C720: Missing Light Sensor?

I recently had access to two Acer C720s – one i3 and on Celeron model – and
I could not pass up the opportunity to crack them open and take a peek at their
internals. [ No, no laptops were harmed in the making of this blog post – they
were going to be opened up for a mobo swap anyway ].

There are a few differences between the early-model and late-model versions
of the Acer C720. Notably, the manufacturer of the battery, the number
of screws used to attach the motherboard, and the presence of an ambient
light sensor (ALS) vary by motherboard model.

Though all C720 chassis come with the light sensor diffuser, not all of them have a functioning light sensor.

This is especially true for the Core i3 version: It has been widely reported
that this model is devoid of an ALS. It is unclear if this is universal, or
if this only applies to late-model versions. In any case, my i3 mobo also lacks
an ALS.

Here are two boards side by side:

The one on the bottom is a Celeron/2G model, mfg date March 2014; the top is a
Core i3/4G model, mfg date December 2014. They look superficially identical, but
this belies the subtle details. Obviously enough, the 2G model lacks 2 extra ram
chips (black squares under label) that the 4G model has.

Less obvious is the lack of an ALS on the i3 model. Linux first alerted me to the
issue when my backlight sensor daemon quit functioning. Dmesg shows
that the ‘isl29018’ module errors out with “__add_probed_i2c_device failed to register device 8-44”

Here is a closeup of the top left of the Celeron motherboard’s bottom (top when installed) side:

Looking even more closely, the board on has a small, surface mount light sensor

As the kernel module indicates, this chip is an Intersil ISL29018 infrared / light sensor. Because of the orientation of the board, the sensor sits directly under the plastic diffuser visible on the laptop’s front, and so can be surface mounted to the mobo itself.

The newer i3 board omits the chip. The Celeron model, which came to market first, never made use of the ALS in ChromeOS, so it is likely the chip was simply left out on the later revisions to save some $. There remains a space on the motherboard labelled for its installation, however. Maybe a quickly solder job with a replacement Intersil chip will add ALS support? (Has anyone out there tried this?)

Interestingly, all C720 boards retain a hall-effect sensor(the small 3-legged black chip, labelled 251RX), which is used to detect when the lid is closed.
Instead of a physical switch attached to the hinge, a magnet in the bezel of the
lid is used to trigger the ‘lid closed’ state. This can be confirmed by placing
a magnet up to the microphone hole near the top-left of the keyboard.

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