CL1B power distribution

david at david at
Tue Apr 28 20:32:59 EDT 2009

On Tue, 28 Apr 2009, 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 would guess probably the power LED as it has the largest opening (and so
> would probably be the best choice for detecting light)

by the way, for those who are wondering what we are talking about. this 
post summarizes how it works (in the comments at, posted by 

if you happen to have a microchip around, just that and an LED and a 
battery will do.
Similar to those LED throwies that have the microchip sitting on the LED's 
legs, the same concept could be used.

Have the microchip reverse bias the LED and then swap the anode to be a 
high impedance input, then loop over checking the value of the LED, it 
should go high after x loops, depending on how much light lands on the 

In the dark, it will take longer (less light -> less photocurrent) so if 
you count how many loops it took and then only turn the LED on when the 
loop count is above the threshhold, you could have the same circuit in 
only 2 components and the battery.

However, if it got very dark the flashing off when the LED is reverse 
biased would probably take long enough that it would be noticeable - you 
could try having a maximum the loop will go up to before it assumes it's 
dark and stops there.

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