post
If you refer to ANY spec sheets for LEDs, you will see that NONE of the commonly available colors use 3V forward voltage as you claim.
Strange, because when I search at Mouser.com for white, surface mount LEDs with a Vf of 2.8-3.2, they show 47 matches, from 13 different manufacturers.
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Old 01-02-2013, 03:32 PM   #66
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Quote:
Originally Posted by mike.s View Post
Strange, because when I search at Mouser.com for white, surface mount LEDs with a Vf of 2.8-3.2, they show 47 matches, from 13 different manufacturers.
... which is relevant in what way for Paperwhite owners?
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Old 01-02-2013, 03:47 PM   #67
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Quote:
Originally Posted by mike.s View Post
Strange, because when I search at Mouser.com for white, surface mount LEDs with a Vf of 2.8-3.2, they show 47 matches, from 13 different manufacturers.
By "commonly available" I was referring to inexpensive common LEDs, which use the voltage ranges I claimed. There are other LED technologies (such as super-bright blue) that do use about 3V, as I also mentioned. What concerns us here though is the specific type of LEDs that are used in the PW. It would be an easy task to measure their voltage drop while they are lit. Please let us know what you find when you measure them. Thanks.

It is certainly possible that Amazon did use super-bright LEDs in the PW even though they cost more and require higher power driver circuitry, especially because they used only four of them and their effect is noticeable even in brightly lit rooms. Fewer LEDS may also simplify the design of the light guide.

But arguing about forward voltage drop is really distracting us from the original suggestion to use a reverse bias to make the LEDs darker than no power to them at all. Reverse biased LEDs do not actually CONSUME light (other that to discharge any internal capacitive charge, which can be useful when using them as photo cells to detect light).

EDIT: Using inexpensive LEDs as optical detectors:
http://www.sensorsynergy.com/helpfulhints.htm
Quote:
Although LEDs are not intended to experience large reverse bias voltages, most can be reverse biased by a few volts (3v to 7v) and operate in the photodiode mode. Make sure you limit the magnitude of the reverse current so that you do not damage the LED.
Another useful reason to reverse bias LEDs is for Charlieplexing them:
http://www.instructables.com/id/Char...s--The-theory/
Quote:
What is 'Charlieplexing'? It is driving lots of LEDs with only a few pins. In case you're wondering Charlieplexing is named after Charles Allen at Maxim who developed the technique.
It seems that Don Lancaster published the details of "Charlieplexing" years before Chares Allen got credit for it. There is an excellent schematic diagram (figure 3) here (and the article preceding it in the PDF is quite illuminating too):
http://www.tinaja.com/glib/muse152.pdf
Quote:
Fig. 3 – 56 LEDS can be driven from one [8-bit] microcomputer port by using this n-connectedness trick. No external decoding is needed!

Last edited by geekmaster; 01-02-2013 at 04:35 PM.
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Old 01-02-2013, 05:14 PM   #68
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But arguing about forward voltage drop is really distracting us
And yet, you insist.
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Old 01-02-2013, 05:57 PM   #69
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And yet, you insist.
You only just now joined us, and your first posts serve only to challenge our well-earned respect and authority in this forum? Please make yourself useful and contribute something valuable, so that you can earn our respect. Your two posts so far seem to be a bit foreboding. I hope to see improved attitude in your future posts.

My recent post showed USEFUL examples of reverse biased LEDs, which actually WAS the current topic of discussion, so your "and yet" comment was inappropriate (besides being generally disrespectful).

I gave you 2 karma points. Now please earn them.

Last edited by geekmaster; 01-02-2013 at 06:15 PM.
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Old 01-02-2013, 08:20 PM   #70
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if [ "$var" = "FrontLight(Intensity) = 1" ]
Can you please explain where the FrontLight(Intensity) came from? Is it an included Kindle function?
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Old 01-02-2013, 08:34 PM   #71
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Can you please explain where the FrontLight(Intensity) came from? Is it an included Kindle function?
It is from a line of text in the file: /sys/devices/system/fl_tps6116x/fl_tps6116x0/fl_intensity

That is a virtual file whose content is dynamically generated by a device driver each time you read from it. In this case, it shows the current intensity level of the FrontLight LEDs.

See this post for more info:
http://www.mobileread.com/forums/showpost.php?p=2252313

In those examples, the cat command is used to read the current intensity (brightness) value, and the echo command is used to write a new value.

Last edited by geekmaster; 01-02-2013 at 08:47 PM.
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Old 01-02-2013, 09:42 PM   #72
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It is from a line of text in the file: /sys/devices/system/fl_tps6116x/fl_tps6116x0/fl_intensity

That is a virtual file whose content is dynamically generated by a device driver each time you read from it.
Thanks. I did play around with it but was mainly writing to the device (without that additional text string). It looked like a shell function but it's just a string copy.
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Old 01-03-2013, 02:05 PM   #73
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Well, if a '0' setting works, no need getting worked up about '-1'.
My question is, if there's some file inside the device that can be modified like this, without the need for jailbraking or using a root kit?
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Old 01-03-2013, 02:40 PM   #74
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Originally Posted by ProDigit View Post
Well, if a '0' setting works, no need getting worked up about '-1'.
My question is, if there's some file inside the device that can be modified like this, without the need for jailbraking or using a root kit?
The files you would need to modify are in the root partition, and you need root access to modify them. So the answer to your question is:
Quote:
No, you cannot modify "some file inside the device" without root access.
Of course, if manually rooting or jailbreaking your kindle really bothers you, there are rumors that you can instead sprinkle some of this "magic powder" on it and you are good to go:


Last edited by geekmaster; 01-03-2013 at 02:57 PM.
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Old 01-03-2013, 07:44 PM   #75
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Quote:
Originally Posted by geekmaster View Post
Enough unsubstantiated claims. Here is some ACCURATE information about LEDs:
http://www.maximintegrated.com/app-n...ex.mvp/id/3070
FYI, a charge pump is a voltage boost convertor that steps the output voltage UP, so it is higher than the supply voltage. It is common practice to do this when driving LEDs from a lithium battery. The kindles also use even higher voltages internally, such as the high-voltage waveform used to electrostatically control the eink display. The battery is not the highest voltage (even for the LEDs).

Note that the referenced LED documentation mentions that LED brightness may be controlled by simple hardware or software-controlled PWM as I mentioned, or it may be controlled by varying the supply CURRENT (not voltage) at the expense of inconsistent emitted light color and more complex control circuitry. PWM results in a more consistent white color at all brightness levels, and does not require extra current-control circuitry for software control.

EDIT: If you refer to ANY spec sheets for LEDs, you will see that NONE of the commonly available colors use 3V forward voltage as you claim. In fact, blue LEDs typically use about 4V, and other colors typically use about 2V. Some super-bright LEDs may have lower reverse breakdown voltages, such as a super-bright blue LED that uses 3.3V. You can measure that voltage drop across the LED while lighting it using a common button cell battery such as a CR2032. The actual battery supply voltage depends on the load, and the internal resistance of the battery will drop the measured voltage to what the LED actually uses. Supplying more current than the LED can dissipate (such as using a benchtop power supply) will burn out the LED if it gets hot enough to initiate thermal runaway. At a minimum, you need a series current limiting resistor to prevent overdriving the LED to destruction. But with small batteries, the internal resistance (and lack of high current capacity) is enough to protect the LED.

You can drive a blue or white LED from a lower voltage battery (such as 3V, or even 1.5V) using a simple boost converter such as a "Joule Thief" (a fun little project in itself):
http://en.wikipedia.org/wiki/Joule_thief

EDIT2: The "tens of volts" reverse breakdown voltage of which you speak applies only to small signal diodes and power rectifier diodes, not LEDs (which typically use 5V reverse breakdown). Zener diodes may have even smaller reverse breakdown (such as 3.3V) depending on how they are doped. Reverse bias beyond breakdown is only destructive with insufficient current limiting (and in fact is the NORMAL mode of operation for Zener diodes).
Forward voltage is different from backward voltage.
There are diodes that have a backward voltage of 97V as well.
I'm not saying that these are the leds installed in the kindle, all I'm saying is that the backward voltage with 'device -1' is not more than a few milli-volts, and would not harm the leds, neither would reversing polarity of full battery voltages cause any damage to the leds.
More than likely battery voltage is around 3.8V; but some e-devices have 2 batteries in series having 7.4V.
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