CL1B power distribution

david at david at
Tue Apr 28 21:02:05 EDT 2009

On Tue, 28 Apr 2009, John Watlington wrote:

> All of our LEDs are dual (one on the inside and one on the outside).
> Instead of running these in parallel, and throwing away the extra
> voltage, I run them in series directly from the battery voltage
> (ever notice that their brightness changes when you plug in
> the charger ?)

hmm, how much power are we talking about saving compared to running them 
in parallel of a chip?

over the life of a battery, how much would the backlight need to be turned 
off to save as much power?

note that if it's really dark, it may be reasonable to dim the backlight, 
so the savings aren't just in bright light.

do we think that we could gain that much time on average?

how about possible savings from driving the LEDs at less than 100% duty 

my gut feeling is that the difference in the power used in driving the 
LEDs from a chip would probably be countered in a very short time by the 
savings from being able to turn off the backlight LED array, and may even 
be able to be countered by just running all the indicator LEDs at lower 
duty cycles when the light is dim.

the only concern I would have is if you can run the two LEDs in parallel 
off of the chip directly.

David Lang

> Cheers,
> wad
> On Apr 28, 2009, at 8:42 PM, david at wrote:
>> On Tue, 28 Apr 2009, John Watlington wrote:
>>> On Apr 28, 2009, at 8:16 PM, david at wrote:
>>>> On Tue, 28 Apr 2009, John Watlington wrote:
>>>>> On Apr 28, 2009, at 7:31 PM, david at wrote:
>>>>>> On Wed, 29 Apr 2009, James Cameron wrote:
>>>>>>> On Tue, Apr 28, 2009 at 10:15:47AM -0400, C. Scott Ananian wrote:
>>>>>>>> I wonder if one could easily support running an LED backwards as an
>>>>>>>> ambient light monitor in Gen 1.5 - it seems that automatically
>>>>>>>> powering off the backlight in bright sunlight would lead to a lot of
>>>>>>>> power savings for most young users.
>>>>>>> I agree that an ambient light detector and automatic adjustment of
>>>>>>> backlight would save power.  It would happen transparently, magically.
>>>>>>> But I don't think the LEDs are often specified in terms of their 
>>>>>>> ambient
>>>>>>> light detection properties.
>>>>>>> Perhaps it would be better to use a photodiode, or light dependent
>>>>>>> resistor.
>>>>>>> Then there's the spectrum of light being received.
>>>>>>> Then there's reflection from the laptop display itself to consider.
>>>>>>> I recall we also once had a discussion on whether the camera could be
>>>>>>> used as an ambient light detector.
>>>>>> you don't want to have to run the camera to detect the light (this will
>>>>>> eat far more power than you would save)
>>>>>> the LED trick has the advantage of not requiring a change to the case,
>>>>>> just a single additional drive pin to be able to run it as a detector.
>>>>> And where would you place said detector LED, without modifying the case 
>>>>> ?
>>>>> (I have the pin...)
>>>> use one of the existing LED's.
>>> I have no intention to use one of the existing LEDs.   They don't run off 
>>> logic level
>>> voltages for power reasons, and adding switches would be more expensive 
>>> than
>>> dedicating an LED.
>>> Hence my question...
>> if it's not reasonable to use an existing LED, then I guess this idea will 
>> need to be scrapped. I think the people proposing the idea were figuring 
>> that in the hardware update one of the LEDs could be re-wired so that it 
>> could be run directly (between two pins, the pin that currently controls 
>> it, and a new one)
>> I will admit to not understanding the power reasons comment. is it that the 
>> LEDs in use draw more power than you want to run through the control chips?
>> David Lang

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