Light sensor on XO-1.75

Richard A. Smith richard at
Tue Aug 30 10:45:10 EDT 2011

On 08/30/2011 09:50 AM, Bert Freudenberg wrote:

>> 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!

Yes. Its not the same LED but its the same light pipe.  We stuck an 
additional LED right next to the WLAN led.  Thats why it as to be turned 
off during the reading period.

> This almost takes the cake over the analog mic hack.

Thanks.  Its a nice inexpensive solution.

Richard A. Smith  <richard at>
One Laptop per Child

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