Light sensor on XO-1.75

[Paul Fox]

bert wrote:
 > 
 > On 30.08.2011, at 01:38, Richard Smith wrote:
 > 
 > > On Mon, Aug 29, 2011 at 1:06 PM, Bert Freudenberg <bert at freudenbergs.de> 
 > wrote:
 > >> On 17.08.2011, at 17:24, Martin Langhoff wrote:
 > >>> Saadia Baloch (4):
 > >>>      Driver to read Outdoor Light Sensor from EC on OLPC.
 > >> 
 > >> So this is exposed at
 > >> 
 > >>        /sys/devices/platform/olpc-ols.0/power_state
 > >> 
 > >> And it seems to sit "behind" the blinking left-most LED? And the blink rate 
 > is proportional to the light sensed? Why?
 > > 
 > > Its proportional because that's how  the sensor works.  The amount of
 > > light changes the time to bleed off the charge stored in the diode
 > > when you reverse bias it.
 > 
 > Ah, so you use the LED itself to sense light? And then time how long a cycle 
 > takes in the EC? Awesome design!

not quite.  we use a different LED to do the sensing.  it's installed
right beside the existing one.  the sensing LED is reverse-biased, so
it doesn't illuminate.

yes, it's a nice trick.  i think it was discussed on this list last year.
it's not very sensitive, but it can tell us the difference between full
sun and not, which is really when we'd want to turn off the backlight.

paul

 > 
 > This almost takes the cake over the analog mic hack.
 > 
 > - Bert -
 > 
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=---------------------
 paul fox, pgf at laptop.org