Ambient light sensing via LED response

[John Watlington]

On May 1, 2009, at 2:28 PM, C. Scott Ananian wrote:

> On Tue, Apr 28, 2009 at 8:38 PM, John Watlington <wad at laptop.org>  
> wrote:
> > I don't have time to take a look at this right now,
> > but we have a A/D input to dedicate to this, if it helps work around
> > the patent.
> >
> > We can talk to MERL if needed.   I probably still know a handfull
> > of people around there.
>
> Oh, yeah, you should be able to wire the top side of the LED  
> directly to the LED and measure the photovoltaic current directly;  
> that's not patented:
>                   battery voltage
>               Q1  |
> ---from EC------|< _____ to A/D
>                   |
>              LED  V
>                   |
>                  GND
>
> The only question is whether the LED can put out enough  
> photovoltaic current to be reliably measured by the A/D.

Ahh, therein lies the challenge!

> Depends on what the input to the A/D looks like, how much  
> capacitance it sees, etc.

Thought the KB3700 (EC) A/D datasheet frustratingly doesn't list any  
such exotic parameter
as input impedance, I asked ENE and they said that the input was high  
impedance CMOS
(think a MOSFET gate, in the wee, wee microamps).   The impedance  
also does not vary
(even though the A/D is muxed).

> An ultralow power versoin of the 339 could fix any problems there,  
> but then your parts count increases.

Sorry, no parts count increases allowed except for one LED,  
resistors, capacitors (basically free), and maybe
one transistor, diode, or NMOS MOSFET (about a penny).   I will throw  
in a couple of EC digital outputs, and
a day of Richard's time in EC code.

> You don't *have* to reverse-bias the LED; that just enhances  
> sensitivity, but distinguishing between
> "outside on a sunny day" and "inside" doesn't exactly require  
> precision; there's at least an order of
> magnitude change in illumination, maybe 2 (http://en.wikipedia.org/ 
> wiki/Lux).

I haven't read the patent, but the advantage to a digitally  
controlled time integration A/D has always
been a high dynamic range by changing the time scale.  I'll let the  
community suggest this
circuit/algorithm.   Someone practiced in the arts might suggest  
measuring the integration of
the reverse leakage current over time, using a digital output to  
clear the integrator, allowing
software to control the time period over which the current is  
integrated to increase the range.

In response to earlier mails on this topic:  the microphone/camera  
LEDs are inviolate.
( Ivan got this one right! )   I will not allow software interference  
with those LEDs.

The power savings resulting from switching the LEDs was significant  
in Gen 1 (on the order of
50 mW in run/suspend).  Even after adding this feature, it will have  
to be integrated into Ohm
and the Control Panel before being useful in power savings.

By the way, has anyone really thought about this feature ?  I grok  
the intent, but you have to make
sure that kids who happen to be in brightly lit rooms (glaring  
fluourescents aren't uncommon)
don't loose their backlight, and wonder why ?   The keyboard lighting  
on my mac is a good idea,
but they only allow adjusting the amount of light output, not the  
sensitivity to ambient light.

And the Mac's light sensor is annoyingly placed where hand movement  
during typing may occlude it,
something we should be able to avoid...
The light pipes for most LEDs, however, are quite large.  I believe  
that a second LED
could be positioned next to an output LED, under a light pipe.     
Perhaps the battery LED is
the best candidate, since it is usually not lit when operating from  
the battery.

Cheers,
wad