Re: [Fwd: Re: Ambient light sensing via LED response]

2009-05-10 Thread C. Scott Ananian
On Wed, May 6, 2009 at 6:14 PM, Reinder de Haan r...@mveas.com wrote:
     Absolutely not.  The A/D is eight bits, with an input range spanning
     0 - 3.3V, so the best you
     can hope for is about 13 mV per LSB.  I would guess actual accuracy
     to be closer to 26 mV.

I think the actual A/D reference voltage is probably *maximum* 3.3V.
What's the *minimum* A/D reference voltage?  And can the A/D measure
all the way down to ground?  (Sometimes there's a Vmin for the A/D
input, often around a diode drop above ground.)

 i did some more experiments and at ~20 cm from the halogen lamp i doest
 matter is i turn the backlight full on of off .. i dont have color
 anymore
 at that distance the bare led gave about 250mv (maybe a bit more into a
 very high R fet gate...)

250mv should be a count of about 20 from the A/D.  That's plenty for
this purpose.  Heck, a count of 1 would be sufficient, as long as it
was repeatable. ;-)

So, it seems like all that's required is one wire from the top of the
LED to the A/D input, and wiring Vref directly to +3.3.  Adding a
zener and a resistor for a lower Vref would probably improve precision
and accuracy.

Everything else is software  tweaking some constants.
  --scott

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Re: [Fwd: Re: Ambient light sensing via LED response]

2009-05-10 Thread Reinder de Haan


C. Scott Ananian wrote:
 On Wed, May 6, 2009 at 6:14 PM, Reinder de Haan r...@mveas.com wrote:
 Absolutely not.  The A/D is eight bits, with an input range spanning
 0 - 3.3V, so the best you
 can hope for is about 13 mV per LSB.  I would guess actual accuracy
 to be closer to 26 mV.
 
 I think the actual A/D reference voltage is probably *maximum* 3.3V.
 What's the *minimum* A/D reference voltage?  And can the A/D measure
 all the way down to ground?  (Sometimes there's a Vmin for the A/D
 input, often around a diode drop above ground.)
 
 i did some more experiments and at ~20 cm from the halogen lamp i doest
 matter is i turn the backlight full on of off .. i dont have color
 anymore
 at that distance the bare led gave about 250mv (maybe a bit more into a
 very high R fet gate...)


that 250mv is without the light guide and lcd cover installed!
with the light guide in place its more like ~70mv ...
0.07/3.3*256= 5.4 LSB and then the Rin of the ADC must be 10Mohm...

 
 250mv should be a count of about 20 from the A/D.  That's plenty for
 this purpose.  Heck, a count of 1 would be sufficient, as long as it
 was repeatable. ;-)
 
 So, it seems like all that's required is one wire from the top of the
 LED to the A/D input, and wiring Vref directly to +3.3.  Adding a
 zener and a resistor for a lower Vref would probably improve precision
 and accuracy.
 
 Everything else is software  tweaking some constants.
   --scott
 

if i find the time somewhere this week i might test it ... otherwise it
will be during of after paris..

Reinder
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Re: [Fwd: Re: Ambient light sensing via LED response]

2009-05-10 Thread C. Scott Ananian
On Sun, May 10, 2009 at 5:01 PM, Reinder de Haan r...@mveas.com wrote:
 C. Scott Ananian wrote:
 On Wed, May 6, 2009 at 6:14 PM, Reinder de Haan r...@mveas.com wrote:
     Absolutely not.  The A/D is eight bits, with an input range spanning
     0 - 3.3V, so the best you
     can hope for is about 13 mV per LSB.  I would guess actual accuracy
     to be closer to 26 mV.

 I think the actual A/D reference voltage is probably *maximum* 3.3V.
 What's the *minimum* A/D reference voltage?  And can the A/D measure
 all the way down to ground?  (Sometimes there's a Vmin for the A/D
 input, often around a diode drop above ground.)

 i did some more experiments and at ~20 cm from the halogen lamp i doest
 matter is i turn the backlight full on of off .. i dont have color
 anymore
 at that distance the bare led gave about 250mv (maybe a bit more into a
 very high R fet gate...)


 that 250mv is without the light guide and lcd cover installed!
 with the light guide in place its more like ~70mv ...
 0.07/3.3*256= 5.4 LSB and then the Rin of the ADC must be 10Mohm...

5 LSB is still okay, and it could be more if the A/D Vref can be
dropped below 3.3V.
 --scott

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Re: [Fwd: Re: Ambient light sensing via LED response]

2009-05-10 Thread Mark Bauer


Hi guys,

How can I help with this,  I still teach electrical engineering at the
University of Nebraska, and before that I have about 25 years in
the field.  Mostly embedded microcontrollers and stuff.  I do have
several XOs to work with.

I would have jumped in earlier but the end of the semester is a bit
hectic.

Mark




On May 10, 2009  Sunday, at 4:13:47:0, C. Scott Ananian wrote:

 On Sun, May 10, 2009 at 5:01 PM, Reinder de Haan r...@mveas.com wrote:
 C. Scott Ananian wrote:
 On Wed, May 6, 2009 at 6:14 PM, Reinder de Haan r...@mveas.com  
 wrote:
 Absolutely not.  The A/D is eight bits, with an input range  
 spanning
 0 - 3.3V, so the best you
 can hope for is about 13 mV per LSB.  I would guess actual  
 accuracy
 to be closer to 26 mV.

 I think the actual A/D reference voltage is probably *maximum* 3.3V.
 What's the *minimum* A/D reference voltage?  And can the A/D measure
 all the way down to ground?  (Sometimes there's a Vmin for the A/D
 input, often around a diode drop above ground.)

 i did some more experiments and at ~20 cm from the halogen lamp i  
 doest
 matter is i turn the backlight full on of off .. i dont have color
 anymore
 at that distance the bare led gave about 250mv (maybe a bit more  
 into a
 very high R fet gate...)


 that 250mv is without the light guide and lcd cover installed!
 with the light guide in place its more like ~70mv ...
 0.07/3.3*256= 5.4 LSB and then the Rin of the ADC must be 10Mohm...

 5 LSB is still okay, and it could be more if the A/D Vref can be
 dropped below 3.3V.
 --scott

 -- 
 ( http://cscott.net/ )
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Re: Ambient light sensing via LED response

2009-05-06 Thread NoiseEHC

 But why do you say you would need 1 mV accuracy ?   Bright sunlight  
 is far stronger than
 the light sources he used.
   
I am not an engineer so forgive me if I am saying something stupid, but 
is not the goal to switch off the backlight if and only if there is no 
difference between the switched on and switched off state? I mean that 
this implies that you have to measure the mV at a light level when you 
cannot see the difference (what can be a much fainter light than the sun).
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[Fwd: Re: Ambient light sensing via LED response]

2009-05-06 Thread Reinder de Haan


 Original Message 
Subject: Re: Ambient light sensing via LED response
Date: Thu, 07 May 2009 00:02:49 +0200
From: Reinder de Haan r...@mveas.com
To: raf...@laptop.org
References: 200904290017.n3t0hl2v006...@new.toad.com
c5d4a4e5-b183-4b7d-bafb-3488e15f7...@laptop.org
c6d9bea0905011128j2d5cdf3cw975cfad790480...@mail.gmail.com
675d3c5c-95ed-4ee9-84bc-3b5164675...@laptop.org
c6d9bea0905042207i458ed9b1if74e0e16d24f5...@mail.gmail.com
4a00a073.8020...@mveas.com
a80d16920905051544s525b7039nf7e87a4a3c754...@mail.gmail.com
75dc0458-925b-4792-9aee-a24b9b1f3...@laptop.org
a80d16920905051952h3d82d15brb8f690cf1c402...@mail.gmail.com



Rafael Enrique Ortiz Guerrero wrote:
  
 
 
 On Tue, May 5, 2009 at 8:34 PM, John Watlington w...@laptop.org
 mailto:w...@laptop.org wrote:
 
 
 On May 5, 2009, at 6:44 PM, Rafael Enrique Ortiz Guerrero wrote:
 
 These measurements are really cool.
 
 But the question remains in whether the ADC could have a
 resolution of 1mv?, i mean in light of these measurements is
 necessary to have an ADC that can reliable sense these
 variations and then with that basis  have a transfer function
 and add it to the algorithm.
 
 
 Absolutely not.  The A/D is eight bits, with an input range spanning
 0 - 3.3V, so the best you
 can hope for is about 13 mV per LSB.  I would guess actual accuracy
 to be closer to 26 mV. 
 
 
 But why do you say you would need 1 mV accuracy ?   Bright sunlight
 is far stronger than
 the light sources he used.
 
 
 i don't know if the measurements at sunlight would show the same
 variations.. we would have to make new measurements, but for experience,
 the variations of voltage regarding light sensing are not of
 considerable amounts, so if the accuracy is 26mv, we would have to see
 if a perceptible change in ambient light could be of a higher magnitude
 than 26mv, if not the accuracy could be lost..
 
 

i did some more experiments and at ~20 cm from the halogen lamp i doest
matter is i turn the backlight full on of off .. i dont have color
anymore
at that distance the bare led gave about 250mv (maybe a bit more into a
very high R fet gate...)

i guess it would be around 70mv whit the light guide in front of it..
(i guess this be more (150mv?) with a very high R (~25 Mohm?) fet gate
as only load)

i would add a simple amp (mosfet? or maybe cheap opamp ?)
to amplify ~X10 so 300mv would be full scale..
my memory is dusty but that should be posible with 2 resistors an a fet
? gain would be posible to change by replacing one of the resistors with
a bigger / lower value...

maybe a trasistor is better bc the leds is acts as a current source ?

ideas welcome

if i can get the partnr of a fet/transistor that would be usable (and
cheap enough) i could maybe order a couple of them from say farnell and
hack something together here

is there a free or hackable input on the current EC ? (or is that
different from the ec that will be in 1.5 ?


 (haven't checked the specs though..)
 
 
 Having the data sheet for the EC controller doesn't help --- 8 bits
 and recommended operating
 voltage for the analog reference voltage is about all it provides.  
 I had to ask a chinese speaker
 to call the app. engineer to find out the input impedance...
 
 Ok, thanks :).
  
 
 
 
 
 On Tue, May 5, 2009 at 3:24 PM, Reinder de Haan r...@mveas.com
 mailto:r...@mveas.com wrote:
 Hallo,
 
 C. Scott Ananian wrote:
 snip
 
  A last resort would be hooking up a MOSFET as a simple
 amplifier --
  again, you're not worried about linearity or any such
 niceties, but
  you'd still need a good match for your MOSFET's threshold
 voltage...
  some real measurements to replace the WAGes would go a long way.
--scott
 
 
 measured on a B1 XO1 laptop (where the leds and the series
 resistor are
 wired in parallel it seam) :
 
 almost dark: 0mv
 ~3meter away from a 8w PL; bare led: ~2mv
 ~50 cm below ~25W halogen desk lamp; bare led: ~40mv
 bright white led directly on bare led ~200mv
 bright white led directly on light guide of the bat.led (lcd
 side) ~50mv
 ~50 cm below ~25w halogen desk lamp (~75* angle to the axis of
 the light
 guide) ~5mv
 
 i measure this between  GND of the laptop the led side of the series
 resistor.
 all leds seam to be about the same.. i did not compare the different
 light guides.
 the main battery and DC power where removed, the RTC baterry was
 still
 in place.
 
 
 the meter i measured this with was fixed in the 2000mv range
 and was abou 10Mohm when i connected it to another meter in
 resistance
 mode; the (volt) meter read 250mv at that time

[Fwd: Re: Ambient light sensing via LED response]

2009-05-06 Thread Reinder E.N. de Haan


 Original Message 
Subject: Re: Ambient light sensing via LED response
Date: Thu, 07 May 2009 00:02:49 +0200
From: Reinder de Haan r...@mveas.com
To: raf...@laptop.org
References: 200904290017.n3t0hl2v006...@new.toad.com
c5d4a4e5-b183-4b7d-bafb-3488e15f7...@laptop.org
c6d9bea0905011128j2d5cdf3cw975cfad790480...@mail.gmail.com
675d3c5c-95ed-4ee9-84bc-3b5164675...@laptop.org
c6d9bea0905042207i458ed9b1if74e0e16d24f5...@mail.gmail.com
4a00a073.8020...@mveas.com
a80d16920905051544s525b7039nf7e87a4a3c754...@mail.gmail.com
75dc0458-925b-4792-9aee-a24b9b1f3...@laptop.org
a80d16920905051952h3d82d15brb8f690cf1c402...@mail.gmail.com



Rafael Enrique Ortiz Guerrero wrote:
  
 
 
 On Tue, May 5, 2009 at 8:34 PM, John Watlington w...@laptop.org
 mailto:w...@laptop.org wrote:
 
 
 On May 5, 2009, at 6:44 PM, Rafael Enrique Ortiz Guerrero wrote:
 
 These measurements are really cool.
 
 But the question remains in whether the ADC could have a
 resolution of 1mv?, i mean in light of these measurements is
 necessary to have an ADC that can reliable sense these
 variations and then with that basis  have a transfer function
 and add it to the algorithm.
 
 
 Absolutely not.  The A/D is eight bits, with an input range spanning
 0 - 3.3V, so the best you
 can hope for is about 13 mV per LSB.  I would guess actual accuracy
 to be closer to 26 mV. 
 
 
 But why do you say you would need 1 mV accuracy ?   Bright sunlight
 is far stronger than
 the light sources he used.
 
 
 i don't know if the measurements at sunlight would show the same
 variations.. we would have to make new measurements, but for experience,
 the variations of voltage regarding light sensing are not of
 considerable amounts, so if the accuracy is 26mv, we would have to see
 if a perceptible change in ambient light could be of a higher magnitude
 than 26mv, if not the accuracy could be lost..
 
 

i did some more experiments and at ~20 cm from the halogen lamp i doest
matter is i turn the backlight full on of off .. i dont have color
anymore
at that distance the bare led gave about 250mv (maybe a bit more into a
very high R fet gate...)

i guess it would be around 70mv whit the light guide in front of it..
(i guess this be more (150mv?) with a very high R (~25 Mohm?) fet gate
as only load)

i would add a simple amp (mosfet? or maybe cheap opamp ?)
to amplify ~X10 so 300mv would be full scale..
my memory is dusty but that should be posible with 2 resistors an a fet
? gain would be posible to change by replacing one of the resistors with
a bigger / lower value...

maybe a trasistor is better bc the leds is acts as a current source ?

ideas welcome

if i can get the partnr of a fet/transistor that would be usable (and
cheap enough) i could maybe order a couple of them from say farnell and
hack something together here

is there a free or hackable input on the current EC ? (or is that
different from the ec that will be in 1.5 ?


 (haven't checked the specs though..)
 
 
 Having the data sheet for the EC controller doesn't help --- 8 bits
 and recommended operating
 voltage for the analog reference voltage is about all it provides.  
 I had to ask a chinese speaker
 to call the app. engineer to find out the input impedance...
 
 Ok, thanks :).
  
 
 
 
 
 On Tue, May 5, 2009 at 3:24 PM, Reinder de Haan r...@mveas.com
 mailto:r...@mveas.com wrote:
 Hallo,
 
 C. Scott Ananian wrote:
 snip
 
  A last resort would be hooking up a MOSFET as a simple
 amplifier --
  again, you're not worried about linearity or any such
 niceties, but
  you'd still need a good match for your MOSFET's threshold
 voltage...
  some real measurements to replace the WAGes would go a long way.
--scott
 
 
 measured on a B1 XO1 laptop (where the leds and the series
 resistor are
 wired in parallel it seam) :
 
 almost dark: 0mv
 ~3meter away from a 8w PL; bare led: ~2mv
 ~50 cm below ~25W halogen desk lamp; bare led: ~40mv
 bright white led directly on bare led ~200mv
 bright white led directly on light guide of the bat.led (lcd
 side) ~50mv
 ~50 cm below ~25w halogen desk lamp (~75* angle to the axis of
 the light
 guide) ~5mv
 
 i measure this between  GND of the laptop the led side of the series
 resistor.
 all leds seam to be about the same.. i did not compare the different
 light guides.
 the main battery and DC power where removed, the RTC baterry was
 still
 in place.
 
 
 the meter i measured this with was fixed in the 2000mv range
 and was abou 10Mohm when i connected it to another meter in
 resistance
 mode; the (volt) meter read 250mv at that time

Re: Ambient light sensing via LED response

2009-05-05 Thread Reinder de Haan
Hallo,

C. Scott Ananian wrote:
snip

 A last resort would be hooking up a MOSFET as a simple amplifier --
 again, you're not worried about linearity or any such niceties, but
 you'd still need a good match for your MOSFET's threshold voltage...
 some real measurements to replace the WAGes would go a long way.
   --scott
 

measured on a B1 XO1 laptop (where the leds and the series resistor are
wired in parallel it seam) :

almost dark: 0mv
~3meter away from a 8w PL; bare led: ~2mv
~50 cm below ~25W halogen desk lamp; bare led: ~40mv
bright white led directly on bare led ~200mv
bright white led directly on light guide of the bat.led (lcd side) ~50mv
~50 cm below ~25w halogen desk lamp (~75* angle to the axis of the light
guide) ~5mv

i measure this between  GND of the laptop the led side of the series
resistor.
all leds seam to be about the same.. i did not compare the different
light guides.
the main battery and DC power where removed, the RTC baterry was still
in place.


the meter i measured this with was fixed in the 2000mv range
and was abou 10Mohm when i connected it to another meter in resistance
mode; the (volt) meter read 250mv at that time.

adding a 10Mohm resistor across the meter halved the
~50 cm below ~25W halogen desk lamp; bare led: ~40mv reading to
~20mv

strangely enough when i add that resistor in series the meter said ~60mv
so voltage on the leds must have been ~120mv? i will have to investigate
this at a later time...

if you want more measurement doen feel free to ask.. i also have a XO1
(production model ?C1?) that i could measure.

i dont have a lux meter :-(

Greetings,
Reinder








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Re: Ambient light sensing via LED response

2009-05-05 Thread Rafael Enrique Ortiz Guerrero
These measurements are really cool.

But the question remains in whether the ADC could have a resolution of 1mv?,
i mean in light of these measurements is necessary to have an ADC that can
reliable sense these variations and then with that basis  have a transfer
function and add it to the algorithm.

(haven't checked the specs though..)

My two cents..;).

Rafael Ortiz


On Tue, May 5, 2009 at 3:24 PM, Reinder de Haan r...@mveas.com wrote:

 Hallo,

 C. Scott Ananian wrote:
 snip

  A last resort would be hooking up a MOSFET as a simple amplifier --
  again, you're not worried about linearity or any such niceties, but
  you'd still need a good match for your MOSFET's threshold voltage...
  some real measurements to replace the WAGes would go a long way.
--scott
 

 measured on a B1 XO1 laptop (where the leds and the series resistor are
 wired in parallel it seam) :

 almost dark: 0mv
 ~3meter away from a 8w PL; bare led: ~2mv
 ~50 cm below ~25W halogen desk lamp; bare led: ~40mv
 bright white led directly on bare led ~200mv
 bright white led directly on light guide of the bat.led (lcd side) ~50mv
 ~50 cm below ~25w halogen desk lamp (~75* angle to the axis of the light
 guide) ~5mv

 i measure this between  GND of the laptop the led side of the series
 resistor.
 all leds seam to be about the same.. i did not compare the different
 light guides.
 the main battery and DC power where removed, the RTC baterry was still
 in place.


 the meter i measured this with was fixed in the 2000mv range
 and was abou 10Mohm when i connected it to another meter in resistance
 mode; the (volt) meter read 250mv at that time.

 adding a 10Mohm resistor across the meter halved the
 ~50 cm below ~25W halogen desk lamp; bare led: ~40mv reading to
 ~20mv

 strangely enough when i add that resistor in series the meter said ~60mv
 so voltage on the leds must have been ~120mv? i will have to investigate
 this at a later time...

 if you want more measurement doen feel free to ask.. i also have a XO1
 (production model ?C1?) that i could measure.

 i dont have a lux meter :-(

 Greetings,
 Reinder








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Re: Ambient light sensing via LED response

2009-05-05 Thread John Watlington

On May 5, 2009, at 6:44 PM, Rafael Enrique Ortiz Guerrero wrote:

 These measurements are really cool.

 But the question remains in whether the ADC could have a resolution  
 of 1mv?, i mean in light of these measurements is necessary to have  
 an ADC that can reliable sense these variations and then with that  
 basis  have a transfer function and add it to the algorithm.

Absolutely not.  The A/D is eight bits, with an input range spanning  
0 - 3.3V, so the best you
can hope for is about 13 mV per LSB.  I would guess actual accuracy  
to be closer to 26 mV.

But why do you say you would need 1 mV accuracy ?   Bright sunlight  
is far stronger than
the light sources he used.

 (haven't checked the specs though..)

Having the data sheet for the EC controller doesn't help --- 8 bits  
and recommended operating
voltage for the analog reference voltage is about all it provides.
I had to ask a chinese speaker
to call the app. engineer to find out the input impedance...

 My two cents..;).

 Rafael Ortiz


 On Tue, May 5, 2009 at 3:24 PM, Reinder de Haan r...@mveas.com wrote:
 Hallo,

 C. Scott Ananian wrote:
 snip

  A last resort would be hooking up a MOSFET as a simple amplifier --
  again, you're not worried about linearity or any such niceties, but
  you'd still need a good match for your MOSFET's threshold voltage...
  some real measurements to replace the WAGes would go a long way.
--scott
 

 measured on a B1 XO1 laptop (where the leds and the series resistor  
 are
 wired in parallel it seam) :

 almost dark: 0mv
 ~3meter away from a 8w PL; bare led: ~2mv
 ~50 cm below ~25W halogen desk lamp; bare led: ~40mv
 bright white led directly on bare led ~200mv
 bright white led directly on light guide of the bat.led (lcd side)  
 ~50mv
 ~50 cm below ~25w halogen desk lamp (~75* angle to the axis of the  
 light
 guide) ~5mv

 i measure this between  GND of the laptop the led side of the series
 resistor.
 all leds seam to be about the same.. i did not compare the different
 light guides.
 the main battery and DC power where removed, the RTC baterry was still
 in place.


 the meter i measured this with was fixed in the 2000mv range
 and was abou 10Mohm when i connected it to another meter in resistance
 mode; the (volt) meter read 250mv at that time.

 adding a 10Mohm resistor across the meter halved the
 ~50 cm below ~25W halogen desk lamp; bare led: ~40mv reading to
 ~20mv

 strangely enough when i add that resistor in series the meter said  
 ~60mv
 so voltage on the leds must have been ~120mv? i will have to  
 investigate
 this at a later time...

 if you want more measurement doen feel free to ask.. i also have a XO1
 (production model ?C1?) that i could measure.

 i dont have a lux meter :-(

 Greetings,
 Reinder








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Re: Ambient light sensing via LED response

2009-05-05 Thread Rafael Enrique Ortiz Guerrero
On Tue, May 5, 2009 at 8:34 PM, John Watlington w...@laptop.org wrote:


 On May 5, 2009, at 6:44 PM, Rafael Enrique Ortiz Guerrero wrote:

  These measurements are really cool.

 But the question remains in whether the ADC could have a resolution of
 1mv?, i mean in light of these measurements is necessary to have an ADC that
 can reliable sense these variations and then with that basis  have a
 transfer function and add it to the algorithm.


 Absolutely not.  The A/D is eight bits, with an input range spanning 0 -
 3.3V, so the best you
 can hope for is about 13 mV per LSB.  I would guess actual accuracy to be
 closer to 26 mV.


 But why do you say you would need 1 mV accuracy ?   Bright sunlight is far
 stronger than
 the light sources he used.


i don't know if the measurements at sunlight would show the same
variations.. we would have to make new measurements, but for experience, the
variations of voltage regarding light sensing are not of considerable
amounts, so if the accuracy is 26mv, we would have to see if a perceptible
change in ambient light could be of a higher magnitude than 26mv, if not the
accuracy could be lost..







  (haven't checked the specs though..)


 Having the data sheet for the EC controller doesn't help --- 8 bits and
 recommended operating
 voltage for the analog reference voltage is about all it provides.   I had
 to ask a chinese speaker
 to call the app. engineer to find out the input impedance...

 Ok, thanks :).





 On Tue, May 5, 2009 at 3:24 PM, Reinder de Haan r...@mveas.com wrote:
 Hallo,

 C. Scott Ananian wrote:
 snip

  A last resort would be hooking up a MOSFET as a simple amplifier --
  again, you're not worried about linearity or any such niceties, but
  you'd still need a good match for your MOSFET's threshold voltage...
  some real measurements to replace the WAGes would go a long way.
--scott
 

 measured on a B1 XO1 laptop (where the leds and the series resistor are
 wired in parallel it seam) :

 almost dark: 0mv
 ~3meter away from a 8w PL; bare led: ~2mv
 ~50 cm below ~25W halogen desk lamp; bare led: ~40mv
 bright white led directly on bare led ~200mv
 bright white led directly on light guide of the bat.led (lcd side) ~50mv
 ~50 cm below ~25w halogen desk lamp (~75* angle to the axis of the light
 guide) ~5mv

 i measure this between  GND of the laptop the led side of the series
 resistor.
 all leds seam to be about the same.. i did not compare the different
 light guides.
 the main battery and DC power where removed, the RTC baterry was still
 in place.


 the meter i measured this with was fixed in the 2000mv range
 and was abou 10Mohm when i connected it to another meter in resistance
 mode; the (volt) meter read 250mv at that time.

 adding a 10Mohm resistor across the meter halved the
 ~50 cm below ~25W halogen desk lamp; bare led: ~40mv reading to
 ~20mv

 strangely enough when i add that resistor in series the meter said ~60mv
 so voltage on the leds must have been ~120mv? i will have to investigate
 this at a later time...

 if you want more measurement doen feel free to ask.. i also have a XO1
 (production model ?C1?) that i could measure.

 i dont have a lux meter :-(

 Greetings,
 Reinder








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Re: Ambient light sensing via LED response

2009-05-04 Thread Peter Robinson
 we had a dedicated light sensor on the last product i worked on
 (which, in retrospect, given that it had 15 watts (!) of
 backlight seems like maybe the wrong place to have started trying
 to save power ;-).  i implemented a proof-of-concept algorithm
 for automatic control, but found it a little weird -- it always
 felt like there was something wrong with the backlight as it
 shifted up and down in brightness, even with a fair amount of
 averaging and/or delay.  plus we never resolved how it should
 interact with the actual brightness buttons.  after all, if you
 adjust manually, you're probably unhappy with the setting that
 was chosen automatically.  so should manual control abort any
 automatic algorithm?  or maybe the buttons should tune the
 algorithm?  how?

The ALS on the Dell Latitude notebooks have either an auto setting
(activated using FN + Left Arrow) and Brightness Up and Down (FN + Up
or Down). When you manually adjust the brightness it disables the ABL
until you re-enable it.

Peter
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Re: Ambient light sensing via LED response

2009-05-04 Thread C. Scott Ananian
On Sat, May 2, 2009 at 8:38 AM,  p...@laptop.org wrote:
   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.

My suggestion would be binary on/off control, with the level set high
enough that you will be very certain that you are outside on a sunny
day.  I'm personally not interested in trying to say a few watts in
marginal lighting conditions -- just that the kid who's outside all
day in the full sunshine using their laptop -- and who can't tell that
the backlight is on anyway, since the sunlight's so bright -- isn't
held responsible for turning off something they can't see.
 --scott

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Re: Ambient light sensing via LED response

2009-05-04 Thread C. Scott Ananian
On Fri, May 1, 2009 at 11:59 PM, John Watlington w...@laptop.org wrote:
 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).

Hm.. What's the input voltage range -- ie, how small can it be?  Even
in full brightness, you're not going to see more than the LED's
forward voltage drop -- which admittedly can be up in the 2 volt
range, depending on your color, but you really want to be working well
below the forward voltage drop, so that the self-discharge current
through the LED isn't significant.  See

http://en.wikipedia.org/wiki/Solar_cell#Equivalent_circuit_of_a_solar_cell

At low voltage, I_D will be small, and assuming infinite input
impedance, I will be zero, so your sensitivity is set by R_SH, the
equivalent shunt resistance of the LED (where you want R_SH as high as
possible).

 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.

Really, the easiest way to do this is just to run a wire from the
positive side of the LED to the A/D, set the A/D reference voltage as
low as possible, and then take it out into the sun and measure what
you've got.  If you generate measurable voltage at too dim a light
level, it's easy to add a shunt resistor (although you'll probably
want to switch the shunt resistor in only when the LED is off), but to
get more voltage...

I think you could probably add a capacitor in parallel to the LED to
integrate the photocurrent over time to generate higher voltage, but
bottom-line your A/D has to be comfortable with voltages around the
voltage drop of the LED, since you'll never generate more than that.
Your maximum voltage even with the paralleled capacitor will be
limited by the self-discharge through the diode; again, it should be
straightforward to put a low ESR cap of a few nF in parallel, take it
outside, and see what voltage you generate.

A last resort would be hooking up a MOSFET as a simple amplifier --
again, you're not worried about linearity or any such niceties, but
you'd still need a good match for your MOSFET's threshold voltage...
some real measurements to replace the WAGes would go a long way.
  --scott

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Re: Ambient light sensing via LED response

2009-05-04 Thread david

On Tue, 5 May 2009, C. Scott Ananian wrote:


On Fri, May 1, 2009 at 11:59 PM, John Watlington w...@laptop.org wrote:

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).


Hm.. What's the input voltage range -- ie, how small can it be?  Even
in full brightness, you're not going to see more than the LED's
forward voltage drop -- which admittedly can be up in the 2 volt
range, depending on your color, but you really want to be working well
below the forward voltage drop, so that the self-discharge current
through the LED isn't significant.  See

http://en.wikipedia.org/wiki/Solar_cell#Equivalent_circuit_of_a_solar_cell

At low voltage, I_D will be small, and assuming infinite input
impedance, I will be zero, so your sensitivity is set by R_SH, the
equivalent shunt resistance of the LED (where you want R_SH as high as
possible).


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.


Really, the easiest way to do this is just to run a wire from the
positive side of the LED to the A/D, set the A/D reference voltage as
low as possible, and then take it out into the sun and measure what
you've got.  If you generate measurable voltage at too dim a light
level, it's easy to add a shunt resistor (although you'll probably
want to switch the shunt resistor in only when the LED is off), but to
get more voltage...

I think you could probably add a capacitor in parallel to the LED to
integrate the photocurrent over time to generate higher voltage, but
bottom-line your A/D has to be comfortable with voltages around the
voltage drop of the LED, since you'll never generate more than that.
Your maximum voltage even with the paralleled capacitor will be
limited by the self-discharge through the diode; again, it should be
straightforward to put a low ESR cap of a few nF in parallel, take it
outside, and see what voltage you generate.

A last resort would be hooking up a MOSFET as a simple amplifier --
again, you're not worried about linearity or any such niceties, but
you'd still need a good match for your MOSFET's threshold voltage...
some real measurements to replace the WAGes would go a long way.


this sounds far more complicated, and far more expensive (in parts) than 
the initial proposal to reverse bias the LED and then run a loop to see 
how long it takes for the leakage current to switch the low side to a 
high.


if you are just looking for the on-off (like you describe above), you 
don't even need to do a busy loop. do a calibration at some point, then 
reverse bias the LED, come back at a calculated time later and see if the 
pin is high or not. if it is, you are in bright light and can turn off the 
backlight.


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Re: Ambient light sensing via LED response

2009-05-02 Thread pgf
john wrote:
  
  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.

i've been wondering about usage, as well.

we had a dedicated light sensor on the last product i worked on
(which, in retrospect, given that it had 15 watts (!) of
backlight seems like maybe the wrong place to have started trying
to save power ;-).  i implemented a proof-of-concept algorithm
for automatic control, but found it a little weird -- it always
felt like there was something wrong with the backlight as it
shifted up and down in brightness, even with a fair amount of
averaging and/or delay.  plus we never resolved how it should
interact with the actual brightness buttons.  after all, if you
adjust manually, you're probably unhappy with the setting that
was chosen automatically.  so should manual control abort any
automatic algorithm?  or maybe the buttons should tune the
algorithm?  how?

i'm suspect there are answers to all of these, but one of the
feature's champions should be thinking about it...

(in the end we never finished or released the photosensor
support, because it turned out the usage pattern of the device
(an automotive diagnostic tool) meant it was almost always
fully powered, either from the wall or the vehicle it was
troubleshooting.)

paul
=-
 paul fox, p...@laptop.org
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Re: Ambient light sensing via LED response

2009-05-01 Thread C. Scott Ananian
On Tue, Apr 28, 2009 at 8:38 PM, John Watlington w...@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.  Depends on what the input to the A/D
looks like, how much capacitance it sees, etc.  An ultralow power versoin of
the 339 could fix any problems there, but then your parts count increases.
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).
 --scott

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Re: Ambient light sensing via LED response

2009-05-01 Thread Nate Ridderman
On Fri, May 1, 2009 at 2:28 PM, C. Scott Ananian csc...@laptop.org wrote:

 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.  Depends on what the input to
 the A/D looks like, how much capacitance it sees, etc.  An ultralow power
 versoin of the 339 could fix any problems there, but then your parts count
 increases.  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).


A reverse biased LED doesn't output much current at all - even photo sensor
diodes that are tuned for the job. Have a look at this page for some ambient
light sensors that you would find in a cell phone -
http://www.avagotech.com/pages/en/optical_sensors/ambient_light_photo_sensors/light_sensor_photo_sensor/.
The basic variety has a photodiode and a small current amplifier. The output
current is logarithmically related to the lux level. If hook a series
resistor to the output, the voltage across vs lux is also logarithmic.
Besides the current amplifier, these devices have other advantages. First,
they have a spectral sensitivity that's tuned to the human eye, so infrared
light (say a campfire) will not skew the reading. Second, they have a large
optical window so the readings won't change when you tilt your phone/laptop
slightly. These little sensors are less than $0.25 in large quantities
(perhaps by quite a bit), and they are being used more and more in cell
phones to reduce backlight power in low light environments. It's well worth
the price if you get 10% more battery life on average. Quantifying the
impact on battery life is tricky because there are a lot of assumptions.

Anyways, a normal LED might work under certain circumstances. I don't have
the experience to say one way or another.

Thanks,
Nate
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Re: Ambient light sensing via LED response

2009-05-01 Thread 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 w...@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

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Re: Ambient light sensing via LED response

2009-05-01 Thread John Watlington

The back-bias voltage is a sensitive topic.
If you could guarantee periodic clearing of the integrator,
I could provide up to 10V or so.   Otherwise, it should
probably be limited to +3.3V.

wad

On May 1, 2009, at 11:59 PM, John Watlington wrote:

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

 On Tue, Apr 28, 2009 at 8:38 PM, John Watlington w...@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


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