01-02-2013, 10:09 AM | #61 | ||
Carpe diem, c'est la vie.
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Quote:
http://www.maximintegrated.com/app-n...ex.mvp/id/3070 Quote:
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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01-02-2013, 10:23 AM | #62 |
Wizard
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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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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:
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 |
01-02-2013, 03:06 PM | #64 |
Wizard
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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. |
01-02-2013, 03:09 PM | #65 |
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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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01-02-2013, 03:32 PM | #66 |
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01-02-2013, 03:47 PM | #67 | ||||
Carpe diem, c'est la vie.
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Quote:
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:
http://www.instructables.com/id/Char...s--The-theory/ Quote:
http://www.tinaja.com/glib/muse152.pdf Quote:
Last edited by geekmaster; 01-02-2013 at 04:35 PM. |
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01-02-2013, 05:14 PM | #68 |
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01-02-2013, 05:57 PM | #69 |
Carpe diem, c'est la vie.
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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. |
01-02-2013, 08:20 PM | #70 |
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01-02-2013, 08:34 PM | #71 | |
Carpe diem, c'est la vie.
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Quote:
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: https://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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01-02-2013, 09:42 PM | #72 |
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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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01-03-2013, 02:05 PM | #73 |
Karmaniac
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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? |
01-03-2013, 02:40 PM | #74 | ||
Carpe diem, c'est la vie.
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Quote:
Quote:
Last edited by geekmaster; 01-03-2013 at 02:57 PM. |
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01-03-2013, 07:44 PM | #75 | |
Karmaniac
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Quote:
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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