[support-gang] XO brightness key operation

James Cameron quozl at laptop.org
Mon Apr 9 21:33:01 EDT 2012

G'day Kevin

I was responsible for the idea to use an LED as a cheap light sensor on
the XO-1.75, built the first prototype, and have been following the
feature since.

To use an LED as a light sensor requires a special connection method;
either the LED must be mounted in reverse, or both terminals of the LED
must be directly connected to software driven input/output pins on a
microcontroller or CPU.

Mounting the LED in reverse prevents it from being used as an indicator.

Using two software driven pins is an additional cost if there is a
limit on the number of pins available.

The embedded controller has a limit on the current it can supply to

None of the LEDs on XO-1 or XO-1.5 are connected in a way that supports
their use as a light sensor.

All of the LEDs on XO-1 or XO-1.5 are connected to driving transistors
or MOSFETs.  This is so that two LEDs, front and back of the hinged
section, can be driven at the same time, with a tiny current draw from
the embedded controller.  But driving these LEDs in this way also
effectively prevents their use as light sensors.

I suggest that if you want to experiment with using LEDs as light
sensors, that you get yourself an Arduino or similar, and play with
that.  It is much easier than opening an XO and changing the wiring.

James Cameron

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